This document describes a set of Emacs Lisp facilities borrowed from Common Lisp. All the facilities are described here in detail. While this document does not assume any prior knowledge of Common Lisp, it does assume a basic familiarity with Emacs Lisp.
Common Lisp is a huge language, and Common Lisp systems tend to be massive and extremely complex. Emacs Lisp, by contrast, is rather minimalist in the choice of Lisp features it offers the programmer. As Emacs Lisp programmers have grown in number, and the applications they write have grown more ambitious, it has become clear that Emacs Lisp could benefit from many of the conveniences of Common Lisp.
The CL package adds a number of Common Lisp functions and control structures to Emacs Lisp. While not a 100% complete implementation of Common Lisp, CL adds enough functionality to make Emacs Lisp programming significantly more convenient.
Some Common Lisp features have been omitted from this package for various reasons:
assoc function is incompatible with the
Common Lisp assoc. In such cases, this package usually
adds the suffix `*' to the function name of the Common
Lisp version of the function (e.g., assoc*).
The package described here was written by Dave Gillespie, `daveg@synaptics.com'. It is a total rewrite of the original 1986 `cl.el' package by Cesar Quiroz. Most features of the the Quiroz package have been retained; any incompatibilities are noted in the descriptions below. Care has been taken in this version to ensure that each function is defined efficiently, concisely, and with minimal impact on the rest of the Emacs environment.
Lisp code that uses features from the CL package should include at the beginning:
(require 'cl)
If you want to ensure that the new (Gillespie) version of CL
is the one that is present, add an additional (require 'cl-19)
call:
(require 'cl) (require 'cl-19)
The second call will fail (with "`cl-19.el' not found") if the old `cl.el' package was in use.
It is safe to arrange to load CL at all times, e.g.,
in your `.emacs' file. But it's a good idea, for portability,
to (require 'cl) in your code even if you do this.
The Common Lisp package is organized into four files:
cadr function won't need to pay
the overhead of loading the more advanced functions.
delete-if and assoc*.
The file `cl.el' includes all necessary autoload
commands for the functions and macros in the other three files.
All you have to do is (require 'cl), and `cl.el'
will take care of pulling in the other files when they are
needed.
There is another file, `cl-compat.el', which defines some
routines from the older `cl.el' package that are no longer
present in the new package. This includes internal routines
like setelt and zip-lists, deprecated features
like defkeyword, and an emulation of the old-style
multiple-values feature. See section Old CL Compatibility.
Installation of the CL package is simple: Just put the
byte-compiled files `cl.elc', `cl-extra.elc',
`cl-seq.elc', `cl-macs.elc', and `cl-compat.elc'
into a directory on your load-path.
There are no special requirements to compile this package: The files do not have to be loaded before they are compiled, nor do they need to be compiled in any particular order.
You may choose to put the files into your main `lisp/'
directory, replacing the original `cl.el' file there. Or,
you could put them into a directory that comes before `lisp/'
on your load-path so that the old `cl.el' is
effectively hidden.
Also, format the `cl.texinfo' file and put the resulting Info files in the `info/' directory or another suitable place.
You may instead wish to leave this package's components all in
their own directory, and then add this directory to your
load-path and (Emacs 19 only) Info-directory-list.
Add the directory to the front of the list so the old CL
package and its documentation are hidden.
Except where noted, all functions defined by this package have the same names and calling conventions as their Common Lisp counterparts.
Following is a complete list of functions whose names were changed from Common Lisp, usually to avoid conflicts with Emacs. In each case, a `*' has been appended to the Common Lisp name to obtain the Emacs name:
defun* defsubst* defmacro* function* member* assoc* rassoc* get* remove* delete* mapcar* sort* floor* ceiling* truncate* round* mod* rem* random* last*
Internal function and variable names in the package are prefixed
by cl-. Here is a complete list of functions not
prefixed by cl- which were not taken from Common Lisp:
member delete remove remq rassoc floatp-safe lexical-let lexical-let* callf callf2 letf letf* defsubst* defalias add-hook eval-when-compile
(Most of these are Emacs 19 features provided to Emacs 18 users,
or introduced, like remq, for reasons of symmetry
with similar features.)
The following simple functions and macros are defined in `cl.el'; they do not cause other components like `cl-extra' to be loaded.
eql floatp-safe abs endp evenp oddp plusp minusp butlast nbutlast caar .. cddddr list* ldiff rest first .. tenth member [1] copy-list subst mapcar* [2] adjoin [3] acons pairlis when unless pop [4] push [4] pushnew [3,4] incf [4] decf [4] proclaim declaim add-hook
[1] This is the Emacs 19-compatible function, not member*.
[2] Only for one sequence argument or two list arguments.
[3] Only if :test is eq, equal, or unspecified,
and :key is not used.
[4] Only when place is a plain variable name.
@chapno=4
This section describes features of the CL package which have to
do with programs as a whole: advanced argument lists for functions,
and the eval-when construct.
@secno=1
Emacs Lisp's notation for argument lists of functions is a subset of
the Common Lisp notation. As well as the familiar &optional
and &rest markers, Common Lisp allows you to specify default
values for optional arguments, and it provides the additional markers
&key and &aux.
Since argument parsing is built-in to Emacs, there is no way for this package to implement Common Lisp argument lists seamlessly. Instead, this package defines alternates for several Lisp forms which you must use if you need Common Lisp argument lists.
defun form, except
that arglist is allowed to be a full Common Lisp argument
list. Also, the function body is enclosed in an implicit block
called name; see section Blocks and Exits.
defun*, except that the function that
is defined is automatically proclaimed inline, i.e.,
calls to it may be expanded into in-line code by the byte compiler.
This is analogous to the defsubst form in Emacs 19;
defsubst* uses a different method (compiler macros) which
works in all version of Emacs, and also generates somewhat more
efficient inline expansions. In particular, defsubst*
arranges for the processing of keyword arguments, default values,
etc., to be done at compile-time whenever possible.
defmacro form,
except that arglist is allowed to be a full Common Lisp
argument list. The &environment keyword is supported as
described in Steele. The &whole keyword is supported only
within destructured lists (see below); top-level &whole
cannot be implemented with the current Emacs Lisp interpreter.
The macro expander body is enclosed in an implicit block called
name.
function form,
except that if the argument is a lambda form then that
form may use a full Common Lisp argument list.
Also, all forms (such as defsetf and flet) defined
in this package that include arglists in their syntax allow
full Common Lisp argument lists.
Note that it is not necessary to use defun* in
order to have access to most CL features in your function.
These features are always present; defun*'s only
difference from defun is its more flexible argument
lists and its implicit block.
The full form of a Common Lisp argument list is
(var... &optional (var initform svar)... &rest var &key ((keyword var) initform svar)... &aux (var initform)...)
Each of the five argument list sections is optional. The svar, initform, and keyword parts are optional; if they are omitted, then `(var)' may be written simply `var'.
The first section consists of zero or more required arguments. These arguments must always be specified in a call to the function; there is no difference between Emacs Lisp and Common Lisp as far as required arguments are concerned.
The second section consists of optional arguments. These
arguments may be specified in the function call; if they are not,
initform specifies the default value used for the argument.
(No initform means to use nil as the default.) The
initform is evaluated with the bindings for the preceding
arguments already established; (a &optional (b (1+ a)))
matches one or two arguments, with the second argument defaulting
to one plus the first argument. If the svar is specified,
it is an auxiliary variable which is bound to t if the optional
argument was specified, or to nil if the argument was omitted.
If you don't use an svar, then there will be no way for your
function to tell whether it was called with no argument, or with
the default value passed explicitly as an argument.
The third section consists of a single rest argument. If
more arguments were passed to the function than are accounted for
by the required and optional arguments, those extra arguments are
collected into a list and bound to the "rest" argument variable.
Common Lisp's &rest is equivalent to that of Emacs Lisp.
Common Lisp accepts &body as a synonym for &rest in
macro contexts; this package accepts it all the time.
The fourth section consists of keyword arguments. These are optional arguments which are specified by name rather than positionally in the argument list. For example,
(defun* foo (a &optional b &key c d (e 17)))
defines a function which may be called with one, two, or more
arguments. The first two arguments are bound to a and
b in the usual way. The remaining arguments must be
pairs of the form :c, :d, or :e followed
by the value to be bound to the corresponding argument variable.
(Symbols whose names begin with a colon are called keywords,
and they are self-quoting in the same way as nil and
t.)
For example, the call (foo 1 2 :d 3 :c 4) sets the five
arguments to 1, 2, 4, 3, and 17, respectively. If the same keyword
appears more than once in the function call, the first occurrence
takes precedence over the later ones. Note that it is not possible
to specify keyword arguments without specifying the optional
argument b as well, since (foo 1 :c 2) would bind
b to the keyword :c, then signal an error because
2 is not a valid keyword.
If a keyword symbol is explicitly specified in the argument list as shown in the above diagram, then that keyword will be used instead of just the variable name prefixed with a colon. You can specify a keyword symbol which does not begin with a colon at all, but such symbols will not be self-quoting; you will have to quote them explicitly with an apostrophe in the function call.
Ordinarily it is an error to pass an unrecognized keyword to
a function, e.g., (foo 1 2 :c 3 :goober 4). You can ask
Lisp to ignore unrecognized keywords, either by adding the
marker &allow-other-keys after the keyword section
of the argument list, or by specifying an :allow-other-keys
argument in the call whose value is non-nil. If the
function uses both &rest and &key at the same time,
the "rest" argument is bound to the keyword list as it appears
in the call. For example:
(defun* find-thing (thing &rest rest &key need &allow-other-keys)
(or (apply 'member* thing thing-list :allow-other-keys t rest)
(if need (error "Thing not found"))))
This function takes a :need keyword argument, but also
accepts other keyword arguments which are passed on to the
member* function. allow-other-keys is used to
keep both find-thing and member* from complaining
about each others' keywords in the arguments.
As a (significant) performance optimization, this package
implements the scan for keyword arguments by calling memq
to search for keywords in a "rest" argument. Technically
speaking, this is incorrect, since memq looks at the
odd-numbered values as well as the even-numbered keywords.
The net effect is that if you happen to pass a keyword symbol
as the value of another keyword argument, where that
keyword symbol happens to equal the name of a valid keyword
argument of the same function, then the keyword parser will
become confused. This minor bug can only affect you if you
use keyword symbols as general-purpose data in your program;
this practice is strongly discouraged in Emacs Lisp.
The fifth section of the argument list consists of auxiliary
variables. These are not really arguments at all, but simply
variables which are bound to nil or to the specified
initforms during execution of the function. There is no
difference between the following two functions, except for a
matter of stylistic taste:
(defun* foo (a b &aux (c (+ a b)) d)
body)
(defun* foo (a b)
(let ((c (+ a b)) d)
body))
Argument lists support destructuring. In Common Lisp,
destructuring is only allowed with defmacro; this package
allows it with defun* and other argument lists as well.
In destructuring, any argument variable (var in the above
diagram) can be replaced by a list of variables, or more generally,
a recursive argument list. The corresponding argument value must
be a list whose elements match this recursive argument list.
For example:
(defmacro* dolist ((var listform &optional resultform)
&rest body)
...)
This says that the first argument of dolist must be a list
of two or three items; if there are other arguments as well as this
list, they are stored in body. All features allowed in
regular argument lists are allowed in these recursive argument lists.
In addition, the clause `&whole var' is allowed at the
front of a recursive argument list. It binds var to the
whole list being matched; thus (&whole all a b) matches
a list of two things, with a bound to the first thing,
b bound to the second thing, and all bound to the
list itself. (Common Lisp allows &whole in top-level
defmacro argument lists as well, but Emacs Lisp does not
support this usage.)
One last feature of destructuring is that the argument list may be
dotted, so that the argument list (a b . c) is functionally
equivalent to (a b &rest c).
If the optimization quality safety is set to 0
(see section Declarations), error checking for wrong number of
arguments and invalid keyword arguments is disabled. By default,
argument lists are rigorously checked.
Normally, the byte-compiler does not actually execute the forms in
a file it compiles. For example, if a file contains (setq foo t),
the act of compiling it will not actually set foo to t.
This is true even if the setq was a top-level form (i.e., not
enclosed in a defun or other form). Sometimes, though, you
would like to have certain top-level forms evaluated at compile-time.
For example, the compiler effectively evaluates defmacro forms
at compile-time so that later parts of the file can refer to the
macros that are defined.
compile, load, and eval (or their long-winded
ANSI equivalents, :compile-toplevel, :load-toplevel,
and :execute).
The eval-when form is handled differently depending on
whether or not it is being compiled as a top-level form.
Specifically, it gets special treatment if it is being compiled
by a command such as byte-compile-file which compiles files
or buffers of code, and it appears either literally at the
top level of the file or inside a top-level progn.
For compiled top-level eval-whens, the body forms are
executed at compile-time if compile is in the situations
list, and the forms are written out to the file (to be executed
at load-time) if load is in the situations list.
For non-compiled-top-level forms, only the eval situation is
relevant. (This includes forms executed by the interpreter, forms
compiled with byte-compile rather than byte-compile-file,
and non-top-level forms.) The eval-when acts like a
progn if eval is specified, and like nil
(ignoring the body forms) if not.
The rules become more subtle when eval-whens are nested;
consult Steele (second edition) for the gruesome details (and
some gruesome examples).
Some simple examples:
;; Top-level forms in foo.el: (eval-when (compile) (setq foo1 'bar)) (eval-when (load) (setq foo2 'bar)) (eval-when (compile load) (setq foo3 'bar)) (eval-when (eval) (setq foo4 'bar)) (eval-when (eval compile) (setq foo5 'bar)) (eval-when (eval load) (setq foo6 'bar)) (eval-when (eval compile load) (setq foo7 'bar))
When `foo.el' is compiled, these variables will be set during the compilation itself:
foo1 foo3 foo5 foo7 ; `compile'
When `foo.elc' is loaded, these variables will be set:
foo2 foo3 foo6 foo7 ; `load'
And if `foo.el' is loaded uncompiled, these variables will be set:
foo4 foo5 foo6 foo7 ; `eval'
If these seven eval-whens had been, say, inside a defun,
then the first three would have been equivalent to nil and the
last four would have been equivalent to the corresponding setqs.
Note that (eval-when (load eval) ...) is equivalent
to (progn ...) in all contexts. The compiler treats
certain top-level forms, like defmacro (sort-of) and
require, as if they were wrapped in (eval-when
(compile load eval) ...).
Emacs 19 includes two special forms related to eval-when.
One of these, eval-when-compile, is not quite equivalent to
any eval-when construct and is described below. This package
defines a version of eval-when-compile for the benefit of
Emacs 18 users.
The other form, (eval-and-compile ...), is exactly
equivalent to `(eval-when (compile load eval) ...)' and
so is not itself defined by this package.
eval-when-compile is just like `eval-when
(compile eval)'. In other contexts, eval-when-compile
allows code to be evaluated once at compile-time for efficiency
or other reasons.
This form is similar to the `#.' syntax of true Common Lisp.
Early Common Lisp had a `#,' syntax that was similar to
this, but ANSI Common Lisp replaced it with load-time-value
and gave it more well-defined semantics.
In a compiled file, load-time-value arranges for form
to be evaluated when the `.elc' file is loaded and then used
as if it were a quoted constant. In code compiled by
byte-compile rather than byte-compile-file, the
effect is identical to eval-when-compile. In uncompiled
code, both eval-when-compile and load-time-value
act exactly like progn.
