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5. Control Structure

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.

5.1 Assignment  The `psetq' form
5.2 Generalized Variables  `setf', `incf', `push', etc.
5.3 Variable Bindings  `progv', `lexical-let', `flet', `macrolet'
5.4 Conditionals  `when', `unless', `case', `typecase'
5.5 Blocks and Exits  `block', `return', `return-from'
5.6 Iteration  `do', `dotimes', `dolist', `do-symbols'
5.7 Loop Facility  The Common Lisp `loop' macro
5.8 Multiple Values  `values', `multiple-value-bind', etc.


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5.1 Assignment

The psetq form is just like setq, except that multiple assignments are done in parallel rather than sequentially.

Macro: psetq [symbol form]...
This 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 section 5.2.2 Modify Macros.)

psetq always returns nil.


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5.2 Generalized Variables

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.

5.2.1 Basic Setf  `setf' and place forms
5.2.2 Modify Macros  `incf', `push', `rotatef', `letf', `callf', etc.
5.2.3 Customizing Setf  `define-modify-macro', `defsetf', `define-setf-method'


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5.2.1 Basic Setf

The setf macro is the most basic way to operate on generalized variables.

Macro: setf [place form]...
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 legally appear in the place argument of setf:

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.


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5.2.2 Modify Macros

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.

Macro: psetf [place form]...
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).

Macro: incf place &optional x
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).

Macro: decf place &optional x
This macro decrements the number stored in place by one, or by x if specified.

Macro: pop place
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.

Macro: push x place
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.

Macro: pushnew x place &key :test :test-not :key
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 section 11.3 Lists as Sets.

Macro: shiftf place... newvalue
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.

Macro: rotatef place...
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.

The following macros were invented for this package; they have no analogues in Common Lisp.

Macro: letf (bindings...) forms...
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.

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

Macro: callf function place args...
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 section 5.2.3 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.

Macro: callf2 function arg1 place args...
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).

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 5.3.4 Macro Bindings.


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5.2.3 Customizing Setf

Common Lisp defines three macros, define-modify-macro, defsetf, and define-setf-method, that allow the user to extend generalized variables in various ways.

Macro: define-modify-macro name arglist function [doc-string]
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.

Macro: defsetf access-fn update-fn
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)

Macro: defsetf access-fn arglist (store-var) forms...
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))

Macro: define-setf-method access-fn arglist forms...
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.

Function: get-setf-method place &optional env
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.

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.


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5.3 Variable Bindings

These Lisp forms make bindings to variables and function names, analogous to Lisp's built-in let form.

See section 5.2.2 Modify Macros, for the letf and letf* forms which are also related to variable bindings.

5.3.1 Dynamic Bindings  The `progv' form
5.3.2 Lexical Bindings  `lexical-let' and lexical closures
5.3.3 Function Bindings  `flet' and `labels'
5.3.4 Macro Bindings  `macrolet' and `symbol-macrolet'


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5.3.1 Dynamic 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.

Macro: progv symbols values forms...
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.


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5.3.2 Lexical Bindings

The CL package defines the following macro which more closely follows the Common Lisp let form:

Macro: lexical-let (bindings...) forms...
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 (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.

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


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5.3.3 Function Bindings

These forms make let-like bindings to functions instead of variables. Normally you should use labels, it is less expensive in compiled code and avoids the problems of dynamic scope.

Macro: labels (bindings...) forms...
This form establishes lexical-let-style bindings on the function cells of symbols rather than on their 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 available within forms, within the body of name, and within the body of any other functions in bindings. This allows the establishment of recursive and mutually-referential functions.

These functions are not available by name at run-time to code textually outside of the labels form, though they may be passed to other code by value. Since labels makes lexical rather than dynamic bindings, bindings of functions like + and list that have byte codes will succeed--that is, calls to such functions within form will reflect the bindings within the labels form, something not true of flet, which see.

Within forms, to access a bound function as a callable object, quote its name using #'name, as in the following example.

 
(labels ((1+ (number)
           "Return 1.0 added to NUMBER"
           (+ number 1.0)))
  (map 'vector #'1+ '(10 9 8 7 6 5 4 3 2 1)))
  => [11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0]

Functions defined by labels 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 3. Program Structure.

