2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-1999, 2001-2013 Free Software
5 @c See the file elisp.texi for copying conditions.
6 @node Control Structures
7 @chapter Control Structures
8 @cindex special forms for control structures
9 @cindex control structures
11 A Lisp program consists of a set of @dfn{expressions}, or
12 @dfn{forms} (@pxref{Forms}). We control the order of execution of
13 these forms by enclosing them in @dfn{control structures}. Control
14 structures are special forms which control when, whether, or how many
15 times to execute the forms they contain.
18 The simplest order of execution is sequential execution: first form
19 @var{a}, then form @var{b}, and so on. This is what happens when you
20 write several forms in succession in the body of a function, or at top
21 level in a file of Lisp code---the forms are executed in the order
22 written. We call this @dfn{textual order}. For example, if a function
23 body consists of two forms @var{a} and @var{b}, evaluation of the
24 function evaluates first @var{a} and then @var{b}. The result of
25 evaluating @var{b} becomes the value of the function.
27 Explicit control structures make possible an order of execution other
30 Emacs Lisp provides several kinds of control structure, including
31 other varieties of sequencing, conditionals, iteration, and (controlled)
32 jumps---all discussed below. The built-in control structures are
33 special forms since their subforms are not necessarily evaluated or not
34 evaluated sequentially. You can use macros to define your own control
35 structure constructs (@pxref{Macros}).
38 * Sequencing:: Evaluation in textual order.
39 * Conditionals:: @code{if}, @code{cond}, @code{when}, @code{unless}.
40 * Combining Conditions:: @code{and}, @code{or}, @code{not}.
41 * Iteration:: @code{while} loops.
42 * Nonlocal Exits:: Jumping out of a sequence.
48 Evaluating forms in the order they appear is the most common way
49 control passes from one form to another. In some contexts, such as in a
50 function body, this happens automatically. Elsewhere you must use a
51 control structure construct to do this: @code{progn}, the simplest
52 control construct of Lisp.
54 A @code{progn} special form looks like this:
58 (progn @var{a} @var{b} @var{c} @dots{})
63 and it says to execute the forms @var{a}, @var{b}, @var{c}, and so on, in
64 that order. These forms are called the @dfn{body} of the @code{progn} form.
65 The value of the last form in the body becomes the value of the entire
66 @code{progn}. @code{(progn)} returns @code{nil}.
68 @cindex implicit @code{progn}
69 In the early days of Lisp, @code{progn} was the only way to execute
70 two or more forms in succession and use the value of the last of them.
71 But programmers found they often needed to use a @code{progn} in the
72 body of a function, where (at that time) only one form was allowed. So
73 the body of a function was made into an ``implicit @code{progn}'':
74 several forms are allowed just as in the body of an actual @code{progn}.
75 Many other control structures likewise contain an implicit @code{progn}.
76 As a result, @code{progn} is not used as much as it was many years ago.
77 It is needed now most often inside an @code{unwind-protect}, @code{and},
78 @code{or}, or in the @var{then}-part of an @code{if}.
80 @defspec progn forms@dots{}
81 This special form evaluates all of the @var{forms}, in textual
82 order, returning the result of the final form.
86 (progn (print "The first form")
87 (print "The second form")
88 (print "The third form"))
89 @print{} "The first form"
90 @print{} "The second form"
91 @print{} "The third form"
92 @result{} "The third form"
97 Two other constructs likewise evaluate a series of forms but return
100 @defspec prog1 form1 forms@dots{}
101 This special form evaluates @var{form1} and all of the @var{forms}, in
102 textual order, returning the result of @var{form1}.
106 (prog1 (print "The first form")
107 (print "The second form")
108 (print "The third form"))
109 @print{} "The first form"
110 @print{} "The second form"
111 @print{} "The third form"
112 @result{} "The first form"
116 Here is a way to remove the first element from a list in the variable
117 @code{x}, then return the value of that former element:
120 (prog1 (car x) (setq x (cdr x)))
124 @defspec prog2 form1 form2 forms@dots{}
125 This special form evaluates @var{form1}, @var{form2}, and all of the
126 following @var{forms}, in textual order, returning the result of
131 (prog2 (print "The first form")
132 (print "The second form")
133 (print "The third form"))
134 @print{} "The first form"
135 @print{} "The second form"
136 @print{} "The third form"
137 @result{} "The second form"
143 @section Conditionals
144 @cindex conditional evaluation
146 Conditional control structures choose among alternatives. Emacs Lisp
147 has four conditional forms: @code{if}, which is much the same as in
148 other languages; @code{when} and @code{unless}, which are variants of
149 @code{if}; and @code{cond}, which is a generalized case statement.
151 @defspec if condition then-form else-forms@dots{}
152 @code{if} chooses between the @var{then-form} and the @var{else-forms}
153 based on the value of @var{condition}. If the evaluated @var{condition} is
154 non-@code{nil}, @var{then-form} is evaluated and the result returned.
155 Otherwise, the @var{else-forms} are evaluated in textual order, and the
156 value of the last one is returned. (The @var{else} part of @code{if} is
157 an example of an implicit @code{progn}. @xref{Sequencing}.)
159 If @var{condition} has the value @code{nil}, and no @var{else-forms} are
160 given, @code{if} returns @code{nil}.