(defun report ()
(insert "This function was executed on: "
(current-time-string)
", compiled on: "
(eval-when-compile (current-time-string))
;; or '#.(current-time-string) in real Common Lisp
", and loaded on: "
(load-time-value (current-time-string))))
Byte-compiled, the above defun will result in the following code (or its compiled equivalent, of course) in the `.elc' file:
(setq --temp-- (current-time-string))
(defun report ()
(insert "This function was executed on: "
(current-time-string)
", compiled on: "
'"Wed Jun 23 18:33:43 1993"
", and loaded on: "
--temp--))
This section describes a feature from GNU Emacs 19 which this package makes available in other versions of Emacs.
fset, except that in GNU Emacs 19 it also
records the setting in load-history so that it can be undone
by a later unload-feature.
In other versions of Emacs, defalias is a synonym for
fset.
This section describes functions for testing whether various facts are true or false.
The CL package defines a version of the Common Lisp typep
predicate.
(typep foo 'integer) is equivalent to (integerp foo).
The type argument to the above function is either a symbol or a list beginning with a symbol.
t stands for the union of all types.
(typep object t) is always true. Likewise, the
type symbol nil stands for nothing at all, and
(typep object nil) is always false.
null represents the symbol nil.
Thus (typep object 'null) is equivalent to
(null object).
real is a synonym for number, and
fixnum is a synonym for integer.
character and string-char match
integers in the range from 0 to 255.
float uses the floatp-safe predicate
defined by this package rather than floatp, so it will work
correctly even in Emacs versions without floating-point support.
(integer low high) represents all
integers between low and high, inclusive. Either bound
may be a list of a single integer to specify an exclusive limit,
or a * to specify no limit. The type (integer * *)
is thus equivalent to integer.
float, real, or
number represent numbers of that type falling in a particular
range.
and, or, and not form
combinations of types. For example, (or integer (float 0 *))
represents all objects that are integers or non-negative floats.
member or member* represent
objects eql to any of the following values. For example,
(member 1 2 3 4) is equivalent to (integer 1 4),
and (member nil) is equivalent to null.
(satisfies predicate) represent
all objects for which predicate returns true when called
with that object as an argument.
The following function and macro (not technically predicates) are
related to typep.
typep, it is simply returned. Otherwise, certain types of
conversions will be made: If type is any sequence type
(string, list, etc.) then object will be
converted to that type if possible. If type is
character, then strings of length one and symbols with
one-character names can be coerced. If type is float,
then integers can be coerced in versions of Emacs that support
floats. In all other circumstances, coerce signals an
error.
defmacro in many ways; when name is encountered
as a type name, the body forms are evaluated and should
return a type specifier that is equivalent to the type. The
arglist is a Common Lisp argument list of the sort accepted
by defmacro*. The type specifier `(name args...)'
is expanded by calling the expander with those arguments; the type
symbol `name' is expanded by calling the expander with
no arguments. The arglist is processed the same as for
defmacro* except that optional arguments without explicit
defaults use * instead of nil as the "default"
default. Some examples:
(deftype null () '(satisfies null)) ; predefined (deftype list () '(or null cons)) ; predefined (deftype unsigned-byte (&optional bits) (list 'integer 0 (if (eq bits '*) bits (1- (lsh 1 bits))))) (unsigned-byte 8) == (integer 0 255) (unsigned-byte) == (integer 0 *) unsigned-byte == (integer 0 *)
The last example shows how the Common Lisp unsigned-byte
type specifier could be implemented if desired; this package does
not implement unsigned-byte by default.
The typecase and check-type macros also use type
names. See section Conditionals. See section Assertions and Errors. The map,
concatenate, and merge functions take type-name
arguments to specify the type of sequence to return. See section Sequences.
This package defines two Common Lisp predicates, eql and
equalp.
eq, except that if a
and b are numbers of the same type, it compares them for numeric
equality (as if by equal instead of eq). This makes a
difference only for versions of Emacs that are compiled with
floating-point support, such as Emacs 19. Emacs floats are allocated
objects just like cons cells, which means that (eq 3.0 3.0)
will not necessarily be true--if the two 3.0s were allocated
separately, the pointers will be different even though the numbers are
the same. But (eql 3.0 3.0) will always be true.
The types of the arguments must match, so (eql 3 3.0) is
still false.
Note that Emacs integers are "direct" rather than allocated, which
basically means (eq 3 3) will always be true. Thus eq
and eql behave differently only if floating-point numbers are
involved, and are indistinguishable on Emacs versions that don't
support floats.
There is a slight inconsistency with Common Lisp in the treatment of
positive and negative zeros. Some machines, notably those with IEEE
standard arithmetic, represent +0 and -0 as distinct
values. Normally this doesn't matter because the standard specifies
that (= 0.0 -0.0) should always be true, and this is indeed
what Emacs Lisp and Common Lisp do. But the Common Lisp standard
states that (eql 0.0 -0.0) and (equal 0.0 -0.0) should
be false on IEEE-like machines; Emacs Lisp does not do this, and in
fact the only known way to distinguish between the two zeros in Emacs
Lisp is to format them and check for a minus sign.
equal. In
particular, it compares strings case-insensitively, and it compares
numbers without regard to type (so that (equalp 3 3.0) is
true). Vectors and conses are compared recursively. All other
objects are compared as if by equal.
This function differs from Common Lisp equalp in several
respects. First, Common Lisp's equalp also compares
characters case-insensitively, which would be impractical
in this package since Emacs does not distinguish between integers
and characters. In keeping with the idea that strings are less
vector-like in Emacs Lisp, this package's equalp also will
not compare strings against vectors of integers. Finally, Common
Lisp's equalp compares hash tables without regard to
ordering, whereas this package simply compares hash tables in
terms of their underlying structure (which means vectors for Lucid
Emacs 19 hash tables, or lists for other hash tables).
Also note that the Common Lisp functions member and assoc
use eql to compare elements, whereas Emacs Lisp follows the
MacLisp tradition and uses equal for these two functions.
In Emacs, use member* and assoc* to get functions
which use eql for comparisons.
The features described in the following sections implement
various advanced control structures, including the powerful
setf facility and a number of looping and conditional
constructs.
The psetq form is just like setq, except that multiple
assignments are done in parallel rather than sequentially.
setq. Given several symbol
and form pairs, it evaluates all the forms in advance
and then stores the corresponding variables afterwards.
(setq x 2 y 3)
(setq x (+ x y) y (* x y))
x
=> 5
y ; y was computed after x was set.
=> 15
(setq x 2 y 3)
(psetq x (+ x y) y (* x y))
x
=> 5
y ; y was computed before x was set.
=> 6
The simplest use of psetq is (psetq x y y x), which
exchanges the values of two variables. (The rotatef form
provides an even more convenient way to swap two variables;
see section Modify Macros.)
psetq always returns nil.
A "generalized variable" or "place form" is one of the many places in Lisp memory where values can be stored. The simplest place form is a regular Lisp variable. But the cars and cdrs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values are stored.
The setf form is like setq, except that it accepts
arbitrary place forms on the left side rather than just
symbols. For example, (setf (car a) b) sets the car of
a to b, doing the same operation as (setcar a b)
but without having to remember two separate functions for setting
and accessing every type of place.
Generalized variables are analogous to "lvalues" in the C
language, where `x = a[i]' gets an element from an array
and `a[i] = x' stores an element using the same notation.
Just as certain forms like a[i] can be lvalues in C, there
is a set of forms that can be generalized variables in Lisp.
The setf macro is the most basic way to operate on generalized
variables.
setq. setf returns the value of the last
form.
The following Lisp forms will work as generalized variables, and
so may legally appear in the place argument of setf:
(setf x y) is
exactly equivalent to (setq x y), and setq itself is
strictly speaking redundant now that setf exists. Many
programmers continue to prefer setq for setting simple
variables, though, purely for stylistic or historical reasons.
The macro (setf x y) actually expands to (setq x y),
so there is no performance penalty for using it in compiled code.
car cdr caar .. cddddr nth rest first .. tenth aref elt nthcdr symbol-function symbol-value symbol-plist get get* getf gethash subseqNote that for
nthcdr and getf, the list argument
of the function must itself be a valid place form. For
example, (setf (nthcdr 0 foo) 7) will set foo itself
to 7. Note that push and pop on an nthcdr
place can be used to insert or delete at any position in a list.
The use of nthcdr as a place form is an extension
to standard Common Lisp.
setf-able.
(Some of these are defined only in Emacs 19 or only in Lucid Emacs.)
buffer-file-name marker-position buffer-modified-p match-data buffer-name mouse-position buffer-string overlay-end buffer-substring overlay-get current-buffer overlay-start current-case-table point current-column point-marker current-global-map point-max current-input-mode point-min current-local-map process-buffer current-window-configuration process-filter default-file-modes process-sentinel default-value read-mouse-position documentation-property screen-height extent-data screen-menubar extent-end-position screen-width extent-start-position selected-window face-background selected-screen face-background-pixmap selected-frame face-font standard-case-table face-foreground syntax-table face-underline-p window-buffer file-modes window-dedicated-p frame-height window-display-table frame-parameters window-height frame-visible-p window-hscroll frame-width window-point get-register window-start getenv window-width global-key-binding x-get-cut-buffer keymap-parent x-get-cutbuffer local-key-binding x-get-secondary-selection mark x-get-selection mark-markerMost of these have directly corresponding "set" functions, like
use-local-map for current-local-map, or goto-char
for point. A few, like point-min, expand to longer
sequences of code when they are setf'd ((narrow-to-region
x (point-max)) in this case).
(substring subplace n [m]),
where subplace is itself a legal generalized variable whose
current value is a string, and where the value stored is also a
string. The new string is spliced into the specified part of the
destination string. For example:
(setq a (list "hello" "world"))
=> ("hello" "world")
(cadr a)
=> "world"
(substring (cadr a) 2 4)
=> "rl"
(setf (substring (cadr a) 2 4) "o")
=> "o"
(cadr a)
=> "wood"
a
=> ("hello" "wood")
The generalized variable buffer-substring, listed above,
also works in this way by replacing a portion of the current buffer.
(apply 'func ...) or
(apply (function func) ...), where func
is a setf-able function whose store function is "suitable"
in the sense described in Steele's book; since none of the standard
Emacs place functions are suitable in this sense, this feature is
only interesting when used with places you define yourself with
define-setf-method or the long form of defsetf.
setf
is applied to the resulting form.
defsetf or define-setf-method
has been made.
Using any forms other than these in the place argument to
setf will signal an error.
The setf macro takes care to evaluate all subforms in
the proper left-to-right order; for example,
(setf (aref vec (incf i)) i)
looks like it will evaluate (incf i) exactly once, before the
following access to i; the setf expander will insert
temporary variables as necessary to ensure that it does in fact work
this way no matter what setf-method is defined for aref.
(In this case, aset would be used and no such steps would
be necessary since aset takes its arguments in a convenient
order.)
However, if the place form is a macro which explicitly evaluates its arguments in an unusual order, this unusual order will be preserved. Adapting an example from Steele, given
(defmacro wrong-order (x y) (list 'aref y x))
the form (setf (wrong-order a b) 17) will
evaluate b first, then a, just as in an actual call
to wrong-order.
This package defines a number of other macros besides setf
that operate on generalized variables. Many are interesting and
useful even when the place is just a variable name.
setf what psetq is to setq:
When several places and forms are involved, the
assignments take place in parallel rather than sequentially.
Specifically, all subforms are evaluated from left to right, then
all the assignments are done (in an undefined order).
(incf i) is equivalent to (setq i (1+ i)), and
(incf (car x) 2) is equivalent to (setcar x (+ (car x) 2)).
Once again, care is taken to preserve the "apparent" order of evaluation. For example,
(incf (aref vec (incf i)))
appears to increment i once, then increment the element of
vec addressed by i; this is indeed exactly what it
does, which means the above form is not equivalent to the
"obvious" expansion,
(setf (aref vec (incf i)) (1+ (aref vec (incf i)))) ; Wrong!
but rather to something more like
(let ((temp (incf i))) (setf (aref vec temp) (1+ (aref vec temp))))
Again, all of this is taken care of automatically by incf and
the other generalized-variable macros.
As a more Emacs-specific example of incf, the expression
(incf (point) n) is essentially equivalent to
(forward-char n).
(prog1 (car place)
(setf place (cdr place))), except that it takes care
to evaluate all subforms only once.
(setf place (cons
x place)), except for evaluation of the subforms.
eql to any
existing element of the list. The optional keyword arguments
are interpreted in the same way as for adjoin.
See section Lists as Sets.
(shiftf a b c
d) is equivalent to
(prog1
a
(psetf a b
b c
c d))
except that the subforms of a, b, and c are actually evaluated only once each and in the apparent order.
(rotatef a b c d) is equivalent to
(psetf a b
b c
c d
d a)
except for the evaluation of subforms. rotatef always
returns nil. Note that (rotatef a b)
conveniently exchanges a and b.
The following macros were invented for this package; they have no analogues in Common Lisp.
let, but for generalized variables
rather than just symbols. Each binding should be of the form
(place value); the original contents of the
places are saved, the values are stored in them, and
then the body forms are executed. Afterwards, the places
are set back to their original saved contents. This cleanup happens
even if the forms exit irregularly due to a throw or an
error.
For example,
(letf (((point) (point-min))
(a 17))
...)
moves "point" in the current buffer to the beginning of the buffer,
and also binds a to 17 (as if by a normal let, since
a is just a regular variable). After the body exits, a
is set back to its original value and point is moved back to its
original position.
Note that letf on (point) is not quite like a
save-excursion, as the latter effectively saves a marker
which tracks insertions and deletions in the buffer. Actually,
a letf of (point-marker) is much closer to this
behavior. (point and point-marker are equivalent
as setf places; each will accept either an integer or a
marker as the stored value.)
Since generalized variables look like lists, let's shorthand
of using `foo' for `(foo nil)' as a binding would
be ambiguous in letf and is not allowed.
However, a binding specifier may be a one-element list
`(place)', which is similar to `(place
place)'. In other words, the place is not disturbed
on entry to the body, and the only effect of the letf is
to restore the original value of place afterwards. (The
redundant access-and-store suggested by the (place
place) example does not actually occur.)
In most cases, the place must have a well-defined value on
entry to the letf form. The only exceptions are plain
variables and calls to symbol-value and symbol-function.
If the symbol is not bound on entry, it is simply made unbound by
makunbound or fmakunbound on exit.
letf what let* is to let:
It does the bindings in sequential rather than parallel order.
(incf place
n) is the same as (callf + place n).
Some more examples:
(callf abs my-number) (callf concat (buffer-name) "<" (int-to-string n) ">") (callf union happy-people (list joe bob) :test 'same-person)
See section Customizing Setf, for define-modify-macro, a way
to create even more concise notations for modify macros. Note
again that callf is an extension to standard Common Lisp.
callf, except that place is
the second argument of function rather than the
first. For example, (push x place) is
equivalent to (callf2 cons x place).
The callf and callf2 macros serve as building
blocks for other macros like incf, pushnew, and
define-modify-macro. The letf and letf*
macros are used in the processing of symbol macros;
see section Macro Bindings.
Common Lisp defines three macros, define-modify-macro,
defsetf, and define-setf-method, that allow the
user to extend generalized variables in various ways.
incf and decf. The macro name is defined
to take a place argument followed by additional arguments
described by arglist. The call
(name place args...)
will be expanded to
(callf func place args...)
which in turn is roughly equivalent to
(setf place (func place args...))
For example:
(define-modify-macro incf (&optional (n 1)) +) (define-modify-macro concatf (&rest args) concat)
Note that &key is not allowed in arglist, but
&rest is sufficient to pass keywords on to the function.