Macro: flet (bindings...) forms...
This form establishes let-style bindings on the function cells of symbols rather than on the value cells. In contravention of Common Lisp, Emacs Lisp flet establishes dynamic bindings (available at runtime) rather than lexical (available at compile time, but outside of forms, not at runtime). 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 XEmacs primitives, although note that some calls to primitive functions internal to XEmacs 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 XEmacs will not be caught since they make direct C-language calls to the message routines rather than going through the Lisp message function.

This is equally true for functions with associated byte codes, since they are also not accessed through the Lisp function slot. The byte compiler will warn in both these cases.


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5.3.4 Macro Bindings

These forms create local macros and "symbol macros."

Macro: macrolet (bindings...) forms...
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.

Macro: symbol-macrolet (bindings...) forms...
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 section 5.6 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 5.7 Loop Facility, for a description of the loop macro. This package defines a nonstandard in-ref loop clause that works much like my-dolist.


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5.4 Conditionals

These conditional forms augment Emacs Lisp's simple if, and, or, and cond forms.

Macro: when test forms...
This is a variant of 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)

Macro: unless test forms...
This is a variant of 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)

Macro: case keyform clause...
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 LFD, or anything else.

 
(case (read-char)
  (?a (do-a-thing))
  (?b (do-b-thing))
  ((?\r ?\n) (do-ret-thing))
  (t (do-other-thing)))

Macro: ecase keyform clause...
This macro is just like case, except that if the key does not match any of the clauses, an error is signalled rather than simply returning nil.

Macro: typecase keyform clause...
This macro is a version of case that checks for types rather than values. Each clause is of the form `(type body...)'. See section 4.1 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.

Macro: etypecase keyform clause...
This macro is just like typecase, except that if the key does not match any of the clauses, an error is signalled rather than simply returning nil.


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5.5 Blocks and Exits

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.

Macro: block name forms...
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, 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.

Macro: return-from name [result]
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.

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


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5.6 Iteration

The macros described here provide more sophisticated, high-level looping constructs to complement Emacs Lisp's basic while loop.

Macro: loop forms...
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 section 5.7 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.)

Macro: do (spec...) (end-test [result...]) forms...
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)))

Macro: do* (spec...) (end-test [result...]) forms...
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))

Macro: dolist (var list [result]) forms...
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. The loop is surrounded by an implicit nil block.

Macro: dotimes (var count [result]) forms...
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. The loop is surrounded by an implicit nil block.

Macro: do-symbols (var [obarray [result]]) forms...
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.

Macro: do-all-symbols (var [result]) forms...
This is identical to do-symbols except that the obarray argument is omitted; it always iterates over the default obarray.

See section 10.2 Mapping over Sequences, for some more functions for iterating over vectors or lists.


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5.7 Loop Facility

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.

5.7.1 Loop Basics  `loop' macro, basic clause structure
5.7.2 Loop Examples  Working examples of `loop' macro
5.7.3 For Clauses  Clauses introduced by `for' or `as'
5.7.4 Iteration Clauses  `repeat', `while', `thereis', etc.
5.7.5 Accumulation Clauses  `collect', `sum', `maximize', etc.
5.7.6 Other Clauses  `with', `if', `initially', `finally'


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5.7.1 Loop Basics

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.

Macro: loop clauses...
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.)

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.


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5.7.2 Loop Examples

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.


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5.7.3 For Clauses

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
This type of 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
This clause iterates var over all the elements of list, in turn. If you specify the 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
This clause iterates var over all the cons cells of list.

 
(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
This is like a regular 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
This clause iterates var over all the elements of array, which may be a vector or a string.

 
(loop for x across "aeiou"
      do (use-vowel (char-to-string x)))

for var across-ref array
This clause iterates over an array, with var a setf-able reference onto the elements; see in-ref above.

for var being the elements of sequence
This clause iterates over the elements of sequence, which may be a list, vector, or string. Since the type must be determined at run-time, this is somewhat less efficient than 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
This clause iterates over a sequence, with var a setf-able reference onto the elements; see in-ref above.

for var being the symbols [of obarray]
This clause iterates over symbols, either over all interned symbols or over all symbols in obarray. The loop is executed with var bound to each symbol in turn. The symbols are visited in an unspecified order.