162 @code{if} is a special form because the branch that is not selected is
163 never evaluated---it is ignored. Thus, in this example,
164 @code{true} is not printed because @code{print} is never called:
176 @defmac when condition then-forms@dots{}
177 This is a variant of @code{if} where there are no @var{else-forms},
178 and possibly several @var{then-forms}. In particular,
181 (when @var{condition} @var{a} @var{b} @var{c})
185 is entirely equivalent to
188 (if @var{condition} (progn @var{a} @var{b} @var{c}) nil)
192 @defmac unless condition forms@dots{}
193 This is a variant of @code{if} where there is no @var{then-form}:
196 (unless @var{condition} @var{a} @var{b} @var{c})
200 is entirely equivalent to
203 (if @var{condition} nil
204 @var{a} @var{b} @var{c})
208 @defspec cond clause@dots{}
209 @code{cond} chooses among an arbitrary number of alternatives. Each
210 @var{clause} in the @code{cond} must be a list. The @sc{car} of this
211 list is the @var{condition}; the remaining elements, if any, the
212 @var{body-forms}. Thus, a clause looks like this:
215 (@var{condition} @var{body-forms}@dots{})
218 @code{cond} tries the clauses in textual order, by evaluating the
219 @var{condition} of each clause. If the value of @var{condition} is
220 non-@code{nil}, the clause ``succeeds''; then @code{cond} evaluates its
221 @var{body-forms}, and returns the value of the last of @var{body-forms}.
222 Any remaining clauses are ignored.
224 If the value of @var{condition} is @code{nil}, the clause ``fails'', so
225 the @code{cond} moves on to the following clause, trying its @var{condition}.
227 A clause may also look like this:
234 Then, if @var{condition} is non-@code{nil} when tested, the @code{cond}
235 form returns the value of @var{condition}.
237 If every @var{condition} evaluates to @code{nil}, so that every clause
238 fails, @code{cond} returns @code{nil}.
240 The following example has four clauses, which test for the cases where
241 the value of @code{x} is a number, string, buffer and symbol,
246 (cond ((numberp x) x)
249 (setq temporary-hack x) ; @r{multiple body-forms}
250 (buffer-name x)) ; @r{in one clause}
251 ((symbolp x) (symbol-value x)))
255 Often we want to execute the last clause whenever none of the previous
256 clauses was successful. To do this, we use @code{t} as the
257 @var{condition} of the last clause, like this: @code{(t
258 @var{body-forms})}. The form @code{t} evaluates to @code{t}, which is
259 never @code{nil}, so this clause never fails, provided the @code{cond}
260 gets to it at all. For example:
265 (cond ((eq a 'hack) 'foo)
272 This @code{cond} expression returns @code{foo} if the value of @code{a}
273 is @code{hack}, and returns the string @code{"default"} otherwise.
276 Any conditional construct can be expressed with @code{cond} or with
277 @code{if}. Therefore, the choice between them is a matter of style.
282 (if @var{a} @var{b} @var{c})
284 (cond (@var{a} @var{b}) (t @var{c}))
289 * Pattern matching case statement::
292 @node Pattern matching case statement
293 @subsection Pattern matching case statement
295 @cindex pattern matching
297 To compare a particular value against various possible cases, the macro
298 @code{pcase} can come handy. It takes the following form:
301 (pcase @var{exp} @var{branch}1 @var{branch}2 @var{branch}3 @dots{})
304 where each @var{branch} takes the form @code{(@var{upattern}
305 @var{body-forms}@dots{})}.
307 It will first evaluate @var{exp} and then compare the value against each
308 @var{upattern} to see which @var{branch} to use, after which it will run the
309 corresponding @var{body-forms}. A common use case is to distinguish
310 between a few different constant values:
313 (pcase (get-return-code x)
314 (`success (message "Done!"))
315 (`would-block (message "Sorry, can't do it now"))
316 (`read-only (message "The shmliblick is read-only"))
317 (`access-denied (message "You do not have the needed rights"))
318 (code (message "Unknown return code %S" code)))
321 In the last clause, @code{code} is a variable that gets bound to the value that
322 was returned by @code{(get-return-code x)}.
324 To give a more complex example, a simple interpreter for a little
325 expression language could look like:
328 (defun evaluate (exp env)
330 (`(add ,x ,y) (+ (evaluate x env) (evaluate y env)))
331 (`(call ,fun ,arg) (funcall (evaluate fun) (evaluate arg env)))
332 (`(fn ,arg ,body) (lambda (val)
333 (evaluate body (cons (cons arg val) env))))
335 ((pred symbolp) (cdr (assq exp env)))
336 (_ (error "Unknown expression %S" exp))))
339 Where @code{`(add ,x ,y)} is a pattern that checks that @code{exp} is a three
340 element list starting with the symbol @code{add}, then extracts the second and
341 third elements and binds them to the variables @code{x} and @code{y}.
342 @code{(pred numberp)} is a pattern that simply checks that @code{exp}
343 is a number, and @code{_} is the catch-all pattern that matches anything.
345 There are two kinds of patterns involved in @code{pcase}, called
346 @emph{U-patterns} and @emph{Q-patterns}. The @var{upattern} mentioned above
347 are U-patterns and can take the following forms:
350 @item `@var{qpattern}
351 This is one of the most common form of patterns. The intention is to mimic the
352 backquote macro: this pattern matches those values that could have been built
353 by such a backquote expression. Since we're pattern matching rather than
354 building a value, the unquote does not indicate where to plug an expression,
355 but instead it lets one specify a U-pattern that should match the value at
358 More specifically, a Q-pattern can take the following forms:
360 @item (@var{qpattern1} . @var{qpattern2})
361 This pattern matches any cons cell whose @code{car} matches @var{QPATTERN1} and
362 whose @code{cdr} matches @var{PATTERN2}.