Most of the modify macros defined by Common Lisp do not exactly
follow the pattern of define-modify-macro. For example,
push takes its arguments in the wrong order, and pop
is completely irregular. You can define these macros "by hand"
using get-setf-method, or consult the source file
`cl-macs.el' to see how to use the internal setf
building blocks.
defsetf forms. Where
access-fn is the name of a function which accesses a place,
this declares update-fn to be the corresponding store
function. From now on,
(setf (access-fn arg1 arg2 arg3) value)
will be expanded to
(update-fn arg1 arg2 arg3 value)
The update-fn is required to be either a true function, or
a macro which evaluates its arguments in a function-like way. Also,
the update-fn is expected to return value as its result.
Otherwise, the above expansion would not obey the rules for the way
setf is supposed to behave.
As a special (non-Common-Lisp) extension, a third argument of t
to defsetf says that the update-fn's return value is
not suitable, so that the above setf should be expanded to
something more like
(let ((temp value)) (update-fn arg1 arg2 arg3 temp) temp)
Some examples of the use of defsetf, drawn from the standard
suite of setf methods, are:
(defsetf car setcar) (defsetf symbol-value set) (defsetf buffer-name rename-buffer t)
defsetf. It is
rather like defmacro except for the additional store-var
argument. The forms should return a Lisp form which stores
the value of store-var into the generalized variable formed
by a call to access-fn with arguments described by arglist.
The forms may begin with a string which documents the setf
method (analogous to the doc string that appears at the front of a
function).
For example, the simple form of defsetf is shorthand for
(defsetf access-fn (&rest args) (store) (append '(update-fn) args (list store)))
The Lisp form that is returned can access the arguments from
arglist and store-var in an unrestricted fashion;
macros like setf and incf which invoke this
setf-method will insert temporary variables as needed to make
sure the apparent order of evaluation is preserved.
Another example drawn from the standard package:
(defsetf nth (n x) (store) (list 'setcar (list 'nthcdr n x) store))
setf to access-fn with arguments described by
arglist is expanded, the forms are evaluated and
must return a list of five items:
gensym).
This is exactly like the Common Lisp macro of the same name, except that the method returns a list of five values rather than the five values themselves, since Emacs Lisp does not support Common Lisp's notion of multiple return values.
Once again, the forms may begin with a documentation string.
A setf-method should be maximally conservative with regard to
temporary variables. In the setf-methods generated by
defsetf, the second return value is simply the list of
arguments in the place form, and the first return value is a
list of a corresponding number of temporary variables generated
by gensym. Macros like setf and incf which
use this setf-method will optimize away most temporaries that
turn out to be unnecessary, so there is little reason for the
setf-method itself to optimize.
defsetf
or define-setf-method. The result is a list of five
values as described above. You can use this function to build
your own incf-like modify macros. (Actually, it is
better to use the internal functions cl-setf-do-modify
and cl-setf-do-store, which are a bit easier to use and
which also do a number of optimizations; consult the source
code for the incf function for a simple example.)
The argument env specifies the "environment" to be
passed on to macroexpand if get-setf-method should
need to expand a macro in place. It should come from
an &environment argument to the macro or setf-method
that called get-setf-method.
See also the source code for the setf-methods for apply
and substring, each of which works by calling
get-setf-method on a simpler case, then massaging
the result in various ways.
Modern Common Lisp defines a second, independent way to specify
the setf behavior of a function, namely "setf
functions" whose names are lists (setf name)
rather than symbols. For example, (defun (setf foo) ...)
defines the function that is used when setf is applied to
foo. This package does not currently support setf
functions. In particular, it is a compile-time error to use
setf on a form which has not already been defsetf'd
or otherwise declared; in newer Common Lisps, this would not be
an error since the function (setf func) might be
defined later.
@secno=4
These Lisp forms make bindings to variables and function names,
analogous to Lisp's built-in let form.
See section Modify Macros, for the letf and letf* forms which
are also related to variable bindings.
The standard let form binds variables whose names are known
at compile-time. The progv form provides an easy way to
bind variables whose names are computed at run-time.
let-style variable bindings on a
set of variables computed at run-time. The expressions
symbols and values are evaluated, and must return lists
of symbols and values, respectively. The symbols are bound to the
corresponding values for the duration of the body forms.
If values is shorter than symbols, the last few symbols
are made unbound (as if by makunbound) inside the body.
If symbols is shorter than values, the excess values
are ignored.
The CL package defines the following macro which
more closely follows the Common Lisp let form:
let except that the bindings it
establishes are purely lexical. Lexical bindings are similar to
local variables in a language like C: Only the code physically
within the body of the lexical-let (after macro expansion)
may refer to the bound variables.
(setq a 5)
(defun foo (b) (+ a b))
(let ((a 2)) (foo a))
=> 4
(lexical-let ((a 2)) (foo a))
=> 7
In this example, a regular let binding of a actually
makes a temporary change to the global variable a, so foo
is able to see the binding of a to 2. But lexical-let
actually creates a distinct local variable a for use within its
body, without any effect on the global variable of the same name.
The most important use of lexical bindings is to create closures. A closure is a function object that refers to an outside lexical variable. For example:
(defun make-adder (n)
(lexical-let ((n n))
(function (lambda (m) (+ n m)))))
(setq add17 (make-adder 17))
(funcall add17 4)
=> 21
The call (make-adder 17) returns a function object which adds
17 to its argument. If let had been used instead of
lexical-let, the function object would have referred to the
global n, which would have been bound to 17 only during the
call to make-adder itself.
(defun make-counter ()
(lexical-let ((n 0))
(function* (lambda (&optional (m 1)) (incf n m)))))
(setq count-1 (make-counter))
(funcall count-1 3)
=> 3
(funcall count-1 14)
=> 17
(setq count-2 (make-counter))
(funcall count-2 5)
=> 5
(funcall count-1 2)
=> 19
(funcall count-2)
=> 6
Here we see that each call to make-counter creates a distinct
local variable n, which serves as a private counter for the
function object that is returned.
Closed-over lexical variables persist until the last reference to
them goes away, just like all other Lisp objects. For example,
count-2 refers to a function object which refers to an
instance of the variable n; this is the only reference
to that variable, so after (setq count-2 nil) the garbage
collector would be able to delete this instance of n.
Of course, if a lexical-let does not actually create any
closures, then the lexical variables are free as soon as the
lexical-let returns.
Many closures are used only during the extent of the bindings they
refer to; these are known as "downward funargs" in Lisp parlance.
When a closure is used in this way, regular Emacs Lisp dynamic
bindings suffice and will be more efficient than lexical-let
closures:
(defun add-to-list (x list)
(mapcar (function (lambda (y) (+ x y))) list))
(add-to-list 7 '(1 2 5))
=> (8 9 12)
Since this lambda is only used while x is still bound,
it is not necessary to make a true closure out of it.
You can use defun or flet inside a lexical-let
to create a named closure. If several closures are created in the
body of a single lexical-let, they all close over the same
instance of the lexical variable.
The lexical-let form is an extension to Common Lisp. In
true Common Lisp, all bindings are lexical unless declared otherwise.
lexical-let, except that the bindings
are made sequentially in the manner of let*.
These forms make let-like bindings to functions instead
of variables.
let-style bindings on the function
cells of symbols rather than on the value cells. Each binding
must be a list of the form `(name arglist
forms...)', which defines a function exactly as if
it were a defun* form. The function name is defined
accordingly for the duration of the body of the flet; then
the old function definition, or lack thereof, is restored.
While flet in Common Lisp establishes a lexical binding of
name, Emacs Lisp flet makes a dynamic binding. The
result is that flet affects indirect calls to a function as
well as calls directly inside the flet form itself.
You can use flet to disable or modify the behavior of a
function in a temporary fashion. This will even work on Emacs
primitives, although note that some calls to primitive functions
internal to Emacs are made without going through the symbol's
function cell, and so will not be affected by flet. For
example,
(flet ((message (&rest args) (push args saved-msgs))) (do-something))
This code attempts to replace the built-in function message
with a function that simply saves the messages in a list rather
than displaying them. The original definition of message
will be restored after do-something exits. This code will
work fine on messages generated by other Lisp code, but messages
generated directly inside Emacs will not be caught since they make
direct C-language calls to the message routines rather than going
through the Lisp message function.
Functions defined by flet may use the full Common Lisp
argument notation supported by defun*; also, the function
body is enclosed in an implicit block as if by defun*.
See section Program Structure.
labels form is like flet, except that it
makes lexical bindings of the function names rather than
dynamic bindings. (In true Common Lisp, both flet and
labels make lexical bindings of slightly different sorts;
since Emacs Lisp is dynamically bound by default, it seemed
more appropriate for flet also to use dynamic binding.
The labels form, with its lexical binding, is fully
compatible with Common Lisp.)
Lexical scoping means that all references to the named
functions must appear physically within the body of the
labels form. References may appear both in the body
forms of labels itself, and in the bodies of
the functions themselves. Thus, labels can define
local recursive functions, or mutually-recursive sets of
functions.
A "reference" to a function name is either a call to that
function, or a use of its name quoted by quote or
function to be passed on to, say, mapcar.
These forms create local macros and "symbol macros."
flet, but for macros instead of
functions. Each binding is a list of the same form as the
arguments to defmacro* (i.e., a macro name, argument list,
and macro-expander forms). The macro is defined accordingly for
use within the body of the macrolet.
Because of the nature of macros, macrolet is lexically
scoped even in Emacs Lisp: The macrolet binding will
affect only calls that appear physically within the body
forms, possibly after expansion of other macros in the
body.
(setq bar '(5 . 9))
(symbol-macrolet ((foo (car bar)))
(incf foo))
bar
=> (6 . 9)
A setq of a symbol macro is treated the same as a setf.
I.e., (setq foo 4) in the above would be equivalent to
(setf foo 4), which in turn expands to (setf (car bar) 4).
Likewise, a let or let* binding a symbol macro is
treated like a letf or letf*. This differs from true
Common Lisp, where the rules of lexical scoping cause a let
binding to shadow a symbol-macrolet binding. In this package,
only lexical-let and lexical-let* will shadow a symbol
macro.
There is no analogue of defmacro for symbol macros; all symbol
macros are local. A typical use of symbol-macrolet is in the
expansion of another macro:
(defmacro* my-dolist ((x list) &rest body)
(let ((var (gensym)))
(list 'loop 'for var 'on list 'do
(list* 'symbol-macrolet (list (list x (list 'car var)))
body))))
(setq mylist '(1 2 3 4))
(my-dolist (x mylist) (incf x))
mylist
=> (2 3 4 5)
In this example, the my-dolist macro is similar to dolist
(see section Iteration) except that the variable x becomes a true
reference onto the elements of the list. The my-dolist call
shown here expands to
(loop for G1234 on mylist do
(symbol-macrolet ((x (car G1234)))
(incf x)))
which in turn expands to
(loop for G1234 on mylist do (incf (car G1234)))
See section Loop Facility, for a description of the loop macro.
This package defines a nonstandard in-ref loop clause that
works much like my-dolist.
These conditional forms augment Emacs Lisp's simple if,
and, or, and cond forms.
if where there are no "else" forms,
and possibly several "then" forms. In particular,
(when test a b c)
is entirely equivalent to
(if test (progn a b c) nil)
if where there are no "then" forms,
and possibly several "else" forms:
(unless test a b c)
is entirely equivalent to
(when (not test) a b c)
eql. If no clause
matches, the case form returns nil. The clauses are
of the form
(keylist body-forms...)
where keylist is a list of key values. If there is exactly
one value, and it is not a cons cell or the symbol nil or
t, then it can be used by itself as a keylist without
being enclosed in a list. All key values in the case form
must be distinct. The final clauses may use t in place of
a keylist to indicate a default clause that should be taken
if none of the other clauses match. (The symbol otherwise
is also recognized in place of t. To make a clause that
matches the actual symbol t, nil, or otherwise,
enclose the symbol in a list.)
For example, this expression reads a keystroke, then does one of four things depending on whether it is an `a', a `b', a RET or C-j, or anything else.
(case (read-char) (?a (do-a-thing)) (?b (do-b-thing)) ((?\r ?\n) (do-ret-thing)) (t (do-other-thing)))
case, except that if the key does
not match any of the clauses, an error is signaled rather than
simply returning nil.
case that checks for types
rather than values. Each clause is of the form
`(type body...)'. See section Type Predicates,
for a description of type specifiers. For example,
(typecase x (integer (munch-integer x)) (float (munch-float x)) (string (munch-integer (string-to-int x))) (t (munch-anything x)))
The type specifier t matches any type of object; the word
otherwise is also allowed. To make one clause match any of
several types, use an (or ...) type specifier.
typecase, except that if the key does
not match any of the clauses, an error is signaled rather than
simply returning nil.
Common Lisp blocks provide a non-local exit mechanism very
similar to catch and throw, but lexically rather than
dynamically scoped. This package actually implements block
in terms of catch; however, the lexical scoping allows the
optimizing byte-compiler to omit the costly catch step if the
body of the block does not actually return-from the block.
progn. However,
if any of the forms execute (return-from name),
they will jump out and return directly from the block form.
The block returns the result of the last form unless
a return-from occurs.
The block/return-from mechanism is quite similar to
the catch/throw mechanism. The main differences are
that block names are unevaluated symbols, rather than forms
(such as quoted symbols) which evaluate to a tag at run-time; and
also that blocks are lexically scoped whereas catch/throw
are dynamically scoped. This means that functions called from the
body of a catch can also throw to the catch,
but the return-from referring to a block name must appear
physically within the forms that make up the body of the block.
They may not appear within other called functions, although they may
appear within macro expansions or lambdas in the body. Block
names and catch names form independent name-spaces.
In true Common Lisp, defun and defmacro surround
the function or expander bodies with implicit blocks with the
same name as the function or macro. This does not occur in Emacs
Lisp, but this package provides defun* and defmacro*
forms which do create the implicit block.
The Common Lisp looping constructs defined by this package,
such as loop and dolist, also create implicit blocks
just as in Common Lisp.
Because they are implemented in terms of Emacs Lisp catch
and throw, blocks have the same overhead as actual
catch constructs (roughly two function calls). However,
Zawinski and Furuseth's optimizing byte compiler (standard in
Emacs 19) will optimize away the catch if the block does
not in fact contain any return or return-from calls
that jump to it. This means that do loops and defun*
functions which don't use return don't pay the overhead to
support it.
block.
Otherwise, nil is returned.
(return-from nil result).
Common Lisp loops like do and dolist implicitly enclose
themselves in nil blocks.
The macros described here provide more sophisticated, high-level
looping constructs to complement Emacs Lisp's basic while
loop.
loop and the extremely powerful and flexible feature known as
the Loop Facility or Loop Macro. This more advanced
facility is discussed in the following section; see section Loop Facility.
The simple form of loop is described here.
If loop is followed by zero or more Lisp expressions,
then (loop exprs...) simply creates an infinite
loop executing the expressions over and over. The loop is
enclosed in an implicit nil block. Thus,
(loop (foo) (if (no-more) (return 72)) (bar))
is exactly equivalent to
(block nil (while t (foo) (if (no-more) (return 72)) (bar)))
If any of the expressions are plain symbols, the loop is instead interpreted as a Loop Macro specification as described later. (This is not a restriction in practice, since a plain symbol in the above notation would simply access and throw away the value of a variable.)
(var [init [step]])
The loop works as follows: First, each var is bound to the
associated init value as if by a let form. Then, in
each iteration of the loop, the end-test is evaluated; if
true, the loop is finished. Otherwise, the body forms are
evaluated, then each var is set to the associated step
expression (as if by a psetq form) and the next iteration
begins. Once the end-test becomes true, the result
forms are evaluated (with the vars still bound to their
values) to produce the result returned by do.
The entire do loop is enclosed in an implicit nil
block, so that you can use (return) to break out of the
loop at any time.
If there are no result forms, the loop returns nil.