As an example,

 
(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
This clause iterates over the entries in hash-table. For each hash table entry, var is bound to the entry's key. If you write `the hash-values' instead, var is bound to the values of the entries. The clause may be followed by the additional term `using (hash-values var2)' (where 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
This clause iterates over the entries in keymap. In GNU Emacs 18 and 19, keymaps are either alists or vectors, and key-codes are integers or symbols. In XEmacs, keymaps are a special new data type, and key-codes are symbols or lists of symbols. The iteration does not enter nested keymaps or inherited (parent) keymaps. You can use `the key-bindings' to access the commands bound to the keys rather than the key codes, and you can add a using clause to access both the codes and the bindings together.

for var being the key-seqs of keymap
This clause iterates over all key sequences defined by keymap and its nested keymaps, where var takes on values which are strings in Emacs 18 or vectors in Emacs 19. The strings or vectors are reused for each iteration, so you must copy them if you wish to keep them permanently. You can add a `using (key-bindings ...)' clause to get the command bindings as well.

for var being the overlays [of buffer] ...
This clause iterates over the Emacs 19 "overlays" or XEmacs "extents" of a buffer (the clause 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] ...
This clause iterates over all intervals of a buffer with constant text properties. The variable var will be bound to conses of start and end positions, where one start position is always equal to the previous end position. The clause allows 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
This clause iterates over all frames, i.e., X window system windows open on Emacs files. This clause works only under Emacs 19. The clause 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]
This clause iterates over the windows (in the Emacs sense) of the current frame, or of the specified frame. (In Emacs 18 there is only ever one frame, and the of term is not allowed there.)

for var being the buffers
This clause iterates over all buffers in Emacs. It is equivalent to `for var in (buffer-list)'.

for var = expr1 then expr2
This clause does a general iteration. The first time through the loop, var will be bound to expr1. On the second and successive iterations it will be set by evaluating expr2 (which may refer to the old value of var). For example, these two loops are effectively the same:

 
(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.


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5.7.4 Iteration Clauses

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
This clause simply counts up to the specified number using an internal temporary variable. The loops

 
(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
This clause stops the loop when the specified condition (any Lisp expression) becomes 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
This clause stops the loop when the specified condition is true, i.e., non-nil.

always condition
This clause stops the loop when the specified condition is 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
This clause is like always, except that the loop returns t if any conditions were false, or nil otherwise.

thereis condition
This clause stops the loop when the specified form is non-nil; in this case, it returns that non-nil value. If all the values were nil, the loop returns nil.


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5.7.5 Accumulation Clauses

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
This clause collects the values of form into a list. Several examples of collect appear elsewhere in this manual.

The word collecting is a synonym for collect, and likewise for the other accumulation clauses.

append form
This clause collects lists of values into a result list using append.

nconc form
This clause collects lists of values into a result list by destructively modifying the lists rather than copying them.

concat form
This clause concatenates the values of the specified form into a string. (It and the following clause are extensions to standard Common Lisp.)

vconcat form
This clause concatenates the values of the specified form into a vector.

count form
This clause counts the number of times the specified form evaluates to a non-nil value.

sum form
This clause accumulates the sum of the values of the specified form, which must evaluate to a number.

maximize form
This clause accumulates the maximum value of the specified form, which must evaluate to a number. The return value is undefined if maximize is executed zero times.

minimize form
This clause accumulates the minimum value of the specified 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)


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5.7.6 Other Clauses

This section describes the remaining loop clauses.

with var = value
This clause binds a variable to a value around the loop, but otherwise leaves the variable alone during the loop. The following loops are basically equivalent:

 
(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
This clause executes the following loop clause only if the specified condition is true. The following clause should be an accumulation, 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
This clause is just a synonym for if.

unless condition clause
The unless clause is just like if except that the sense of the condition is reversed.

named name
This clause gives a name other than nil to the implicit block surrounding the loop. The name is the symbol to be used as the block name.

initially [do] forms...
This keyword introduces one or more Lisp forms which will be executed before the loop itself begins (but after any variables requested by 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...
This introduces Lisp forms which will be executed after the loop finishes (say, on request of a 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
This says that form should be executed after the loop is done to obtain a return value. (Without this, or some other clause like 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...
The word 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
This clause causes the loop to return immediately. The following Lisp form is evaluated to give the return value of the 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.


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5.8 Multiple Values

This functionality has been moved to core XEmacs, and is documented in the XEmacs Lisp reference, @xref{(lispref.info)Multiple values}.


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