Common Lisp Extensions

This form is identical to the regular 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 Blocks and Exits.

This is just like 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; 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.

This is identical to the regular 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.

This is identical to the regular function form, except that if the argument is a lambda form then that form may use a full Common Lisp argument list.

This form controls when the body forms are evaluated. The situations list may contain any set of the symbols 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) ...).

The forms are evaluated at compile-time; at execution time, this form acts like a quoted constant of the resulting value. Used at top-level, 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.

The form is evaluated at load-time; at execution time, this form acts like a quoted constant of the resulting value.

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--))
     

Check if object is of type type, where type is a (quoted) type name of the sort used by Common Lisp. For example, (typep foo 'integer) is equivalent to (integerp foo).

This function attempts to convert object to the specified type. If object is already of that type as determined by 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.

This macro defines a new type called name. It is similar to 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.

This function is almost the same as 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. 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.

This function is a more flexible version of 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.

This special form (actually a macro) is used to assign to several variables simultaneously. Given only one symbol and form, it has the same effect as 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 Modify Macros.)

psetq always returns nil.

This macro evaluates form and stores it in place, which must be a valid generalized variable form. If there are several place and form pairs, the assignments are done sequentially just as with setq. setf returns the value of the last form.

The following Lisp forms will work as generalized variables, and so may appear in the place argument of setf:

• A symbol naming a variable. In other words, (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.
• A call to any of the following Lisp functions:

                 car                 cdr                 caar .. cddddr
                 nth                 rest                first .. tenth
                 aref                elt                 nthcdr
                 symbol-function     symbol-value        symbol-plist
                 get                 get*                getf
                 gethash             subseq
            

  Note 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.

• The following Emacs-specific functions are also setf-able.

                 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-marker
            

  Most 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).

• A call of the form (substring subplace n [m]), where subplace is itself a valid 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.

• A call of the form (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.
• A macro call, in which case the macro is expanded and setf is applied to the resulting form.
• Any form for which a 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 macro is to 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).

This macro increments the number stored in place by one, or by x if specified. The incremented value is returned. For example, (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).

This macro decrements the number stored in place by one, or by x if specified.

This macro removes and returns the first element of the list stored in place. It is analogous to (prog1 (car place) (setf place (cdr place))), except that it takes care to evaluate all subforms only once.

This macro inserts x at the front of the list stored in place. It is analogous to (setf place (cons x place)), except for evaluation of the subforms.

This macro inserts x at the front of the list stored in place, but only if x was not eql to any existing element of the list. The optional keyword arguments are interpreted in the same way as for adjoin. See Lists as Sets.

This macro shifts the places left by one, shifting in the value of newvalue (which may be any Lisp expression, not just a generalized variable), and returning the value shifted out of the first place. Thus, (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.

This macro rotates the places left by one in circular fashion. Thus, (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.

This macro is analogous to 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.

This macro is to letf what let* is to let: It does the bindings in sequential rather than parallel order.

This is the “generic” modify macro. It calls function, which should be an unquoted function name, macro name, or lambda. It passes place and args as arguments, and assigns the result back to place. For example, (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 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.

This macro is like 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).

This macro defines a “read-modify-write” macro similar to 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.

This is the simpler of two 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)
     

This is the second, more complex, form of 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))
     

This is the most general way to create new place forms. When a setf to access-fn with arguments described by arglist is expanded, the forms are evaluated and must return a list of five items:

1. A list of temporary variables.
2. A list of value forms corresponding to the temporary variables above. The temporary variables will be bound to these value forms as the first step of any operation on the generalized variable.
3. A list of exactly one store variable (generally obtained from a call to gensym).
4. A Lisp form which stores the contents of the store variable into the generalized variable, assuming the temporaries have been bound as described above.
5. A Lisp form which accesses the contents of the generalized variable, assuming the temporaries have been bound.

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.

This function returns the setf-method for place, by invoking the definition previously recorded by 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.

This form establishes 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.

This form is exactly like 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 (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.

This form is just like lexical-let, except that the bindings are made sequentially in the manner of let*.

This form establishes 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 Program Structure.

The 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.

This form is analogous to 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.

This form creates symbol macros, which are macros that look like variable references rather than function calls. Each binding is a list `(var expansion)'; any reference to var within the body forms is replaced by expansion.

          (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 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 Loop Facility, for a description of the loop macro. This package defines a nonstandard in-ref loop clause that works much like my-dolist.

This macro evaluates keyform, then compares it with the key values listed in the various clauses. Whichever clause matches the key is executed; comparison is done by 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)))
     

This macro is just like case, except that if the key does not match any of the clauses, an error is signaled rather than simply returning nil.

This macro is a version of case that checks for types rather than values. Each clause is of the form `(type body...)'. See 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.

This macro is just like typecase, except that if the key does not match any of the clauses, an error is signaled rather than simply returning nil.

The forms are evaluated as if by a 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, the optimizing byte compiler 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.

This macro returns from the block named name, which must be an (unevaluated) symbol. If a result form is specified, it is evaluated to produce the result returned from the block. Otherwise, nil is returned.

This macro is exactly like (return-from nil result). Common Lisp loops like do and dolist implicitly enclose themselves in nil blocks.

The CL package supports both the simple, old-style meaning of 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 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.)

This macro creates a general iterative loop. Each spec is of the form

          (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)))
     

This is to 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))
     

This is a more specialized loop which iterates across the elements of a list. list should evaluate to a list; the body forms are executed with var bound to each element of the list in turn. Finally, the result form (or nil) is evaluated with var bound to nil to produce the result returned by the loop. Unlike with Emacs's built in dolist, the loop is surrounded by an implicit nil block.

This is a more specialized loop which iterates a specified number of times. The body is executed with var bound to the integers from zero (inclusive) to count (exclusive), in turn. Then the 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. Unlike with Emacs's built in dolist, the loop is surrounded by an implicit nil block.

This loop iterates over all interned symbols. If obarray is specified and is not 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.

This is identical to do-symbols except that the obarray argument is omitted; it always iterates over the default obarray.

A loop construct consists of a series of clauses, each introduced by a symbol like 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.)