If a given var has no step form, it is bound to its
init value but not otherwise modified during the do
loop (unless the code explicitly modifies it); this case is just
a shorthand for putting a (let ((var init)) ...)
around the loop. If init is also omitted it defaults to
nil, and in this case a plain `var' can be used
in place of `(var)', again following the analogy with
let.
This example (from Steele) illustrates a loop which applies the
function f to successive pairs of values from the lists
foo and bar; it is equivalent to the call
(mapcar* 'f foo bar). Note that this loop has no body
forms at all, performing all its work as side effects of
the rest of the loop.
(do ((x foo (cdr x))
(y bar (cdr y))
(z nil (cons (f (car x) (car y)) z)))
((or (null x) (null y))
(nreverse z)))
do what let* is to let. In
particular, the initial values are bound as if by let*
rather than let, and the steps are assigned as if by
setq rather than psetq.
Here is another way to write the above loop:
(do* ((xp foo (cdr xp))
(yp bar (cdr yp))
(x (car xp) (car xp))
(y (car yp) (car yp))
z)
((or (null xp) (null yp))
(nreverse z))
(push (f x y) z))
nil) is evaluated
with var bound to nil to produce the result returned by
the loop. The loop is surrounded by an implicit nil block.
result form is evaluated with var bound to the total
number of iterations that were done (i.e., (max 0 count))
to get the return value for the loop form. The loop is surrounded
by an implicit nil block.
nil, it loops over all symbols in
that obarray. For each symbol, the body forms are evaluated
with var bound to that symbol. The symbols are visited in
an unspecified order. Afterward the result form, if any,
is evaluated (with var bound to nil) to get the return
value. The loop is surrounded by an implicit nil block.
do-symbols except that the obarray
argument is omitted; it always iterates over the default obarray.
See section Mapping over Sequences, for some more functions for iterating over vectors or lists.
A common complaint with Lisp's traditional looping constructs is
that they are either too simple and limited, such as Common Lisp's
dotimes or Emacs Lisp's while, or too unreadable and
obscure, like Common Lisp's do loop.
To remedy this, recent versions of Common Lisp have added a new
construct called the "Loop Facility" or "loop macro,"
with an easy-to-use but very powerful and expressive syntax.
The loop macro essentially creates a mini-language within
Lisp that is specially tailored for describing loops. While this
language is a little strange-looking by the standards of regular Lisp,
it turns out to be very easy to learn and well-suited to its purpose.
Since loop is a macro, all parsing of the loop language
takes place at byte-compile time; compiled loops are just
as efficient as the equivalent while loops written longhand.
for or do. Clauses
are simply strung together in the argument list of loop,
with minimal extra parentheses. The various types of clauses
specify initializations, such as the binding of temporary
variables, actions to be taken in the loop, stepping actions,
and final cleanup.
Common Lisp specifies a certain general order of clauses in a loop:
(loop name-clause
var-clauses...
action-clauses...)
The name-clause optionally gives a name to the implicit
block that surrounds the loop. By default, the implicit block
is named nil. The var-clauses specify what
variables should be bound during the loop, and how they should
be modified or iterated throughout the course of the loop. The
action-clauses are things to be done during the loop, such
as computing, collecting, and returning values.
The Emacs version of the loop macro is less restrictive about
the order of clauses, but things will behave most predictably if
you put the variable-binding clauses with, for, and
repeat before the action clauses. As in Common Lisp,
initially and finally clauses can go anywhere.
Loops generally return nil by default, but you can cause
them to return a value by using an accumulation clause like
collect, an end-test clause like always, or an
explicit return clause to jump out of the implicit block.
(Because the loop body is enclosed in an implicit block, you can
also use regular Lisp return or return-from to
break out of the loop.)
The following sections give some examples of the Loop Macro in
action, and describe the particular loop clauses in great detail.
Consult the second edition of Steele's Common Lisp, the Language,
for additional discussion and examples of the loop macro.
Before listing the full set of clauses that are allowed, let's
look at a few example loops just to get a feel for the loop
language.
(loop for buf in (buffer-list)
collect (buffer-file-name buf))
This loop iterates over all Emacs buffers, using the list
returned by buffer-list. For each buffer buf,
it calls buffer-file-name and collects the results into
a list, which is then returned from the loop construct.
The result is a list of the file names of all the buffers in
Emacs' memory. The words for, in, and collect
are reserved words in the loop language.
(loop repeat 20 do (insert "Yowsa\n"))
This loop inserts the phrase "Yowsa" twenty times in the current buffer.
(loop until (eobp) do (munch-line) (forward-line 1))
This loop calls munch-line on every line until the end
of the buffer. If point is already at the end of the buffer,
the loop exits immediately.
(loop do (munch-line) until (eobp) do (forward-line 1))
This loop is similar to the above one, except that munch-line
is always called at least once.
(loop for x from 1 to 100
for y = (* x x)
until (>= y 729)
finally return (list x (= y 729)))
This more complicated loop searches for a number x whose
square is 729. For safety's sake it only examines x
values up to 100; dropping the phrase `to 100' would
cause the loop to count upwards with no limit. The second
for clause defines y to be the square of x
within the loop; the expression after the = sign is
reevaluated each time through the loop. The until
clause gives a condition for terminating the loop, and the
finally clause says what to do when the loop finishes.
(This particular example was written less concisely than it
could have been, just for the sake of illustration.)
Note that even though this loop contains three clauses (two
fors and an until) that would have been enough to
define loops all by themselves, it still creates a single loop
rather than some sort of triple-nested loop. You must explicitly
nest your loop constructs if you want nested loops.
Most loops are governed by one or more for clauses.
A for clause simultaneously describes variables to be
bound, how those variables are to be stepped during the loop,
and usually an end condition based on those variables.
The word as is a synonym for the word for. This
word is followed by a variable name, then a word like from
or across that describes the kind of iteration desired.
In Common Lisp, the phrase being the sometimes precedes
the type of iteration; in this package both being and
the are optional. The word each is a synonym
for the, and the word that follows it may be singular
or plural: `for x being the elements of y' or
`for x being each element of y'. Which form you use
is purely a matter of style.
The variable is bound around the loop as if by let:
(setq i 'happy)
(loop for i from 1 to 10 do (do-something-with i))
i
=> happy
for var from expr1 to expr2 by expr3
for clause creates a counting loop. Each of
the three sub-terms is optional, though there must be at least one
term so that the clause is marked as a counting clause.
The three expressions are the starting value, the ending value, and
the step value, respectively, of the variable. The loop counts
upwards by default (expr3 must be positive), from expr1
to expr2 inclusively. If you omit the from term, the
loop counts from zero; if you omit the to term, the loop
counts forever without stopping (unless stopped by some other
loop clause, of course); if you omit the by term, the loop
counts in steps of one.
You can replace the word from with upfrom or
downfrom to indicate the direction of the loop. Likewise,
you can replace to with upto or downto.
For example, `for x from 5 downto 1' executes five times
with x taking on the integers from 5 down to 1 in turn.
Also, you can replace to with below or above,
which are like upto and downto respectively except
that they are exclusive rather than inclusive limits:
(loop for x to 10 collect x)
=> (0 1 2 3 4 5 6 7 8 9 10)
(loop for x below 10 collect x)
=> (0 1 2 3 4 5 6 7 8 9)
The by value is always positive, even for downward-counting
loops. Some sort of from value is required for downward
loops; `for x downto 5' is not a legal loop clause all by
itself.
for var in list by function
by term, then function
is used to traverse the list instead of cdr; it must be a
function taking one argument. For example:
(loop for x in '(1 2 3 4 5 6) collect (* x x))
=> (1 4 9 16 25 36)
(loop for x in '(1 2 3 4 5 6) by 'cddr collect (* x x))
=> (1 9 25)
for var on list by function
(loop for x on '(1 2 3 4) collect x)
=> ((1 2 3 4) (2 3 4) (3 4) (4))
With by, there is no real reason that the on expression
must be a list. For example:
(loop for x on first-animal by 'next-animal collect x)where
(next-animal x) takes an "animal" x and returns
the next in the (assumed) sequence of animals, or nil if
x was the last animal in the sequence.
for var in-ref list by function
in clause, but var becomes
a setf-able "reference" onto the elements of the list
rather than just a temporary variable. For example,
(loop for x in-ref my-list do (incf x))increments every element of
my-list in place. This clause
is an extension to standard Common Lisp.
for var across array
(loop for x across "aeiou"
do (use-vowel (char-to-string x)))
for var across-ref array
setf-able
reference onto the elements; see in-ref above.
for var being the elements of sequence
in or
across. The clause may be followed by the additional term
`using (index var2)' to cause var2 to be bound to
the successive indices (starting at 0) of the elements.
This clause type is taken from older versions of the loop macro,
and is not present in modern Common Lisp. The `using (sequence ...)'
term of the older macros is not supported.
for var being the elements of-ref sequence
setf-able
reference onto the elements; see in-ref above.
for var being the symbols [of obarray]
(loop for sym being the symbols
when (fboundp sym)
when (string-match "^map" (symbol-name sym))
collect sym)
returns a list of all the functions whose names begin with `map'.
The Common Lisp words external-symbols and present-symbols
are also recognized but are equivalent to symbols in Emacs Lisp.
Due to a minor implementation restriction, it will not work to have
more than one for clause iterating over symbols, hash tables,
keymaps, overlays, or intervals in a given loop. Fortunately,
it would rarely if ever be useful to do so. It is legal to mix
one of these types of clauses with other clauses like for ... to
or while.
for var being the hash-keys of hash-table
hash-values
is the opposite word of the word following the) to cause
var and var2 to be bound to the two parts of each
hash table entry.
for var being the key-codes of keymap
using
clause to access both the codes and the bindings together.
for var being the key-seqs of keymap
for var being the overlays [of buffer] ...
extents is synonymous
with overlays). Under Emacs 18, this clause iterates zero
times. If the of term is omitted, the current buffer is used.
This clause also accepts optional `from pos' and
`to pos' terms, limiting the clause to overlays which
overlap the specified region.
for var being the intervals [of buffer] ...
of,
from, to, and property terms, where the latter
term restricts the search to just the specified property. The
of term may specify either a buffer or a string. This
clause is useful only in GNU Emacs 19; in other versions, all
buffers and strings consist of a single interval.
for var being the frames
screens is a synonym for frames. The frames
are visited in next-frame order starting from
selected-frame.
for var being the windows [of frame]
of term is not
allowed there.)
for var being the buffers
for var = expr1 then expr2
(loop for x on my-list by 'cddr do ...) (loop for x = my-list then (cddr x) while x do ...)Note that this type of
for clause does not imply any sort
of terminating condition; the above example combines it with a
while clause to tell when to end the loop.
If you omit the then term, expr1 is used both for
the initial setting and for successive settings:
(loop for x = (random) when (> x 0) return x)This loop keeps taking random numbers from the
(random)
function until it gets a positive one, which it then returns.
If you include several for clauses in a row, they are
treated sequentially (as if by let* and setq).
You can instead use the word and to link the clauses,
in which case they are processed in parallel (as if by let
and psetq).
(loop for x below 5 for y = nil then x collect (list x y))
=> ((0 nil) (1 1) (2 2) (3 3) (4 4))
(loop for x below 5 and y = nil then x collect (list x y))
=> ((0 nil) (1 0) (2 1) (3 2) (4 3))
In the first loop, y is set based on the value of x
that was just set by the previous clause; in the second loop,
x and y are set simultaneously so y is set
based on the value of x left over from the previous time
through the loop.
Another feature of the loop macro is destructuring,
similar in concept to the destructuring provided by defmacro.
The var part of any for clause can be given as a list
of variables instead of a single variable. The values produced
during loop execution must be lists; the values in the lists are
stored in the corresponding variables.
(loop for (x y) in '((2 3) (4 5) (6 7)) collect (+ x y))
=> (5 9 13)
In loop destructuring, if there are more values than variables
the trailing values are ignored, and if there are more variables
than values the trailing variables get the value nil.
If nil is used as a variable name, the corresponding
values are ignored. Destructuring may be nested, and dotted
lists of variables like (x . y) are allowed.
Aside from for clauses, there are several other loop clauses
that control the way the loop operates. They might be used by
themselves, or in conjunction with one or more for clauses.
repeat integer
(loop repeat n do ...) (loop for temp to n do ...)are identical except that the second one forces you to choose a name for a variable you aren't actually going to use.
while condition
nil. For example, the following two
loops are equivalent, except for the implicit nil block
that surrounds the second one:
(while cond forms...) (loop while cond do forms...)
until condition
nil.
always condition
nil.
Unlike while, it stops the loop using return nil so that
the finally clauses are not executed. If all the conditions
were non-nil, the loop returns t:
(if (loop for size in size-list always (> size 10))
(some-big-sizes)
(no-big-sizes))
never condition
always, except that the loop returns
t if any conditions were false, or nil otherwise.
thereis condition
nil;
in this case, it returns that non-nil value. If all the
values were nil, the loop returns nil.
These clauses cause the loop to accumulate information about the
specified Lisp form. The accumulated result is returned
from the loop unless overridden, say, by a return clause.
collect form
collect appear elsewhere in this manual.
The word collecting is a synonym for collect, and
likewise for the other accumulation clauses.
append form
append.
nconc form
concat form
vconcat form
count form
nil value.
sum form
maximize form
maximize is executed zero times.
minimize form
Accumulation clauses can be followed by `into var' to
cause the data to be collected into variable var (which is
automatically let-bound during the loop) rather than an
unnamed temporary variable. Also, into accumulations do
not automatically imply a return value. The loop must use some
explicit mechanism, such as finally return, to return
the accumulated result.
It is legal for several accumulation clauses of the same type to accumulate into the same place. From Steele:
(loop for name in '(fred sue alice joe june)
for kids in '((bob ken) () () (kris sunshine) ())
collect name
append kids)
=> (fred bob ken sue alice joe kris sunshine june)
This section describes the remaining loop clauses.
with var = value
(loop with x = 17 do ...) (let ((x 17)) (loop do ...)) (loop for x = 17 then x do ...)Naturally, the variable var might be used for some purpose in the rest of the loop. For example:
(loop for x in my-list with res = nil do (push x res)
finally return res)
This loop inserts the elements of my-list at the front of
a new list being accumulated in res, then returns the
list res at the end of the loop. The effect is similar
to that of a collect clause, but the list gets reversed
by virtue of the fact that elements are being pushed onto the
front of res rather than the end.
If you omit the = term, the variable is initialized to
nil. (Thus the `= nil' in the above example is
unnecessary.)
Bindings made by with are sequential by default, as if
by let*. Just like for clauses, with clauses
can be linked with and to cause the bindings to be made by
let instead.
if condition clause
do, return, if, or unless clause.
Several clauses may be linked by separating them with and.
These clauses may be followed by else and a clause or clauses
to execute if the condition was false. The whole construct may
optionally be followed by the word end (which may be used to
disambiguate an else or and in a nested if).