364 This pattern matches any atom @code{equal} to @var{atom}.
365 @item ,@var{upattern}
366 This pattern matches any object that matches the @var{upattern}.
370 A mere symbol in a U-pattern matches anything, and additionally let-binds this
371 symbol to the value that it matched, so that you can later refer to it, either
372 in the @var{body-forms} or also later in the pattern.
374 This so-called @emph{don't care} pattern matches anything, like the previous
375 one, but unlike symbol patterns it does not bind any variable.
376 @item (pred @var{pred})
377 This pattern matches if the function @var{pred} returns non-@code{nil} when
378 called with the object being matched.
379 @item (or @var{upattern1} @var{upattern2}@dots{})
380 This pattern matches as soon as one of the argument patterns succeeds.
381 All argument patterns should let-bind the same variables.
382 @item (and @var{upattern1} @var{upattern2}@dots{})
383 This pattern matches only if all the argument patterns succeed.
384 @item (guard @var{exp})
385 This pattern ignores the object being examined and simply succeeds if @var{exp}
386 evaluates to non-@code{nil} and fails otherwise. It is typically used inside
387 an @code{and} pattern. For example, @code{(and x (guard (< x 10)))}
388 is a pattern which matches any number smaller than 10 and let-binds it to
389 the variable @code{x}.
392 @node Combining Conditions
393 @section Constructs for Combining Conditions
395 This section describes three constructs that are often used together
396 with @code{if} and @code{cond} to express complicated conditions. The
397 constructs @code{and} and @code{or} can also be used individually as
398 kinds of multiple conditional constructs.
401 This function tests for the falsehood of @var{condition}. It returns
402 @code{t} if @var{condition} is @code{nil}, and @code{nil} otherwise.
403 The function @code{not} is identical to @code{null}, and we recommend
404 using the name @code{null} if you are testing for an empty list.
407 @defspec and conditions@dots{}
408 The @code{and} special form tests whether all the @var{conditions} are
409 true. It works by evaluating the @var{conditions} one by one in the
412 If any of the @var{conditions} evaluates to @code{nil}, then the result
413 of the @code{and} must be @code{nil} regardless of the remaining
414 @var{conditions}; so @code{and} returns @code{nil} right away, ignoring
415 the remaining @var{conditions}.
417 If all the @var{conditions} turn out non-@code{nil}, then the value of
418 the last of them becomes the value of the @code{and} form. Just
419 @code{(and)}, with no @var{conditions}, returns @code{t}, appropriate
420 because all the @var{conditions} turned out non-@code{nil}. (Think
421 about it; which one did not?)
423 Here is an example. The first condition returns the integer 1, which is
424 not @code{nil}. Similarly, the second condition returns the integer 2,
425 which is not @code{nil}. The third condition is @code{nil}, so the
426 remaining condition is never evaluated.
430 (and (print 1) (print 2) nil (print 3))
437 Here is a more realistic example of using @code{and}:
441 (if (and (consp foo) (eq (car foo) 'x))
442 (message "foo is a list starting with x"))
447 Note that @code{(car foo)} is not executed if @code{(consp foo)} returns
448 @code{nil}, thus avoiding an error.
450 @code{and} expressions can also be written using either @code{if} or
451 @code{cond}. Here's how:
455 (and @var{arg1} @var{arg2} @var{arg3})
457 (if @var{arg1} (if @var{arg2} @var{arg3}))
459 (cond (@var{arg1} (cond (@var{arg2} @var{arg3}))))
464 @defspec or conditions@dots{}
465 The @code{or} special form tests whether at least one of the
466 @var{conditions} is true. It works by evaluating all the
467 @var{conditions} one by one in the order written.
469 If any of the @var{conditions} evaluates to a non-@code{nil} value, then
470 the result of the @code{or} must be non-@code{nil}; so @code{or} returns
471 right away, ignoring the remaining @var{conditions}. The value it
472 returns is the non-@code{nil} value of the condition just evaluated.
474 If all the @var{conditions} turn out @code{nil}, then the @code{or}
475 expression returns @code{nil}. Just @code{(or)}, with no
476 @var{conditions}, returns @code{nil}, appropriate because all the
477 @var{conditions} turned out @code{nil}. (Think about it; which one
480 For example, this expression tests whether @code{x} is either
481 @code{nil} or the integer zero:
484 (or (eq x nil) (eq x 0))
487 Like the @code{and} construct, @code{or} can be written in terms of
488 @code{cond}. For example:
492 (or @var{arg1} @var{arg2} @var{arg3})
500 You could almost write @code{or} in terms of @code{if}, but not quite:
504 (if @var{arg1} @var{arg1}
505 (if @var{arg2} @var{arg2}
511 This is not completely equivalent because it can evaluate @var{arg1} or
512 @var{arg2} twice. By contrast, @code{(or @var{arg1} @var{arg2}
513 @var{arg3})} never evaluates any argument more than once.
521 Iteration means executing part of a program repetitively. For
522 example, you might want to repeat some computation once for each element
523 of a list, or once for each integer from 0 to @var{n}. You can do this
524 in Emacs Lisp with the special form @code{while}:
526 @defspec while condition forms@dots{}
527 @code{while} first evaluates @var{condition}. If the result is
528 non-@code{nil}, it evaluates @var{forms} in textual order. Then it
529 reevaluates @var{condition}, and if the result is non-@code{nil}, it
530 evaluates @var{forms} again. This process repeats until @var{condition}
531 evaluates to @code{nil}.
533 There is no limit on the number of iterations that may occur. The loop
534 will continue until either @var{condition} evaluates to @code{nil} or
535 until an error or @code{throw} jumps out of it (@pxref{Nonlocal Exits}).
537 The value of a @code{while} form is always @code{nil}.
546 (princ (format "Iteration %d." num))
548 @print{} Iteration 0.
549 @print{} Iteration 1.
550 @print{} Iteration 2.
551 @print{} Iteration 3.
556 To write a ``repeat...until'' loop, which will execute something on each
557 iteration and then do the end-test, put the body followed by the
558 end-test in a @code{progn} as the first argument of @code{while}, as
565 (not (looking-at "^$"))))
570 This moves forward one line and continues moving by lines until it
571 reaches an empty line. It is peculiar in that the @code{while} has no
572 body, just the end test (which also does the real work of moving point).
575 The @code{dolist} and @code{dotimes} macros provide convenient ways to
576 write two common kinds of loops.
578 @defmac dolist (var list [result]) body@dots{}
579 This construct executes @var{body} once for each element of
580 @var{list}, binding the variable @var{var} locally to hold the current
581 element. Then it returns the value of evaluating @var{result}, or
582 @code{nil} if @var{result} is omitted. For example, here is how you
583 could use @code{dolist} to define the @code{reverse} function:
586 (defun reverse (list)
588 (dolist (elt list value)
589 (setq value (cons elt value)))))
593 @defmac dotimes (var count [result]) body@dots{}
594 This construct executes @var{body} once for each integer from 0
595 (inclusive) to @var{count} (exclusive), binding the variable @var{var}
596 to the integer for the current iteration. Then it returns the value
597 of evaluating @var{result}, or @code{nil} if @var{result} is omitted.
598 Here is an example of using @code{dotimes} to do something 100 times:
602 (insert "I will not obey absurd orders\n"))
607 @section Nonlocal Exits
608 @cindex nonlocal exits
610 A @dfn{nonlocal exit} is a transfer of control from one point in a
611 program to another remote point. Nonlocal exits can occur in Emacs Lisp
612 as a result of errors; you can also use them under explicit control.
613 Nonlocal exits unbind all variable bindings made by the constructs being
617 * Catch and Throw:: Nonlocal exits for the program's own purposes.
618 * Examples of Catch:: Showing how such nonlocal exits can be written.
619 * Errors:: How errors are signaled and handled.
620 * Cleanups:: Arranging to run a cleanup form if an error happens.
623 @node Catch and Throw
624 @subsection Explicit Nonlocal Exits: @code{catch} and @code{throw}
626 Most control constructs affect only the flow of control within the
627 construct itself. The function @code{throw} is the exception to this
628 rule of normal program execution: it performs a nonlocal exit on
629 request. (There are other exceptions, but they are for error handling
630 only.) @code{throw} is used inside a @code{catch}, and jumps back to
631 that @code{catch}. For example:
648 The @code{throw} form, if executed, transfers control straight back to
649 the corresponding @code{catch}, which returns immediately. The code
650 following the @code{throw} is not executed. The second argument of
651 @code{throw} is used as the return value of the @code{catch}.
653 The function @code{throw} finds the matching @code{catch} based on the
654 first argument: it searches for a @code{catch} whose first argument is
655 @code{eq} to the one specified in the @code{throw}. If there is more
656 than one applicable @code{catch}, the innermost one takes precedence.
657 Thus, in the above example, the @code{throw} specifies @code{foo}, and
658 the @code{catch} in @code{foo-outer} specifies the same symbol, so that
659 @code{catch} is the applicable one (assuming there is no other matching
660 @code{catch} in between).
662 Executing @code{throw} exits all Lisp constructs up to the matching
663 @code{catch}, including function calls. When binding constructs such
664 as @code{let} or function calls are exited in this way, the bindings
665 are unbound, just as they are when these constructs exit normally
666 (@pxref{Local Variables}). Likewise, @code{throw} restores the buffer
667 and position saved by @code{save-excursion} (@pxref{Excursions}), and
668 the narrowing status saved by @code{save-restriction}. It also runs
669 any cleanups established with the @code{unwind-protect} special form
670 when it exits that form (@pxref{Cleanups}).
672 The @code{throw} need not appear lexically within the @code{catch}
673 that it jumps to. It can equally well be called from another function
674 called within the @code{catch}. As long as the @code{throw} takes place
675 chronologically after entry to the @code{catch}, and chronologically
676 before exit from it, it has access to that @code{catch}. This is why
677 @code{throw} can be used in commands such as @code{exit-recursive-edit}
678 that throw back to the editor command loop (@pxref{Recursive Editing}).
680 @cindex CL note---only @code{throw} in Emacs
682 @b{Common Lisp note:} Most other versions of Lisp, including Common Lisp,
683 have several ways of transferring control nonsequentially: @code{return},
684 @code{return-from}, and @code{go}, for example. Emacs Lisp has only
685 @code{throw}. The @file{cl-lib} library provides versions of some of
686 these. @xref{Blocks and Exits,,,cl,Common Lisp Extensions}.
689 @defspec catch tag body@dots{}
690 @cindex tag on run time stack
691 @code{catch} establishes a return point for the @code{throw} function.
692 The return point is distinguished from other such return points by
693 @var{tag}, which may be any Lisp object except @code{nil}. The argument
694 @var{tag} is evaluated normally before the return point is established.
696 With the return point in effect, @code{catch} evaluates the forms of the
697 @var{body} in textual order. If the forms execute normally (without
698 error or nonlocal exit) the value of the last body form is returned from
701 If a @code{throw} is executed during the execution of @var{body},
702 specifying the same value @var{tag}, the @code{catch} form exits
703 immediately; the value it returns is whatever was specified as the
704 second argument of @code{throw}.