The actual non-nil value of the condition form is available
by the name it in the "then" part. For example:
(setq funny-numbers '(6 13 -1))
=> (6 13 -1)
(loop for x below 10
if (oddp x)
collect x into odds
and if (memq x funny-numbers) return (cdr it) end
else
collect x into evens
finally return (vector odds evens))
=> [(1 3 5 7 9) (0 2 4 6 8)]
(setq funny-numbers '(6 7 13 -1))
=> (6 7 13 -1)
(loop <same thing again>)
=> (13 -1)
Note the use of and to put two clauses into the "then"
part, one of which is itself an if clause. Note also that
end, while normally optional, was necessary here to make
it clear that the else refers to the outermost if
clause. In the first case, the loop returns a vector of lists
of the odd and even values of x. In the second case, the
odd number 7 is one of the funny-numbers so the loop
returns early; the actual returned value is based on the result
of the memq call.
when condition clause
if.
unless condition clause
unless clause is just like if except that the
sense of the condition is reversed.
named name
nil to the implicit
block surrounding the loop. The name is the symbol to be
used as the block name.
initially [do] forms...
for or with have been bound to their
initial values). initially clauses can appear anywhere;
if there are several, they are executed in the order they appear
in the loop. The keyword do is optional.
finally [do] forms...
for or while).
initially and finally clauses may appear anywhere
in the loop construct, but they are executed (in the specified
order) at the beginning or end, respectively, of the loop.
finally return form
collect or return, the loop will simply
return nil.) Variables bound by for, with,
or into will still contain their final values when form
is executed.
do forms...
do may be followed by any number of Lisp expressions
which are executed as an implicit progn in the body of the
loop. Many of the examples in this section illustrate the use of
do.
return form
loop
form. The finally clauses, if any, are not executed.
Of course, return is generally used inside an if or
unless, as its use in a top-level loop clause would mean
the loop would never get to "loop" more than once.
The clause `return form' is equivalent to
`do (return form)' (or return-from if the loop
was named). The return clause is implemented a bit more
efficiently, though.
While there is no high-level way to add user extensions to loop
(comparable to defsetf for setf, say), this package
does offer two properties called cl-loop-handler and
cl-loop-for-handler which are functions to be called when
a given symbol is encountered as a top-level loop clause or
for clause, respectively. Consult the source code in
file `cl-macs.el' for details.
This package's loop macro is compatible with that of Common
Lisp, except that a few features are not implemented: loop-finish
and data-type specifiers. Naturally, the for clauses which
iterate over keymaps, overlays, intervals, frames, windows, and
buffers are Emacs-specific extensions.
Common Lisp functions can return zero or more results. Emacs Lisp
functions, by contrast, always return exactly one result. This
package makes no attempt to emulate Common Lisp multiple return
values; Emacs versions of Common Lisp functions that return more
than one value either return just the first value (as in
compiler-macroexpand) or return a list of values (as in
get-setf-method). This package does define placeholders
for the Common Lisp functions that work with multiple values, but
in Emacs Lisp these functions simply operate on lists instead.
The values form, for example, is a synonym for list
in Emacs.
let, and then executes the body forms.
If there are more vars than values, the extra vars
are bound to nil. If there are fewer vars than
values, the excess values are ignored.
setq. Extra vars or values are treated the same as
in multiple-value-bind.
The older Quiroz package attempted a more faithful (but still
imperfect) emulation of Common Lisp multiple values. The old
method "usually" simulated true multiple values quite well,
but under certain circumstances would leave spurious return
values in memory where a later, unrelated multiple-value-bind
form would see them.
Since a perfect emulation is not feasible in Emacs Lisp, this package opts to keep it as simple and predictable as possible.
This package implements the various Common Lisp features of
defmacro, such as destructuring, &environment,
and &body. Top-level &whole is not implemented
for defmacro due to technical difficulties.
See section Argument Lists.
Destructuring is made available to the user by way of the following macro:
defmacro argument lists,
including destructuring. (The &environment keyword
is not allowed.) The macro expansion will signal an error
if expr returns a list of the wrong number of arguments
or with incorrect keyword arguments.
This package also includes the Common Lisp define-compiler-macro
facility, which allows you to define compile-time expansions and
optimizations for your functions.
defmacro, except that it only expands
calls to name at compile-time; calls processed by the Lisp
interpreter are not expanded, nor are they expanded by the
macroexpand function.
The argument list may begin with a &whole keyword and a
variable. This variable is bound to the macro-call form itself,
i.e., to a list of the form `(name args...)'.
If the macro expander returns this form unchanged, then the
compiler treats it as a normal function call. This allows
compiler macros to work as optimizers for special cases of a
function, leaving complicated cases alone.
For example, here is a simplified version of a definition that appears as a standard part of this package:
(define-compiler-macro member* (&whole form a list &rest keys)
(if (and (null keys)
(eq (car-safe a) 'quote)
(not (floatp-safe (cadr a))))
(list 'memq a list)
form))
This definition causes (member* a list) to change
to a call to the faster memq in the common case where a
is a non-floating-point constant; if a is anything else, or
if there are any keyword arguments in the call, then the original
member* call is left intact. (The actual compiler macro
for member* optimizes a number of other cases, including
common :test predicates.)
macroexpand, except that it
expands compiler macros rather than regular macros. It returns
form unchanged if it is not a call to a function for which
a compiler macro has been defined, or if that compiler macro
decided to punt by returning its &whole argument. Like
macroexpand, it expands repeatedly until it reaches a form
for which no further expansion is possible.
See section Macro Bindings, for descriptions of the macrolet
and symbol-macrolet forms for making "local" macro
definitions.
Common Lisp includes a complex and powerful "declaration"
mechanism that allows you to give the compiler special hints
about the types of data that will be stored in particular variables,
and about the ways those variables and functions will be used. This
package defines versions of all the Common Lisp declaration forms:
declare, locally, proclaim, declaim,
and the.
Most of the Common Lisp declarations are not currently useful in
Emacs Lisp, as the byte-code system provides little opportunity
to benefit from type information, and special declarations
are redundant in a fully dynamically-scoped Lisp. A few
declarations are meaningful when the optimizing Emacs 19 byte
compiler is being used, however. Under the earlier non-optimizing
compiler, these declarations will effectively be ignored.
proclaim is a function, decl-spec
is evaluated and thus should normally be quoted.
proclaim, except that it takes any number
of decl-spec arguments, and the arguments are unevaluated and
unquoted. The declaim macro also puts an (eval-when
(compile load eval) ...) around the declarations so that they will
be registered at compile-time as well as at run-time. (This is vital,
since normally the declarations are meant to influence the way the
compiler treats the rest of the file that contains the declaim
form.)
progns"
throughout Lisp syntax, such as function bodies, let bodies,
etc. Currently the only declaration understood by declare
is special.
locally is no different from progn.
the is ignored in this package;
in other words, (the type form) is equivalent
to form. Future versions of the optimizing byte-compiler may
make use of this information.
For example, mapcar can map over both lists and arrays. It is
hard for the compiler to expand mapcar into an in-line loop
unless it knows whether the sequence will be a list or an array ahead
of time. With (mapcar 'car (the vector foo)), a future
compiler would have enough information to expand the loop in-line.
For now, Emacs Lisp will treat the above code as exactly equivalent
to (mapcar 'car foo).
Each decl-spec in a proclaim, declaim, or
declare should be a list beginning with a symbol that says
what kind of declaration it is. This package currently understands
special, inline, notinline, optimize,
and warn declarations. (The warn declaration is an
extension of standard Common Lisp.) Other Common Lisp declarations,
such as type and ftype, are silently ignored.
special
special declarations are only advisory. They
simply tell the optimizing byte compiler that the specified
variables are intentionally being referred to without being
bound in the body of the function. The compiler normally emits
warnings for such references, since they could be typographical
errors for references to local variables.
The declaration (declare (special var1 var2)) is
equivalent to (defvar var1) (defvar var2) in the
optimizing compiler, or to nothing at all in older compilers (which
do not warn for non-local references).
In top-level contexts, it is generally better to write
(defvar var) than (declaim (special var)),
since defvar makes your intentions clearer. But the older
byte compilers can not handle defvars appearing inside of
functions, while (declare (special var)) takes care
to work correctly with all compilers.
inline
inline decl-spec lists one or more functions
whose bodies should be expanded "in-line" into calling functions
whenever the compiler is able to arrange for it. For example,
the Common Lisp function cadr is declared inline
by this package so that the form (cadr x) will
expand directly into (car (cdr x)) when it is called
in user functions, for a savings of one (relatively expensive)
function call.
The following declarations are all equivalent. Note that the
defsubst form is a convenient way to define a function
and declare it inline all at once, but it is available only in
Emacs 19.
(declaim (inline foo bar)) (eval-when (compile load eval) (proclaim '(inline foo bar))) (proclaim-inline foo bar) ; Lucid Emacs only (defsubst foo (...) ...) ; instead of defun; Emacs 19 onlyNote: This declaration remains in effect after the containing source file is done. It is correct to use it to request that a function you have defined should be inlined, but it is impolite to use it to request inlining of an external function. In Common Lisp, it is possible to use
(declare (inline ...))
before a particular call to a function to cause just that call to
be inlined; the current byte compilers provide no way to implement
this, so (declare (inline ...)) is currently ignored by
this package.
notinline
notinline declaration lists functions which should
not be inlined after all; it cancels a previous inline
declaration.
optimize
optimize is followed by any number of lists like
(speed 3) or (safety 2). Common Lisp defines several
optimization "qualities"; this package ignores all but speed
and safety. The value of a quality should be an integer from
0 to 3, with 0 meaning "unimportant" and 3 meaning "very important."
The default level for both qualities is 1.
In this package, with the Emacs 19 optimizing compiler, the
speed quality is tied to the byte-compile-optimize
flag, which is set to nil for (speed 0) and to
t for higher settings; and the safety quality is
tied to the byte-compile-delete-errors flag, which is
set to t for (safety 3) and to nil for all
lower settings. (The latter flag controls whether the compiler
is allowed to optimize out code whose only side-effect could
be to signal an error, e.g., rewriting (progn foo bar) to
bar when it is not known whether foo will be bound
at run-time.)
Note that even compiling with (safety 0), the Emacs
byte-code system provides sufficient checking to prevent real
harm from being done. For example, barring serious bugs in
Emacs itself, Emacs will not crash with a segmentation fault
just because of an error in a fully-optimized Lisp program.
The optimize declaration is normally used in a top-level
proclaim or declaim in a file; Common Lisp allows
it to be used with declare to set the level of optimization
locally for a given form, but this will not work correctly with the
current version of the optimizing compiler. (The declare
will set the new optimization level, but that level will not
automatically be unset after the enclosing form is done.)
warn
warn is followed by any
number of "warning qualities," similar in form to optimization
qualities. The currently supported warning types are
redefine, callargs, unresolved, and
free-vars; in the current system, a value of 0 will
disable these warnings and any higher value will enable them.
See the documentation for the optimizing byte compiler for details.
This package defines several symbol-related features that were missing from Emacs Lisp.
These functions augment the standard Emacs Lisp functions get
and put for operating on properties attached to symbols.
There are also functions for working with property lists as
first-class data structures not attached to particular symbols.
get, except that if the property is
not found, the default argument provides the return value.
(The Emacs Lisp get function always uses nil as
the default; this package's get* is equivalent to Common
Lisp's get.)
The get* function is setf-able; when used in this
fashion, the default argument is allowed but ignored.
nil if there was no such property.
(This function was probably omitted from Emacs originally because,
since get did not allow a default, it was very difficult
to distinguish between a missing property and a property whose value
was nil; thus, setting a property to nil was close
enough to remprop for most purposes.)
eq
to property, the following odd-numbered element is returned.
Otherwise, default is returned (or nil if no default
is given).
In particular,
(get sym prop) == (getf (symbol-plist sym) prop)
It is legal to use getf as a setf place, in which case
its place argument must itself be a legal setf place.
The default argument, if any, is ignored in this context.
The effect is to change (via setcar) the value cell in the
list that corresponds to property, or to cons a new property-value
pair onto the list if the property is not yet present.
(put sym prop val) == (setf (getf (symbol-plist sym) prop) val)
The get and get* functions are also setf-able.
The fact that default is ignored can sometimes be useful:
(incf (get* 'foo 'usage-count 0))
Here, symbol foo's usage-count property is incremented
if it exists, or set to 1 (an incremented 0) otherwise.
When not used as a setf form, getf is just a regular
function and its place argument can actually be any Lisp
expression.
setf-able
place expression. It returns true if the property was found. Note
that if property happens to be first on the list, this will
effectively do a (setf place (cddr place)),
whereas if it occurs later, this simply uses setcdr to splice
out the property and value cells.
@secno=2
These functions create unique symbols, typically for use as temporary variables.
make-symbol)
with a unique name. (The name of an uninterned symbol is relevant
only if the symbol is printed.) By default, the name is generated
from an increasing sequence of numbers, `G1000', `G1001',
`G1002', etc. If the optional argument x is a string, that
string is used as a prefix instead of `G'. Uninterned symbols
are used in macro expansions for temporary variables, to ensure that
their names will not conflict with "real" variables in the user's
code.
gensym names.
It is incremented after each use by gensym. In Common Lisp
this is initialized with 0, but this package initializes it with a
random (time-dependent) value to avoid trouble when two files that
each used gensym in their compilation are loaded together.
(Uninterned symbols become interned when the compiler writes them
out to a file and the Emacs loader loads them, so their names have to
be treated a bit more carefully than in Common Lisp where uninterned
symbols remain uninterned after loading.)
gensym, except that it produces a new
interned symbol. If the symbol that is generated already
exists, the function keeps incrementing the counter and trying
again until a new symbol is generated.
The Quiroz `cl.el' package also defined a defkeyword
form for creating self-quoting keyword symbols. This package
automatically creates all keywords that are called for by
&key argument specifiers, and discourages the use of
keywords as data unrelated to keyword arguments, so the
defkeyword form has been discontinued.
@chapno=11
This section defines a few simple Common Lisp operations on numbers which were left out of Emacs Lisp.
@secno=1
These functions return t if the specified condition is
true of the numerical argument, or nil otherwise.
floatp. On other systems, this always returns nil.
@secno=3
These functions perform various arithmetic operations on numbers.
abs only for Emacs 18 versions which don't provide
it as a primitive.)
expt only for Emacs 18 versions which don't
provide it as a primitive.)
floor function.
It is called floor* to avoid name conflicts with the
simpler floor function built-in to Emacs 19.
With one argument, floor* returns a list of two numbers:
The argument rounded down (toward minus infinity) to an integer,
and the "remainder" which would have to be added back to the
first return value to yield the argument again. If the argument
is an integer x, the result is always the list (x 0).
If the argument is an Emacs 19 floating-point number, the first
result is a Lisp integer and the second is a Lisp float between
0 (inclusive) and 1 (exclusive).
With two arguments, floor* divides number by
divisor, and returns the floor of the quotient and the
corresponding remainder as a list of two numbers. If
(floor* x y) returns (q r),
then q*y + r = x, with r
between 0 (inclusive) and r (exclusive). Also, note
that (floor* x) is exactly equivalent to
(floor* x 1).
This function is entirely compatible with Common Lisp's floor
function, except that it returns the two results in a list since
Emacs Lisp does not support multiple-valued functions.
ceiling function,
which is analogous to floor except that it rounds the
argument or quotient of the arguments up toward plus infinity.
The remainder will be between 0 and minus r.
truncate function,
which is analogous to floor except that it rounds the
argument or quotient of the arguments toward zero. Thus it is
equivalent to floor* if the argument or quotient is
positive, or to ceiling* otherwise. The remainder has
the same sign as number.
round function,
which is analogous to floor except that it rounds the
argument or quotient of the arguments to the nearest integer.
In the case of a tie (the argument or quotient is exactly
halfway between two integers), it rounds to the even integer.
floor.
truncate.
These definitions are compatible with those in the Quiroz `cl.el' package, except that this package appends `*' to certain function names to avoid conflicts with existing Emacs 19 functions, and that the mechanism for returning multiple values is different.
@secno=8
This package also provides an implementation of the Common Lisp random number generator. It uses its own additive-congruential algorithm, which is much more likely to give statistically clean random numbers than the simple generators supplied by many operating systems.
random-state object
which holds the state of the random number generator. The
function modifies this state object as a side effect. If
state is omitted, it defaults to the variable
*random-state*, which contains a pre-initialized
random-state object.
random-state
object, used for calls to random* that do not specify an
alternative state object. Since any number of programs in the
Emacs process may be accessing *random-state* in interleaved
fashion, the sequence generated from this variable will be
irreproducible for all intents and purposes.
random-state object.