707 @defun throw tag value
708 The purpose of @code{throw} is to return from a return point previously
709 established with @code{catch}. The argument @var{tag} is used to choose
710 among the various existing return points; it must be @code{eq} to the value
711 specified in the @code{catch}. If multiple return points match @var{tag},
712 the innermost one is used.
714 The argument @var{value} is used as the value to return from that
718 If no return point is in effect with tag @var{tag}, then a @code{no-catch}
719 error is signaled with data @code{(@var{tag} @var{value})}.
722 @node Examples of Catch
723 @subsection Examples of @code{catch} and @code{throw}
725 One way to use @code{catch} and @code{throw} is to exit from a doubly
726 nested loop. (In most languages, this would be done with a ``goto''.)
727 Here we compute @code{(foo @var{i} @var{j})} for @var{i} and @var{j}
739 (throw 'loop (list i j)))
746 If @code{foo} ever returns non-@code{nil}, we stop immediately and return a
747 list of @var{i} and @var{j}. If @code{foo} always returns @code{nil}, the
748 @code{catch} returns normally, and the value is @code{nil}, since that
749 is the result of the @code{while}.
751 Here are two tricky examples, slightly different, showing two
752 return points at once. First, two return points with the same tag,
765 (print (catch2 'hack))
773 Since both return points have tags that match the @code{throw}, it goes to
774 the inner one, the one established in @code{catch2}. Therefore,
775 @code{catch2} returns normally with value @code{yes}, and this value is
776 printed. Finally the second body form in the outer @code{catch}, which is
777 @code{'no}, is evaluated and returned from the outer @code{catch}.
779 Now let's change the argument given to @code{catch2}:
784 (print (catch2 'quux))
791 We still have two return points, but this time only the outer one has
792 the tag @code{hack}; the inner one has the tag @code{quux} instead.
793 Therefore, @code{throw} makes the outer @code{catch} return the value
794 @code{yes}. The function @code{print} is never called, and the
795 body-form @code{'no} is never evaluated.
801 When Emacs Lisp attempts to evaluate a form that, for some reason,
802 cannot be evaluated, it @dfn{signals} an @dfn{error}.
804 When an error is signaled, Emacs's default reaction is to print an
805 error message and terminate execution of the current command. This is
806 the right thing to do in most cases, such as if you type @kbd{C-f} at
807 the end of the buffer.
809 In complicated programs, simple termination may not be what you want.
810 For example, the program may have made temporary changes in data
811 structures, or created temporary buffers that should be deleted before
812 the program is finished. In such cases, you would use
813 @code{unwind-protect} to establish @dfn{cleanup expressions} to be
814 evaluated in case of error. (@xref{Cleanups}.) Occasionally, you may
815 wish the program to continue execution despite an error in a subroutine.
816 In these cases, you would use @code{condition-case} to establish
817 @dfn{error handlers} to recover control in case of error.
819 Resist the temptation to use error handling to transfer control from
820 one part of the program to another; use @code{catch} and @code{throw}
821 instead. @xref{Catch and Throw}.
824 * Signaling Errors:: How to report an error.
825 * Processing of Errors:: What Emacs does when you report an error.
826 * Handling Errors:: How you can trap errors and continue execution.
827 * Error Symbols:: How errors are classified for trapping them.
830 @node Signaling Errors
831 @subsubsection How to Signal an Error
832 @cindex signaling errors
834 @dfn{Signaling} an error means beginning error processing. Error
835 processing normally aborts all or part of the running program and
836 returns to a point that is set up to handle the error
837 (@pxref{Processing of Errors}). Here we describe how to signal an
840 Most errors are signaled ``automatically'' within Lisp primitives
841 which you call for other purposes, such as if you try to take the
842 @sc{car} of an integer or move forward a character at the end of the
843 buffer. You can also signal errors explicitly with the functions
844 @code{error} and @code{signal}.
846 Quitting, which happens when the user types @kbd{C-g}, is not
847 considered an error, but it is handled almost like an error.
850 Every error specifies an error message, one way or another. The
851 message should state what is wrong (``File does not exist''), not how
852 things ought to be (``File must exist''). The convention in Emacs
853 Lisp is that error messages should start with a capital letter, but
854 should not end with any sort of punctuation.
856 @defun error format-string &rest args
857 This function signals an error with an error message constructed by
858 applying @code{format} (@pxref{Formatting Strings}) to
859 @var{format-string} and @var{args}.
861 These examples show typical uses of @code{error}:
865 (error "That is an error -- try something else")
866 @error{} That is an error -- try something else
870 (error "You have committed %d errors" 10)
871 @error{} You have committed 10 errors
875 @code{error} works by calling @code{signal} with two arguments: the
876 error symbol @code{error}, and a list containing the string returned by
879 @strong{Warning:} If you want to use your own string as an error message
880 verbatim, don't just write @code{(error @var{string})}. If @var{string}
881 contains @samp{%}, it will be interpreted as a format specifier, with
882 undesirable results. Instead, use @code{(error "%s" @var{string})}.
885 @defun signal error-symbol data
886 @anchor{Definition of signal}
887 This function signals an error named by @var{error-symbol}. The
888 argument @var{data} is a list of additional Lisp objects relevant to
889 the circumstances of the error.
891 The argument @var{error-symbol} must be an @dfn{error symbol}---a symbol
892 defined with @code{define-error}. This is how Emacs Lisp classifies different
893 sorts of errors. @xref{Error Symbols}, for a description of error symbols,
894 error conditions and condition names.