If state is omitted or nil, it returns a new copy of
*random-state*. This is a copy in the sense that future
sequences of calls to (random* n) and
(random* n s) (where s is the new
random-state object) will return identical sequences of random
numbers.
If state is a random-state object, this function
returns a copy of that object. If state is t, this
function returns a new random-state object seeded from the
date and time. As an extension to Common Lisp, state may also
be an integer in which case the new object is seeded from that
integer; each different integer seed will result in a completely
different sequence of random numbers.
It is legal to print a random-state object to a buffer or
file and later read it back with read. If a program wishes
to use a sequence of pseudo-random numbers which can be reproduced
later for debugging, it can call (make-random-state t) to
get a new sequence, then print this sequence to a file. When the
program is later rerun, it can read the original run's random-state
from the file.
t if object is a
random-state object, or nil otherwise.
This package defines several useful constants having to with numbers.
2^23-1 or 2^25-1.
The following parameters have to do with floating-point numbers. This package determines their values by exercising the computer's floating-point arithmetic in various ways. Because this operation might be slow, the code for initializing them is kept in a separate function that must be called before the parameters can be used.
most-positive-float have been initialized.
Until it is called, these parameters will be nil. If this
version of Emacs does not support floats (e.g., most versions of
Emacs 18), the parameters will remain nil. If the parameters
have already been initialized, the function returns immediately.
The algorithm makes assumptions that will be valid for most modern machines, but will fail if the machine's arithmetic is extremely unusual, e.g., decimal.
Since true Common Lisp supports up to four different floating-point
precisions, it has families of constants like
most-positive-single-float, most-positive-double-float,
most-positive-long-float, and so on. Emacs has only one
floating-point precision, so this package omits the precision word
from the constants' names.
1.79e+308.
(- most-positive-float).)
4.94e-324 if denormals are
supported or 2.22e-308 if not.
2.22e-308. For machines that do not support
the concept of denormalization and gradual underflow, this constant
will always equal least-positive-float.
least-positive-float.
least-positive-normalized-float.
2.22e-16.
1.11e-16.
@chapno=13
Common Lisp defines a number of functions that operate on
sequences, which are either lists, strings, or vectors.
Emacs Lisp includes a few of these, notably elt and
length; this package defines most of the rest.
Many of the sequence functions take keyword arguments; see section Argument Lists. All keyword arguments are optional and, if specified, may appear in any order.
The :key argument should be passed either nil, or a
function of one argument. This key function is used as a filter
through which the elements of the sequence are seen; for example,
(find x y :key 'car) is similar to (assoc* x y):
It searches for an element of the list whose car equals
x, rather than for an element which equals x itself.
If :key is omitted or nil, the filter is effectively
the identity function.
The :test and :test-not arguments should be either
nil, or functions of two arguments. The test function is
used to compare two sequence elements, or to compare a search value
with sequence elements. (The two values are passed to the test
function in the same order as the original sequence function
arguments from which they are derived, or, if they both come from
the same sequence, in the same order as they appear in that sequence.)
The :test argument specifies a function which must return
true (non-nil) to indicate a match; instead, you may use
:test-not to give a function which returns false to
indicate a match. The default test function is :test 'eql.
Many functions which take item and :test or :test-not
arguments also come in -if and -if-not varieties,
where a predicate function is passed instead of item,
and sequence elements match if the predicate returns true on them
(or false in the case of -if-not). For example:
(remove* 0 seq :test '=) == (remove-if 'zerop seq)
to remove all zeros from sequence seq.
Some operations can work on a subsequence of the argument sequence;
these function take :start and :end arguments which
default to zero and the length of the sequence, respectively.
Only elements between start (inclusive) and end
(exclusive) are affected by the operation. The end argument
may be passed nil to signify the length of the sequence;
otherwise, both start and end must be integers, with
0 <= start <= end <= (length seq).
If the function takes two sequence arguments, the limits are
defined by keywords :start1 and :end1 for the first,
and :start2 and :end2 for the second.
A few functions accept a :from-end argument, which, if
non-nil, causes the operation to go from right-to-left
through the sequence instead of left-to-right, and a :count
argument, which specifies an integer maximum number of elements
to be removed or otherwise processed.
The sequence functions make no guarantees about the order in
which the :test, :test-not, and :key functions
are called on various elements. Therefore, it is a bad idea to depend
on side effects of these functions. For example, :from-end
may cause the sequence to be scanned actually in reverse, or it may
be scanned forwards but computing a result "as if" it were scanned
backwards. (Some functions, like mapcar* and every,
do specify exactly the order in which the function is called
so side effects are perfectly acceptable in those cases.)
Strings in GNU Emacs 19 may contain "text properties" as well
as character data. Except as noted, it is undefined whether or
not text properties are preserved by sequence functions. For
example, (remove* ?A str) may or may not preserve
the properties of the characters copied from str into the
result.
These functions "map" the function you specify over the elements
of lists or arrays. They are all variations on the theme of the
built-in function mapcar.
mapcar; given n sequences,
it calls the function with the first elements of each of the sequences
as the n arguments to yield the first element of the result
list, then with the second elements, and so on. The mapping stops as
soon as the shortest sequence runs out. The argument sequences may
be any mixture of lists, strings, and vectors; the return sequence
is always a list.
Common Lisp's mapcar accepts multiple arguments but works
only on lists; Emacs Lisp's mapcar accepts a single sequence
argument. This package's mapcar* works as a compatible
superset of both.
mapcar*, but it returns a sequence of type
result-type rather than a list. result-type must
be one of the following symbols: vector, string,
list (in which case the effect is the same as for
mapcar*), or nil (in which case the results are
thrown away and map returns nil).
cdrs of those lists, and so on, until the
shortest list runs out. The results are returned in the form
of a list. Thus, maplist is like mapcar* except
that it passes in the list pointers themselves rather than the
cars of the advancing pointers.
mapcar*, except that the values
returned by function are ignored and thrown away rather
than being collected into a list. The return value of mapc
is seq, the first sequence.
maplist, except that it throws away
the values returned by function.
mapcar*, except that it concatenates
the return values (which must be lists) using nconc,
rather than simply collecting them into a list.
maplist, except that it concatenates
the return values using nconc.
nil value,
some returns that value, otherwise it returns nil.
Given several sequence arguments, it steps through the sequences
in parallel until the shortest one runs out, just as in
mapcar*. You can rely on the left-to-right order in which
the elements are visited, and on the fact that mapping stops
immediately as soon as predicate returns non-nil.
nil as soon as predicate returns
nil for any element, or t if the predicate was true
for all elements.
nil as soon as predicate returns
a non-nil value for any element, or t if the predicate
was nil for all elements.
nil value as soon as predicate
returns nil for any element, or t if the predicate was
true for all elements.
* and seq is
the list (2 3 4 5). The first two elements of the list are
combined with (* 2 3) = 6; this is combined with the next
element, (* 6 4) = 24, and that is combined with the final
element: (* 24 5) = 120. Note that the * function happens
to be self-reducing, so that (* 2 3 4 5) has the same effect as
an explicit call to reduce.
If :from-end is true, the reduction is right-associative instead
of left-associative:
(reduce '- '(1 2 3 4))
== (- (- (- 1 2) 3) 4) => -8
(reduce '- '(1 2 3 4) :from-end t)
== (- 1 (- 2 (- 3 4))) => -2
If :key is specified, it is a function of one argument which
is called on each of the sequence elements in turn.
If :initial-value is specified, it is effectively added to the
front (or rear in the case of :from-end) of the sequence.
The :key function is not applied to the initial value.
If the sequence, including the initial value, has exactly one element then that element is returned without ever calling function. If the sequence is empty (and there is no initial value), then function is called with no arguments to obtain the return value.
All of these mapping operations can be expressed conveniently in
terms of the loop macro. In compiled code, loop will
be faster since it generates the loop as in-line code with no
function calls.
This section describes a number of Common Lisp functions for operating on sequences.
As an extension to Common Lisp, start and/or end
may be negative, in which case they represent a distance back
from the end of the sequence. This is for compatibility with
Emacs' substring function. Note that subseq is
the only sequence function that allows negative
start and end.
You can use setf on a subseq form to replace a
specified range of elements with elements from another sequence.
The replacement is done as if by replace, described below.
vector, string, or list. The
arguments are always copied, even in cases such as
(concatenate 'list '(1 2 3)) where the result is
identical to an argument.
If seq1 and seq2 are eq, then the replacement
will work correctly even if the regions indicated by the start
and end arguments overlap. However, if seq1 and seq2
are lists which share storage but are not eq, and the
start and end arguments specify overlapping regions, the effect
is undefined.
eq to seq in some circumstances, but the original
seq will not be modified. The :test, :test-not,
and :key arguments define the matching test that is used;
by default, elements eql to item are removed. The
:count argument specifies the maximum number of matching
elements that can be removed (only the leftmost count matches
are removed). The :start and :end arguments specify
a region in seq in which elements will be removed; elements
outside that region are not matched or removed. The :from-end
argument, if true, says that elements should be deleted from the
end of the sequence rather than the beginning (this matters only
if count was also specified).
remove*
for those sequence types. On lists, remove* will copy the
list if necessary to preserve the original list, whereas
delete* will splice out parts of the argument list.
Compare append and nconc, which are analogous
non-destructive and destructive list operations in Emacs Lisp.
The predicate-oriented functions remove-if, remove-if-not,
delete-if, and delete-if-not are defined similarly.
equal to item. The delete function is
built-in to Emacs 19; this package defines it equivalently in Emacs 18.
equal to item. This package defines it for symmetry
with delete, even though remove is not built-in to
Emacs 19.
eq to item. This package defines it for symmetry
with delq, even though remq is not built-in to
Emacs 19.
:test, :test-not, and :key
arguments, only the rightmost one is retained. If :from-end
is true, the leftmost one is retained instead. If :start or
:end is specified, only elements within that subsequence are
examined or removed.
remove-duplicates.
:count,
:start, :end, and :from-end arguments may be
used to limit the number of substitutions made.
substitute; it performs
the substitution using setcar or aset rather than
by returning a changed copy of the sequence.
The substitute-if, substitute-if-not, nsubstitute-if,
and nsubstitute-if-not functions are defined similarly. For
these, a predicate is given in place of the old argument.
These functions search for elements or subsequences in a sequence.
(See also member* and assoc*; see section Lists.)
nil. It returns the leftmost match, unless
:from-end is true, in which case it returns the rightmost
match. The :start and :end arguments may be used to
limit the range of elements that are searched.
find, except that it returns the
integer position in the sequence of the matching item rather than
the item itself. The position is relative to the start of the
sequence as a whole, even if :start is non-zero. The function
returns nil if no matching element was found.
The find-if, find-if-not, position-if,
position-if-not, count-if, and count-if-not
functions are defined similarly.
:test, :test-not,
and :key), the function returns nil. If there is
a mismatch, the function returns the index (relative to seq1)
of the first mismatching element. This will be the leftmost pair of
elements which do not match, or the position at which the shorter of
the two otherwise-matching sequences runs out.
If :from-end is true, then the elements are compared from right
to left starting at (1- end1) and (1- end2).
If the sequences differ, then one plus the index of the rightmost
difference (relative to seq1) is returned.
An interesting example is (mismatch str1 str2 :key 'upcase),
which compares two strings case-insensitively.
:start1 and
:end1.) Only matches which fall entirely within the region
defined by :start2 and :end2 will be considered.
The return value is the index of the leftmost element of the
leftmost match, relative to the start of seq2, or nil
if no matches were found. If :from-end is true, the
function finds the rightmost matching subsequence.
nil) if and only if its first argument
is less than (not equal to) its second argument. For example,
< and string-lessp are suitable predicate functions
for sorting numbers and strings, respectively; > would sort
numbers into decreasing rather than increasing order.
This function differs from Emacs' built-in sort in that it
can operate on any type of sequence, not just lists. Also, it
accepts a :key argument which is used to preprocess data
fed to the predicate function. For example,
(setq data (sort data 'string-lessp :key 'downcase))
sorts data, a sequence of strings, into increasing alphabetical
order without regard to case. A :key function of car
would be useful for sorting association lists.
The sort* function is destructive; it sorts lists by actually
rearranging the cdr pointers in suitable fashion.
In practice, sort* and stable-sort are equivalent
in Emacs Lisp because the underlying sort function is
stable by default. However, this package reserves the right to
use non-stable methods for sort* in the future.
concatenate), has length equal to the sum
of the lengths of the two input sequences. The sequences may be
modified destructively. Order of elements within seq1 and
seq2 is preserved in the interleaving; elements of the two
sequences are compared by predicate (in the sense of
sort) and the lesser element goes first in the result.
When elements are equal, those from seq1 precede those from
seq2 in the result. Thus, if seq1 and seq2 are
both sorted according to predicate, then the result will be
a merged sequence which is (stably) sorted according to
predicate.
The functions described here operate on lists.
This section describes a number of simple operations on lists, i.e., chains of cons cells.
(car (cdr (cdr x))).
Likewise, this package defines all 28 cxxxr functions
where xxx is up to four `a's and/or `d's.
All of these functions are setf-able, and calls to them
are expanded inline by the byte-compiler for maximum efficiency.
(car x). Likewise,
the functions second, third, ..., through
tenth return the given element of the list x.
(cdr x).
null, but
signaling an error if x is neither a nil nor a
cons cell. This package simply defines endp as a synonym
for null.
(length x), except that if x is a circular
list (where the cdr-chain forms a loop rather than terminating
with nil), this function returns nil. (The regular
length function would get stuck if given a circular list.)
cdr is not another cons cell. (For normal lists, the
cdr of the last cons will be nil.) This function
returns nil if x is nil or shorter than
n. Note that the last element of the list is
(car (last x)).
The Emacs function last does the same thing
except that it does not handle the optional argument n.
(append (butlast x n)
(last x n)) will return a list equal to x.
butlast that works by destructively
modifying the cdr of the appropriate element, rather than
making a copy of the list.
cdr of the last cell constructed.
Thus, (list* a b c) is equivalent to
(cons a (cons b c)), and
(list* a b nil) is equivalent to
(list a b).
(Note that this function really is called list* in Common
Lisp; it is not a name invented for this package like member*
or defun*.)
eq to
one of the cons cells of list, then this function returns
a copy of the part of list up to but not including
sublist. For example, (ldiff x (cddr x)) returns
the first two elements of the list x. The result is a
copy; the original list is not modified. If sublist
is not a sublist of list, a copy of the entire list
is returned.
(1 2 . 3) correctly.
copy-sequence (and its alias copy-list),
which copies only along the cdr direction, this function
copies (recursively) along both the car and the cdr
directions. If x is not a cons cell, the function simply
returns x unchanged. If the optional vecp argument
is true, this function copies vectors (recursively) as well as
cons cells.
cars and cdrs are
compared recursively. If neither x nor y is a cons
cell, they are compared by eql, or according to the
specified test. The :key function, if specified, is
applied to the elements of both trees. See section Sequences.
@secno=3
These functions substitute elements throughout a tree of cons
cells. (See section Sequence Functions, for the substitute
function, which works on just the top-level elements of a list.)
cars and cdrs
of the component cons cells. If old is itself a cons cell,
then matching cells in the tree are substituted as usual without
recursively substituting in that cell. Comparisons with old
are done according to the specified test (eql by default).
The :key function is applied to the elements of the tree
but not to old.
subst, except that it works by
destructive modification (by setcar or setcdr)
rather than copying.