896 If the error is not handled, the two arguments are used in printing
897 the error message. Normally, this error message is provided by the
898 @code{error-message} property of @var{error-symbol}. If @var{data} is
899 non-@code{nil}, this is followed by a colon and a comma separated list
900 of the unevaluated elements of @var{data}. For @code{error}, the
901 error message is the @sc{car} of @var{data} (that must be a string).
902 Subcategories of @code{file-error} are handled specially.
904 The number and significance of the objects in @var{data} depends on
905 @var{error-symbol}. For example, with a @code{wrong-type-argument} error,
906 there should be two objects in the list: a predicate that describes the type
907 that was expected, and the object that failed to fit that type.
909 Both @var{error-symbol} and @var{data} are available to any error
910 handlers that handle the error: @code{condition-case} binds a local
911 variable to a list of the form @code{(@var{error-symbol} .@:
912 @var{data})} (@pxref{Handling Errors}).
914 The function @code{signal} never returns.
915 @c (though in older Emacs versions it sometimes could).
919 (signal 'wrong-number-of-arguments '(x y))
920 @error{} Wrong number of arguments: x, y
924 (signal 'no-such-error '("My unknown error condition"))
925 @error{} peculiar error: "My unknown error condition"
930 @cindex user errors, signaling
931 @defun user-error format-string &rest args
932 This function behaves exactly like @code{error}, except that it uses
933 the error symbol @code{user-error} rather than @code{error}. As the
934 name suggests, this is intended to report errors on the part of the
935 user, rather than errors in the code itself. For example,
936 if you try to use the command @code{Info-history-back} (@kbd{l}) to
937 move back beyond the start of your Info browsing history, Emacs
938 signals a @code{user-error}. Such errors do not cause entry to the
939 debugger, even when @code{debug-on-error} is non-@code{nil}.
940 @xref{Error Debugging}.
943 @cindex CL note---no continuable errors
945 @b{Common Lisp note:} Emacs Lisp has nothing like the Common Lisp
946 concept of continuable errors.
949 @node Processing of Errors
950 @subsubsection How Emacs Processes Errors
952 When an error is signaled, @code{signal} searches for an active
953 @dfn{handler} for the error. A handler is a sequence of Lisp
954 expressions designated to be executed if an error happens in part of the
955 Lisp program. If the error has an applicable handler, the handler is
956 executed, and control resumes following the handler. The handler
957 executes in the environment of the @code{condition-case} that
958 established it; all functions called within that @code{condition-case}
959 have already been exited, and the handler cannot return to them.
961 If there is no applicable handler for the error, it terminates the
962 current command and returns control to the editor command loop. (The
963 command loop has an implicit handler for all kinds of errors.) The
964 command loop's handler uses the error symbol and associated data to
965 print an error message. You can use the variable
966 @code{command-error-function} to control how this is done:
968 @defvar command-error-function
969 This variable, if non-@code{nil}, specifies a function to use to
970 handle errors that return control to the Emacs command loop. The
971 function should take three arguments: @var{data}, a list of the same
972 form that @code{condition-case} would bind to its variable;
973 @var{context}, a string describing the situation in which the error
974 occurred, or (more often) @code{nil}; and @var{caller}, the Lisp
975 function which called the primitive that signaled the error.
978 @cindex @code{debug-on-error} use
979 An error that has no explicit handler may call the Lisp debugger. The
980 debugger is enabled if the variable @code{debug-on-error} (@pxref{Error
981 Debugging}) is non-@code{nil}. Unlike error handlers, the debugger runs
982 in the environment of the error, so that you can examine values of
983 variables precisely as they were at the time of the error.
985 @node Handling Errors
986 @subsubsection Writing Code to Handle Errors
987 @cindex error handler
988 @cindex handling errors
990 The usual effect of signaling an error is to terminate the command
991 that is running and return immediately to the Emacs editor command loop.
992 You can arrange to trap errors occurring in a part of your program by
993 establishing an error handler, with the special form
994 @code{condition-case}. A simple example looks like this:
999 (delete-file filename)
1005 This deletes the file named @var{filename}, catching any error and
1006 returning @code{nil} if an error occurs. (You can use the macro
1007 @code{ignore-errors} for a simple case like this; see below.)
1009 The @code{condition-case} construct is often used to trap errors that
1010 are predictable, such as failure to open a file in a call to
1011 @code{insert-file-contents}. It is also used to trap errors that are
1012 totally unpredictable, such as when the program evaluates an expression
1015 The second argument of @code{condition-case} is called the
1016 @dfn{protected form}. (In the example above, the protected form is a
1017 call to @code{delete-file}.) The error handlers go into effect when
1018 this form begins execution and are deactivated when this form returns.
1019 They remain in effect for all the intervening time. In particular, they
1020 are in effect during the execution of functions called by this form, in
1021 their subroutines, and so on. This is a good thing, since, strictly
1022 speaking, errors can be signaled only by Lisp primitives (including
1023 @code{signal} and @code{error}) called by the protected form, not by the
1024 protected form itself.
1026 The arguments after the protected form are handlers. Each handler
1027 lists one or more @dfn{condition names} (which are symbols) to specify
1028 which errors it will handle. The error symbol specified when an error
1029 is signaled also defines a list of condition names. A handler applies
1030 to an error if they have any condition names in common. In the example
1031 above, there is one handler, and it specifies one condition name,
1032 @code{error}, which covers all errors.
1034 The search for an applicable handler checks all the established handlers
1035 starting with the most recently established one. Thus, if two nested
1036 @code{condition-case} forms offer to handle the same error, the inner of
1037 the two gets to handle it.