The subst-if, subst-if-not, nsubst-if, and
nsubst-if-not functions are defined similarly.
subst, except that it takes an
association list alist of old-new pairs.
Each element of the tree (after applying the :key
function, if any), is compared with the cars of
alist; if it matches, it is replaced by the corresponding
cdr.
sublis.
These functions perform operations on lists which represent sets of elements.
equal to item. The member function is
built-in to Emacs 19; this package defines it equivalently in Emacs 18.
See the following function for a Common-Lisp compatible version.
car was
the matching element. Otherwise, it returns nil. Elements
are compared by eql by default; you can use the :test,
:test-not, and :key arguments to modify this behavior.
See section Sequences.
Note that this function's name is suffixed by `*' to avoid
the incompatible member function defined in Emacs 19.
(That function uses equal for comparisons; it is equivalent
to (member* item list :test 'equal).)
The member-if and member-if-not functions
analogously search for elements which satisfy a given predicate.
t if sublist is a sublist of
list, i.e., if sublist is eql to list or to
any of its cdrs.
(cons item list), but only if item
is not already present on the list (as determined by member*).
If a :key argument is specified, it is applied to
item as well as to the elements of list during
the search, on the reasoning that item is "about" to
become part of the list.
union; rather than copying,
it tries to reuse the storage of the argument lists if possible.
intersection. It
tries to reuse storage of list1 rather than copying.
It does not reuse the storage of list2.
set-difference, which will try
to reuse list1 if possible.
set-exclusive-or, which will try
to reuse list1 and list2 if possible.
An association list is a list representing a mapping from one set of values to another; any list whose elements are cons cells is an association list.
car matches (in the sense of :test,
:test-not, and :key, or by comparison with eql)
a given item. It returns the matching element, if any,
otherwise nil. It ignores elements of a-list which
are not cons cells. (This corresponds to the behavior of
assq and assoc in Emacs Lisp; Common Lisp's
assoc ignores nils but considers any other non-cons
elements of a-list to be an error.)
cdr matches
item. If a-list represents a mapping, this applies
the inverse of the mapping to item.
rassoc* with a :test
argument of equal. It is analogous to Emacs Lisp's
standard assoc function, which derives from the MacLisp
rather than the Common Lisp tradition.
The assoc-if, assoc-if-not, rassoc-if,
and rassoc-if-not functions are defined similarly.
Two simple functions for constructing association lists are:
(cons (cons key value) alist).
(nconc (mapcar* 'cons keys values)
alist).
A hash table is a data structure that maps "keys" onto "values." Keys and values can be arbitrary Lisp data objects. Hash tables have the property that the time to search for a given key is roughly constant; simpler data structures like association lists take time proportional to the number of entries in the list.
:test (eql
by default), and which is allocated to fit about :size
elements. The :size argument is purely advisory; the
table will stretch automatically if you store more elements in
it. If :size is omitted, a reasonable default is used.
Common Lisp allows only eq, eql, equal,
and equalp as legal values for the :test argument.
In this package, any reasonable predicate function will work,
though if you use something else you should check the details of
the hashing function described below to make sure it is suitable
for your predicate.
Some versions of Emacs (like Lucid Emacs 19) include a built-in
hash table type; in these versions, make-hash-table with
a test of eq will use these built-in hash tables. In all
other cases, it will return a hash-table object which takes the
form of a list with an identifying "tag" symbol at the front.
All of the hash table functions in this package can operate on
both types of hash table; normally you will never know which
type is being used.
This function accepts the additional Common Lisp keywords
:rehash-size and :rehash-threshold, but it ignores
their values.
nil) is returned.
To store new data in the hash table, use setf on a call to
gethash. If key already exists in the table, the
corresponding value is changed to the stored value. If key
does not already exist, a new entry is added to the table and the
table is reallocated to a larger size if necessary. The default
argument is allowed but ignored in this case. The situation is
exactly analogous to that of get*; see section Property Lists.
t. If key does
not appear in the table, it does nothing and returns nil.
nil. See section Loop Facility, for
an alternate way of iterating over hash tables.
count when
remhash removes an entry. Therefore, the return value of
this function is not dependable if you have used remhash
on the table and the table's test is eq. A slower, but
reliable, way to count the entries is (loop for x being the
hash-keys of table count t).
t if object is a hash table,
nil otherwise. It recognizes both types of hash tables
(both Lucid Emacs built-in tables and tables implemented with
special lists.)
Sometimes when dealing with hash tables it is useful to know the
exact "hash function" that is used. This package implements
hash tables using Emacs Lisp "obarrays," which are the same
data structure that Emacs Lisp uses to keep track of symbols.
Each hash table includes an embedded obarray. Key values given
to gethash are converted by various means into strings,
which are then looked up in the obarray using intern and
intern-soft. The symbol, or "bucket," corresponding to
a given key string includes as its symbol-value an association
list of all key-value pairs which hash to that string. Depending
on the test function, it is possible for many entries to hash to
the same bucket. For example, if the test is eql, then the
symbol foo and two separately built strings "foo" will
create three entries in the same bucket. Search time is linear
within buckets, so hash tables will be most effective if you arrange
not to store too many things that hash the same.
The following algorithm is used to convert Lisp objects to hash strings:
equalp, strings are downcased first.)
symbol-name.
cars; nonempty vectors
are hashed according to their first element.
"*".
Thus, for example, searching among many buffer objects in a hash table will devolve to a (still fairly fast) linear-time search through a single bucket, whereas searching for different symbols will be very fast since each symbol will, in general, hash into its own bucket.
The size of the obarray in a hash table is automatically adjusted as the number of elements increases.
As a special case, make-hash-table with a :size argument
of 0 or 1 will create a hash-table object that uses a single association
list rather than an obarray of many lists. For very small tables this
structure will be more efficient since lookup does not require
converting the key to a string or looking it up in an obarray.
However, such tables are guaranteed to take time proportional to
their size to do a search.
@chapno=18
The Common Lisp structure mechanism provides a general way
to define data types similar to C's struct types. A
structure is a Lisp object containing some number of slots,
each of which can hold any Lisp data object. Functions are
provided for accessing and setting the slots, creating or copying
structure objects, and recognizing objects of a particular structure
type.
In true Common Lisp, each structure type is a new type distinct from all existing Lisp types. Since the underlying Emacs Lisp system provides no way to create new distinct types, this package implements structures as vectors (or lists upon request) with a special "tag" symbol to identify them.
defstruct form defines a new structure type called
name, with the specified slots. (The slots
may begin with a string which documents the structure type.)
In the simplest case, name and each of the slots
are symbols. For example,
(defstruct person name age sex)
defines a struct type called person which contains three
slots. Given a person object p, you can access those
slots by calling (person-name p), (person-age p),
and (person-sex p). You can also change these slots by
using setf on any of these place forms:
(incf (person-age birthday-boy))
You can create a new person by calling make-person,
which takes keyword arguments :name, :age, and
:sex to specify the initial values of these slots in the
new object. (Omitting any of these arguments leaves the corresponding
slot "undefined," according to the Common Lisp standard; in Emacs
Lisp, such uninitialized slots are filled with nil.)
Given a person, (copy-person p) makes a new
object of the same type whose slots are eq to those of p.
Given any Lisp object x, (person-p x) returns
true if x looks like a person, false otherwise. (Again,
in Common Lisp this predicate would be exact; in Emacs Lisp the
best it can do is verify that x is a vector of the correct
length which starts with the correct tag symbol.)
Accessors like person-name normally check their arguments
(effectively using person-p) and signal an error if the
argument is the wrong type. This check is affected by
(optimize (safety ...)) declarations. Safety level 1,
the default, uses a somewhat optimized check that will detect all
incorrect arguments, but may use an uninformative error message
(e.g., "expected a vector" instead of "expected a person").
Safety level 0 omits all checks except as provided by the underlying
aref call; safety levels 2 and 3 do rigorous checking that will
always print a descriptive error message for incorrect inputs.
See section Declarations.
(setq dave (make-person :name "Dave" :sex 'male))
=> [cl-struct-person "Dave" nil male]
(setq other (copy-person dave))
=> [cl-struct-person "Dave" nil male]
(eq dave other)
=> nil
(eq (person-name dave) (person-name other))
=> t
(person-p dave)
=> t
(person-p [1 2 3 4])
=> nil
(person-p "Bogus")
=> nil
(person-p '[cl-struct-person counterfeit person object])
=> t
In general, name is either a name symbol or a list of a name symbol followed by any number of struct options; each slot is either a slot symbol or a list of the form `(slot-name default-value slot-options...)'. The default-value is a Lisp form which is evaluated any time an instance of the structure type is created without specifying that slot's value.
Common Lisp defines several slot options, but the only one
implemented in this package is :read-only. A non-nil
value for this option means the slot should not be setf-able;
the slot's value is determined when the object is created and does
not change afterward.
(defstruct person (name nil :read-only t) age (sex 'unknown))
Any slot options other than :read-only are ignored.
For obscure historical reasons, structure options take a different form than slot options. A structure option is either a keyword symbol, or a list beginning with a keyword symbol possibly followed by arguments. (By contrast, slot options are key-value pairs not enclosed in lists.)
(defstruct (person (:constructor create-person)
(:type list)
:named)
name age sex)
The following structure options are recognized.
:conc-name
(:conc-name p-)
would change this prefix to p-. Specifying nil as an
argument means no prefix, so that the slot names themselves are used
to name the accessor functions.
:constructor
make-name, e.g., make-person. The above
example changes this to create-person. Specifying nil
as an argument means that no standard constructor should be
generated at all.
In the full form of this option, the constructor name is followed
by an arbitrary argument list. See section Program Structure, for a
description of the format of Common Lisp argument lists. All
options, such as &rest and &key, are supported.
The argument names should match the slot names; each slot is
initialized from the corresponding argument. Slots whose names
do not appear in the argument list are initialized based on the
default-value in their slot descriptor. Also, &optional
and &key arguments which don't specify defaults take their
defaults from the slot descriptor. It is legal to include arguments
which don't correspond to slot names; these are useful if they are
referred to in the defaults for optional, keyword, or &aux
arguments which do correspond to slots.
You can specify any number of full-format :constructor
options on a structure. The default constructor is still generated
as well unless you disable it with a simple-format :constructor
option.
(defstruct
(person
(:constructor nil) ; no default constructor
(:constructor new-person (name sex &optional (age 0)))
(:constructor new-hound (&key (name "Rover")
(dog-years 0)
&aux (age (* 7 dog-years))
(sex 'canine))))
name age sex)
The first constructor here takes its arguments positionally rather
than by keyword. (In official Common Lisp terminology, constructors
that work By Order of Arguments instead of by keyword are called
"BOA constructors." No, I'm not making this up.) For example,
(new-person "Jane" 'female) generates a person whose slots
are "Jane", 0, and female, respectively.
The second constructor takes two keyword arguments, :name,
which initializes the name slot and defaults to "Rover",
and :dog-years, which does not itself correspond to a slot
but which is used to initialize the age slot. The sex
slot is forced to the symbol canine with no syntax for
overriding it.
:copier
copy-name. nil
means not to generate a copier function. (In this implementation,
all copier functions are simply synonyms for copy-sequence.)
:predicate
name-p. nil
means not to generate a predicate function. (If the :type
option is used without the :named option, no predicate is
ever generated.)
In true Common Lisp, typep is always able to recognize a
structure object even if :predicate was used. In this
package, typep simply looks for a function called
typename-p, so it will work for structure types
only if they used the default predicate name.
:include
defstruct. The effect is to cause the new
structure type to inherit all of the included structure's slots
(plus, of course, any new slots described by this struct's slot
descriptors). The new structure is considered a "specialization"
of the included one. In fact, the predicate and slot accessors
for the included type will also accept objects of the new type.
If there are extra arguments to the :include option after
the included-structure name, these options are treated as replacement
slot descriptors for slots in the included structure, possibly with
modified default values. Borrowing an example from Steele:
(defstruct person name (age 0) sex)
=> person
(defstruct (astronaut (:include person (age 45)))
helmet-size
(favorite-beverage 'tang))
=> astronaut
(setq joe (make-person :name "Joe"))
=> [cl-struct-person "Joe" 0 nil]
(setq buzz (make-astronaut :name "Buzz"))
=> [cl-struct-astronaut "Buzz" 45 nil nil tang]
(list (person-p joe) (person-p buzz))
=> (t t)
(list (astronaut-p joe) (astronaut-p buzz))
=> (nil t)
(person-name buzz)
=> "Buzz"
(astronaut-name joe)
=> error: "astronaut-name accessing a non-astronaut"
Thus, if astronaut is a specialization of person,
then every astronaut is also a person (but not the
other way around). Every astronaut includes all the slots
of a person, plus extra slots that are specific to
astronauts. Operations that work on people (like person-name)
work on astronauts just like other people.
:print-function
:print-function.
:type
vector or list.
This tells which underlying Lisp data type should be used to implement
the new structure type. Vectors are used by default, but
(:type list) will cause structure objects to be stored as
lists instead.
The vector representation for structure objects has the advantage
that all structure slots can be accessed quickly, although creating
vectors is a bit slower in Emacs Lisp. Lists are easier to create,
but take a relatively long time accessing the later slots.
:named
:type without also using :named will result in a
structure type stored as plain vectors or lists with no identifying
features.
The default, if you don't specify :type explicitly, is to
use named vectors. Therefore, :named is only useful in
conjunction with :type.
(defstruct (person1) name age sex)
(defstruct (person2 (:type list) :named) name age sex)
(defstruct (person3 (:type list)) name age sex)
(setq p1 (make-person1))
=> [cl-struct-person1 nil nil nil]
(setq p2 (make-person2))
=> (person2 nil nil nil)
(setq p3 (make-person3))
=> (nil nil nil)
(person1-p p1)
=> t
(person2-p p2)
=> t
(person3-p p3)
=> error: function person3-p undefined
Since unnamed structures don't have tags, defstruct is not
able to make a useful predicate for recognizing them. Also,
accessors like person3-name will be generated but they
will not be able to do any type checking. The person3-name
function, for example, will simply be a synonym for car in
this case. By contrast, person2-name is able to verify
that its argument is indeed a person2 object before
proceeding.
:initial-offset
nil by the constructors and ignored otherwise. If
the type :includes another type, then :initial-offset
specifies a number of slots to be skipped between the last slot
of the included type and the first new slot.
Except as noted, the defstruct facility of this package is
entirely compatible with that of Common Lisp.
@chapno=23
This section describes two macros that test assertions, i.e., conditions which must be true if the program is operating correctly. Assertions never add to the behavior of a Lisp program; they simply make "sanity checks" to make sure everything is as it should be.
If the optimization property speed has been set to 3, and
safety is less than 3, then the byte-compiler will optimize
away the following assertions. Because assertions might be optimized
away, it is a bad idea for them to include side-effects.
nil value). If so, it returns nil. If the test
is not satisfied, assert signals an error.
A default error message will be supplied which includes test-form.
You can specify a different error message by including a string
argument plus optional extra arguments. Those arguments are simply
passed to error to signal the error.
If the optional second argument show-args is t instead
of nil, then the error message (with or without string)
will also include all non-constant arguments of the top-level
form. For example:
(assert (> x 10) t "x is too small: %d")
This usage of show-args is an extension to Common Lisp. In
true Common Lisp, the second argument gives a list of places
which can be setf'd by the user before continuing from the
error. Since Emacs Lisp does not support continuable errors, it
makes no sense to specify places.
nil. If not, check-type
signals a wrong-type-argument error. The default error message
lists the erroneous value along with type and form
themselves. If string is specified, it is included in the
error message in place of type. For example:
(check-type x (integer 1 *) "a positive integer")
See section Type Predicates, for a description of the type specifiers that may be used for type.