1039 If an error is handled by some @code{condition-case} form, this
1040 ordinarily prevents the debugger from being run, even if
1041 @code{debug-on-error} says this error should invoke the debugger.
1043 If you want to be able to debug errors that are caught by a
1044 @code{condition-case}, set the variable @code{debug-on-signal} to a
1045 non-@code{nil} value. You can also specify that a particular handler
1046 should let the debugger run first, by writing @code{debug} among the
1047 conditions, like this:
1052 (delete-file filename)
1053 ((debug error) nil))
1058 The effect of @code{debug} here is only to prevent
1059 @code{condition-case} from suppressing the call to the debugger. Any
1060 given error will invoke the debugger only if @code{debug-on-error} and
1061 the other usual filtering mechanisms say it should. @xref{Error Debugging}.
1063 @defmac condition-case-unless-debug var protected-form handlers@dots{}
1064 The macro @code{condition-case-unless-debug} provides another way to
1065 handle debugging of such forms. It behaves exactly like
1066 @code{condition-case}, unless the variable @code{debug-on-error} is
1067 non-@code{nil}, in which case it does not handle any errors at all.
1070 Once Emacs decides that a certain handler handles the error, it
1071 returns control to that handler. To do so, Emacs unbinds all variable
1072 bindings made by binding constructs that are being exited, and
1073 executes the cleanups of all @code{unwind-protect} forms that are
1074 being exited. Once control arrives at the handler, the body of the
1075 handler executes normally.
1077 After execution of the handler body, execution returns from the
1078 @code{condition-case} form. Because the protected form is exited
1079 completely before execution of the handler, the handler cannot resume
1080 execution at the point of the error, nor can it examine variable
1081 bindings that were made within the protected form. All it can do is
1082 clean up and proceed.
1084 Error signaling and handling have some resemblance to @code{throw} and
1085 @code{catch} (@pxref{Catch and Throw}), but they are entirely separate
1086 facilities. An error cannot be caught by a @code{catch}, and a
1087 @code{throw} cannot be handled by an error handler (though using
1088 @code{throw} when there is no suitable @code{catch} signals an error
1089 that can be handled).
1091 @defspec condition-case var protected-form handlers@dots{}
1092 This special form establishes the error handlers @var{handlers} around
1093 the execution of @var{protected-form}. If @var{protected-form} executes
1094 without error, the value it returns becomes the value of the
1095 @code{condition-case} form; in this case, the @code{condition-case} has
1096 no effect. The @code{condition-case} form makes a difference when an
1097 error occurs during @var{protected-form}.
1099 Each of the @var{handlers} is a list of the form @code{(@var{conditions}
1100 @var{body}@dots{})}. Here @var{conditions} is an error condition name
1101 to be handled, or a list of condition names (which can include @code{debug}
1102 to allow the debugger to run before the handler); @var{body} is one or more
1103 Lisp expressions to be executed when this handler handles an error.
1104 Here are examples of handlers:
1110 (arith-error (message "Division by zero"))
1112 ((arith-error file-error)
1114 "Either division by zero or failure to open a file"))
1118 Each error that occurs has an @dfn{error symbol} that describes what
1119 kind of error it is, and which describes also a list of condition names
1120 (@pxref{Error Symbols}). Emacs
1121 searches all the active @code{condition-case} forms for a handler that
1122 specifies one or more of these condition names; the innermost matching
1123 @code{condition-case} handles the error. Within this
1124 @code{condition-case}, the first applicable handler handles the error.
1126 After executing the body of the handler, the @code{condition-case}
1127 returns normally, using the value of the last form in the handler body
1128 as the overall value.
1130 @cindex error description
1131 The argument @var{var} is a variable. @code{condition-case} does not
1132 bind this variable when executing the @var{protected-form}, only when it
1133 handles an error. At that time, it binds @var{var} locally to an
1134 @dfn{error description}, which is a list giving the particulars of the
1135 error. The error description has the form @code{(@var{error-symbol}
1136 . @var{data})}. The handler can refer to this list to decide what to
1137 do. For example, if the error is for failure opening a file, the file
1138 name is the second element of @var{data}---the third element of the
1141 If @var{var} is @code{nil}, that means no variable is bound. Then the
1142 error symbol and associated data are not available to the handler.
1144 @cindex rethrow a signal
1145 Sometimes it is necessary to re-throw a signal caught by
1146 @code{condition-case}, for some outer-level handler to catch. Here's
1150 (signal (car err) (cdr err))
1154 where @code{err} is the error description variable, the first argument
1155 to @code{condition-case} whose error condition you want to re-throw.
1156 @xref{Definition of signal}.
1159 @defun error-message-string error-descriptor
1160 This function returns the error message string for a given error
1161 descriptor. It is useful if you want to handle an error by printing the
1162 usual error message for that error. @xref{Definition of signal}.
1165 @cindex @code{arith-error} example
1166 Here is an example of using @code{condition-case} to handle the error
1167 that results from dividing by zero. The handler displays the error
1168 message (but without a beep), then returns a very large number.