Note that in Common Lisp, the first argument to check-type
must be a place suitable for use by setf, because
check-type signals a continuable error that allows the
user to modify place.
The following error-related macro is also defined:
progn, except that
errors are ignored during the forms. More precisely, if
an error is signaled then ignore-errors immediately
aborts execution of the forms and returns nil.
If the forms complete successfully, ignore-errors
returns the result of the last form.
Many of the advanced features of this package, such as defun*,
loop, and setf, are implemented as Lisp macros. In
byte-compiled code, these complex notations will be expanded into
equivalent Lisp code which is simple and efficient. For example,
the forms
(incf i n) (push x (car p))
are expanded at compile-time to the Lisp forms
(setq i (+ i n)) (setcar p (cons x (car p)))
which are the most efficient ways of doing these respective operations
in Lisp. Thus, there is no performance penalty for using the more
readable incf and push forms in your compiled code.
Interpreted code, on the other hand, must expand these macros
every time they are executed. For this reason it is strongly
recommended that code making heavy use of macros be compiled.
(The features labeled "Special Form" instead of "Function" in
this manual are macros.) A loop using incf a hundred times
will execute considerably faster if compiled, and will also
garbage-collect less because the macro expansion will not have
to be generated, used, and thrown away a hundred times.
You can find out how a macro expands by using the
cl-prettyexpand function.
*scratch* buffer and type, say,
(cl-prettyexpand '(loop for x below 10 collect x))
and type C-x C-e immediately after the closing parenthesis; the expansion
(block nil
(let* ((x 0)
(G1004 nil))
(while (< x 10)
(setq G1004 (cons x G1004))
(setq x (+ x 1)))
(nreverse G1004)))
will be inserted into the buffer. (The block macro is
expanded differently in the interpreter and compiler, so
cl-prettyexpand just leaves it alone. The temporary
variable G1004 was created by gensym.)
If the optional argument full is true, then all
macros are expanded, including block, eval-when,
and compiler macros. Expansion is done as if form were
a top-level form in a file being compiled. For example,
(cl-prettyexpand '(pushnew 'x list))
-| (setq list (adjoin 'x list))
(cl-prettyexpand '(pushnew 'x list) t)
-| (setq list (if (memq 'x list) list (cons 'x list)))
(cl-prettyexpand '(caddr (member* 'a list)) t)
-| (car (cdr (cdr (memq 'a list))))
Note that adjoin, caddr, and member* all
have built-in compiler macros to optimize them in common cases.
Common Lisp compliance has in general not been sacrificed for the
sake of efficiency. A few exceptions have been made for cases
where substantial gains were possible at the expense of marginal
incompatibility. One example is the use of memq (which is
treated very efficiently by the byte-compiler) to scan for keyword
arguments; this can become confused in rare cases when keyword
symbols are used as both keywords and data values at once. This
is extremely unlikely to occur in practical code, and the use of
memq allows functions with keyword arguments to be nearly
as fast as functions that use &optional arguments.
The Common Lisp standard (as embodied in Steele's book) uses the
phrase "it is an error if" to indicate a situation which is not
supposed to arise in complying programs; implementations are strongly
encouraged but not required to signal an error in these situations.
This package sometimes omits such error checking in the interest of
compactness and efficiency. For example, do variable
specifiers are supposed to be lists of one, two, or three forms;
extra forms are ignored by this package rather than signaling a
syntax error. The endp function is simply a synonym for
null in this package. Functions taking keyword arguments
will accept an odd number of arguments, treating the trailing
keyword as if it were followed by the value nil.
Argument lists (as processed by defun* and friends)
are checked rigorously except for the minor point just
mentioned; in particular, keyword arguments are checked for
validity, and &allow-other-keys and :allow-other-keys
are fully implemented. Keyword validity checking is slightly
time consuming (though not too bad in byte-compiled code);
you can use &allow-other-keys to omit this check. Functions
defined in this package such as find and member*
do check their keyword arguments for validity.
The byte-compiler that comes with Emacs 18 normally fails to expand
macros that appear in top-level positions in the file (i.e., outside
of defuns or other enclosing forms). This would have
disastrous consequences to programs that used such top-level macros
as defun*, eval-when, and defstruct. To
work around this problem, the CL package patches the Emacs
18 compiler to expand top-level macros. This patch will apply to
your own macros, too, if they are used in a top-level context.
The patch will not harm versions of the Emacs 18 compiler which
have already had a similar patch applied, nor will it affect the
optimizing Emacs 19 byte-compiler written by Jamie Zawinski and
Hallvard Furuseth. The patch is applied to the byte compiler's
code in Emacs' memory, not to the `bytecomp.elc' file
stored on disk.
The Emacs 19 compiler (for Emacs 18) is available from various
Emacs Lisp archive sites such as archive.cis.ohio-state.edu.
Its use is highly recommended; many of the Common Lisp macros emit
code which can be improved by optimization. In particular,
blocks (whether explicit or implicit in constructs like
defun* and loop) carry a fair run-time penalty; the
optimizing compiler removes blocks which are not actually
referenced by return or return-from inside the block.
Following is a list of all known incompatibilities between this package and Common Lisp as documented in Steele (2nd edition).
Certain function names, such as member, assoc, and
floor, were already taken by (incompatible) Emacs Lisp
functions; this package appends `*' to the names of its
Common Lisp versions of these functions.
The word defun* is required instead of defun in order
to use extended Common Lisp argument lists in a function. Likewise,
defmacro* and function* are versions of those forms
which understand full-featured argument lists. The &whole
keyword does not work in defmacro argument lists (except
inside recursive argument lists).
In order to allow an efficient implementation, keyword arguments use
a slightly cheesy parser which may be confused if a keyword symbol
is passed as the value of another keyword argument.
(Specifically, (memq :keyword rest-of-arguments)
is used to scan for :keyword among the supplied
keyword arguments.)
The eql and equal predicates do not distinguish
between IEEE floating-point plus and minus zero. The equalp
predicate has several differences with Common Lisp; see section Predicates.
The setf mechanism is entirely compatible, except that
setf-methods return a list of five values rather than five
values directly. Also, the new "setf function" concept
(typified by (defun (setf foo) ...)) is not implemented.
The do-all-symbols form is the same as do-symbols
with no obarray argument. In Common Lisp, this form would
iterate over all symbols in all packages. Since Emacs obarrays
are not a first-class package mechanism, there is no way for
do-all-symbols to locate any but the default obarray.
The loop macro is complete except that loop-finish
and type specifiers are unimplemented.
The multiple-value return facility treats lists as multiple
values, since Emacs Lisp cannot support multiple return values
directly. The macros will be compatible with Common Lisp if
values or values-list is always used to return to
a multiple-value-bind or other multiple-value receiver;
if values is used without multiple-value-...
or vice-versa the effect will be different from Common Lisp.
Many Common Lisp declarations are ignored, and others match
the Common Lisp standard in concept but not in detail. For
example, local special declarations, which are purely
advisory in Emacs Lisp, do not rigorously obey the scoping rules
set down in Steele's book.
The variable *gensym-counter* starts out with a pseudo-random
value rather than with zero. This is to cope with the fact that
generated symbols become interned when they are written to and
loaded back from a file.
The defstruct facility is compatible, except that structures
are of type :type vector :named by default rather than some
special, distinct type. Also, the :type slot option is ignored.
The second argument of check-type is treated differently.
Following is a list of all known incompatibilities between this package and the older Quiroz `cl.el' package.
This package's emulation of multiple return values in functions is incompatible with that of the older package. That package attempted to come as close as possible to true Common Lisp multiple return values; unfortunately, it could not be 100% reliable and so was prone to occasional surprises if used freely. This package uses a simpler method, namely replacing multiple values with lists of values, which is more predictable though more noticeably different from Common Lisp.
The defkeyword form and keywordp function are not
implemented in this package.
The member, floor, ceiling, truncate,
round, mod, and rem functions are suffixed
by `*' in this package to avoid collision with existing
functions in Emacs 18 or Emacs 19. The older package simply
redefined these functions, overwriting the built-in meanings and
causing serious portability problems with Emacs 19. (Some more
recent versions of the Quiroz package changed the names to
cl-member, etc.; this package defines the latter names as
aliases for member*, etc.)
Certain functions in the old package which were buggy or inconsistent
with the Common Lisp standard are incompatible with the conforming
versions in this package. For example, eql and member
were synonyms for eq and memq in that package, setf
failed to preserve correct order of evaluation of its arguments, etc.
Finally, unlike the older package, this package is careful to
prefix all of its internal names with cl-. Except for a
few functions which are explicitly defined as additional features
(such as floatp-safe and letf), this package does not
export any non-`cl-' symbols which are not also part of Common
Lisp.
cl-compat package
The CL package includes emulations of some features of the
old `cl.el', in the form of a compatibility package
cl-compat. To use it, put (require 'cl-compat) in
your program.
The old package defined a number of internal routines without
cl- prefixes or other annotations. Call to these routines
may have crept into existing Lisp code. cl-compat
provides emulations of the following internal routines:
pair-with-newsyms, zip-lists, unzip-lists,
reassemble-arglists, duplicate-symbols-p,
safe-idiv.
Some setf forms translated into calls to internal
functions that user code might call directly. The functions
setnth, setnthcdr, and setelt fall in
this category; they are defined by cl-compat, but the
best fix is to change to use setf properly.
The cl-compat file defines the keyword functions
keywordp, keyword-of, and defkeyword,
which are not defined by the new CL package because the
use of keywords as data is discouraged.
The build-klist mechanism for parsing keyword arguments
is emulated by cl-compat; the with-keyword-args
macro is not, however, and in any case it's best to change to
use the more natural keyword argument processing offered by
defun*.
Multiple return values are treated differently by the two
Common Lisp packages. The old package's method was more
compatible with true Common Lisp, though it used heuristics
that caused it to report spurious multiple return values in
certain cases. The cl-compat package defines a set
of multiple-value macros that are compatible with the old
CL package; again, they are heuristic in nature, but they
are guaranteed to work in any case where the old package's
macros worked. To avoid name collision with the "official"
multiple-value facilities, the ones in cl-compat have
capitalized names: Values, Values-list,
Multiple-value-bind, etc.
The functions cl-floor, cl-ceiling, cl-truncate,
and cl-round are defined by cl-compat to use the
old-style multiple-value mechanism, just as they did in the old
package. The newer floor* and friends return their two
results in a list rather than as multiple values. Note that
older versions of the old package used the unadorned names
floor, ceiling, etc.; cl-compat cannot use
these names because they conflict with Emacs 19 built-ins.
This package is meant to be used as an extension to Emacs Lisp, not as an Emacs implementation of true Common Lisp. Some of the remaining differences between Emacs Lisp and Common Lisp make it difficult to port large Common Lisp applications to Emacs. For one, some of the features in this package are not fully compliant with ANSI or Steele; see section Common Lisp Compatibility. But there are also quite a few features that this package does not provide at all. Here are some major omissions that you will want watch out for when bringing Common Lisp code into Emacs.
foo in one place and Foo or FOO in another.
Emacs Lisp will treat these as three distinct symbols.
Some Common Lisp code is written entirely in upper case. While Emacs
is happy to let the program's own functions and variables use
this convention, calls to Lisp builtins like if and
defun will have to be changed to lower case.
let
bindings apply only to references physically within their bodies
(or within macro expansions in their bodies). Emacs Lisp, by
contrast, uses dynamic scoping wherein a binding to a
variable is visible even inside functions called from the body.
Variables in Common Lisp can be made dynamically scoped by
declaring them special or using defvar. In Emacs
Lisp it is as if all variables were declared special.
Often you can use code that was written for lexical scoping
even in a dynamically scoped Lisp, but not always. Here is
an example of a Common Lisp code fragment that would fail in
Emacs Lisp:
(defun map-odd-elements (func list)
(loop for x in list
for flag = t then (not flag)
collect (if flag x (funcall func x))))
(defun add-odd-elements (list x)
(map-odd-elements (function (lambda (a) (+ a x))) list))
In Common Lisp, the two functions' usages of x are completely
independent. In Emacs Lisp, the binding to x made by
add-odd-elements will have been hidden by the binding
in map-odd-elements by the time the (+ a x) function
is called.
(This package avoids such problems in its own mapping functions
by using names like cl-x instead of x internally;
as long as you don't use the cl- prefix for your own
variables no collision can occur.)
See section Lexical Bindings, for a description of the lexical-let
form which establishes a Common Lisp-style lexical binding, and some
examples of how it differs from Emacs' regular let.
',
whereas Emacs Lisp's parser just treats quote as a special case.
Some Lisp packages use reader macros to create special syntaxes
for themselves, which the Emacs parser is incapable of reading.
The lack of reader macros, incidentally, is the reason behind
Emacs Lisp's unusual backquote syntax. Since backquotes are
implemented as a Lisp package and not built-in to the Emacs
parser, they are forced to use a regular macro named `
which is used with the standard function/macro call notation.
# that the Emacs Lisp parser
won't understand. For example, `#| ... |#' is an
alternate comment notation, and `#+lucid (foo)' tells
the parser to ignore the (foo) except in Lucid Common
Lisp.
package:symbol or package::symbol.
Emacs Lisp has a single namespace for all interned symbols, and
then uses a naming convention of putting a prefix like cl-
in front of the name. Some Emacs packages adopt the Common Lisp-like
convention of using cl: or cl:: as the prefix.
However, the Emacs parser does not understand colons and just
treats them as part of the symbol name. Thus, while mapcar
and lisp:mapcar may refer to the same symbol in Common
Lisp, they are totally distinct in Emacs Lisp. Common Lisp
programs which refer to a symbol by the full name sometimes
and the short name other times will not port cleanly to Emacs.
Emacs Lisp does have a concept of "obarrays," which are
package-like collections of symbols, but this feature is not
strong enough to be used as a true package mechanism.
format function is quite different between Common
Lisp and Emacs Lisp. It takes an additional "destination"
argument before the format string. A destination of nil
means to format to a string as in Emacs Lisp; a destination
of t means to write to the terminal (similar to
message in Emacs). Also, format control strings are
utterly different; ~ is used instead of % to
introduce format codes, and the set of available codes is
much richer. There are no notations like \n for
string literals; instead, format is used with the
"newline" format code, ~%. More advanced formatting
codes provide such features as paragraph filling, case
conversion, and even loops and conditionals.
While it would have been possible to implement most of Common
Lisp format in this package (under the name format*,
of course), it was not deemed worthwhile. It would have required
a huge amount of code to implement even a decent subset of
format*, yet the functionality it would provide over
Emacs Lisp's format would rarely be useful.
#(a b c) notation in Common Lisp. To further complicate
matters, Emacs 19 introduces its own #( notation for
something entirely different--strings with properties.
#\A
instead of ?A. Also, string= and string-equal
are synonyms in Emacs Lisp whereas the latter is case-insensitive
in Common Lisp.
defconstant
where Emacs Lisp uses defconst. Similarly, make-list
takes its arguments in different ways in the two Lisps but does
exactly the same thing, so this package has not bothered to
implement a Common Lisp-style make-list.
compiler-let, tagbody, prog,
ldb/dpb, parse-integer, cerror.
(defun sum-list (list)
(if list
(+ (car list) (sum-list (cdr list)))
0))
where a more iteratively-minded programmer might write one of
these forms:
(let ((total 0)) (dolist (x my-list) (incf total x)) total) (loop for x in my-list sum x)While this would be mainly a stylistic choice in most Common Lisps, in Emacs Lisp you should be aware that the iterative forms are much faster than recursion. Also, Lisp programmers will want to note that the current Emacs Lisp compiler does not optimize tail recursion.
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