1172 (defun safe-divide (dividend divisor)
1174 ;; @r{Protected form.}
1175 (/ dividend divisor)
1179 (arith-error ; @r{Condition.}
1180 ;; @r{Display the usual message for this error.}
1181 (message "%s" (error-message-string err))
1183 @result{} safe-divide
1188 @print{} Arithmetic error: (arith-error)
1194 The handler specifies condition name @code{arith-error} so that it
1195 will handle only division-by-zero errors. Other kinds of errors will
1196 not be handled (by this @code{condition-case}). Thus:
1201 @error{} Wrong type argument: number-or-marker-p, nil
1205 Here is a @code{condition-case} that catches all kinds of errors,
1206 including those from @code{error}:
1218 ;; @r{This is a call to the function @code{error}.}
1219 (error "Rats! The variable %s was %s, not 35" 'baz baz))
1220 ;; @r{This is the handler; it is not a form.}
1221 (error (princ (format "The error was: %s" err))
1223 @print{} The error was: (error "Rats! The variable baz was 34, not 35")
1228 @defmac ignore-errors body@dots{}
1229 This construct executes @var{body}, ignoring any errors that occur
1230 during its execution. If the execution is without error,
1231 @code{ignore-errors} returns the value of the last form in @var{body};
1232 otherwise, it returns @code{nil}.
1234 Here's the example at the beginning of this subsection rewritten using
1235 @code{ignore-errors}:
1240 (delete-file filename))
1245 @defmac with-demoted-errors body@dots{}
1246 This macro is like a milder version of @code{ignore-errors}. Rather
1247 than suppressing errors altogether, it converts them into messages.
1248 Use this form around code that is not expected to signal errors, but
1249 should be robust if one does occur. Note that this macro uses
1250 @code{condition-case-unless-debug} rather than @code{condition-case}.
1254 @subsubsection Error Symbols and Condition Names
1255 @cindex error symbol
1257 @cindex condition name
1258 @cindex user-defined error
1259 @kindex error-conditions
1260 @kindex define-error
1262 When you signal an error, you specify an @dfn{error symbol} to specify
1263 the kind of error you have in mind. Each error has one and only one
1264 error symbol to categorize it. This is the finest classification of
1265 errors defined by the Emacs Lisp language.
1267 These narrow classifications are grouped into a hierarchy of wider
1268 classes called @dfn{error conditions}, identified by @dfn{condition
1269 names}. The narrowest such classes belong to the error symbols
1270 themselves: each error symbol is also a condition name. There are also
1271 condition names for more extensive classes, up to the condition name
1272 @code{error} which takes in all kinds of errors (but not @code{quit}).
1273 Thus, each error has one or more condition names: @code{error}, the
1274 error symbol if that is distinct from @code{error}, and perhaps some
1275 intermediate classifications.
1277 @defun define-error name message &optional parent
1278 In order for a symbol to be an error symbol, it must be defined with
1279 @code{define-error} which takes a parent condition (defaults to @code{error}).
1280 This parent defines the conditions that this kind of error belongs to.
1281 The transitive set of parents always includes the error symbol itself, and the
1282 symbol @code{error}. Because quitting is not considered an error, the set of
1283 parents of @code{quit} is just @code{(quit)}.
1286 @cindex peculiar error
1287 In addition to its parents, the error symbol has a @var{message} which
1288 is a string to be printed when that error is signaled but not handled. If that
1289 message is not valid, the error message @samp{peculiar error} is used.
1290 @xref{Definition of signal}.
1292 Internally, the set of parents is stored in the @code{error-conditions}
1293 property of the error symbol and the message is stored in the
1294 @code{error-message} property of the error symbol.
1296 Here is how we define a new error symbol, @code{new-error}:
1300 (define-error 'new-error "A new error" 'my-own-errors)
1305 This error has several condition names: @code{new-error}, the narrowest
1306 classification; @code{my-own-errors}, which we imagine is a wider
1307 classification; and all the conditions of @code{my-own-errors} which should
1308 include @code{error}, which is the widest of all.
1310 The error string should start with a capital letter but it should
1311 not end with a period. This is for consistency with the rest of Emacs.
1313 Naturally, Emacs will never signal @code{new-error} on its own; only
1314 an explicit call to @code{signal} (@pxref{Definition of signal}) in
1315 your code can do this:
1319 (signal 'new-error '(x y))
1320 @error{} A new error: x, y
1324 This error can be handled through any of its condition names.
1325 This example handles @code{new-error} and any other errors in the class
1326 @code{my-own-errors}:
1332 (my-own-errors nil))
1336 The significant way that errors are classified is by their condition
1337 names---the names used to match errors with handlers. An error symbol
1338 serves only as a convenient way to specify the intended error message
1339 and list of condition names. It would be cumbersome to give
1340 @code{signal} a list of condition names rather than one error symbol.
1342 By contrast, using only error symbols without condition names would
1343 seriously decrease the power of @code{condition-case}. Condition names
1344 make it possible to categorize errors at various levels of generality
1345 when you write an error handler. Using error symbols alone would
1346 eliminate all but the narrowest level of classification.
1348 @xref{Standard Errors}, for a list of the main error symbols
1349 and their conditions.
1352 @subsection Cleaning Up from Nonlocal Exits
1354 The @code{unwind-protect} construct is essential whenever you
1355 temporarily put a data structure in an inconsistent state; it permits
1356 you to make the data consistent again in the event of an error or
1357 throw. (Another more specific cleanup construct that is used only for
1358 changes in buffer contents is the atomic change group; @ref{Atomic
1361 @defspec unwind-protect body-form cleanup-forms@dots{}
1362 @cindex cleanup forms
1363 @cindex protected forms
1364 @cindex error cleanup
1366 @code{unwind-protect} executes @var{body-form} with a guarantee that
1367 the @var{cleanup-forms} will be evaluated if control leaves
1368 @var{body-form}, no matter how that happens. @var{body-form} may
1369 complete normally, or execute a @code{throw} out of the
1370 @code{unwind-protect}, or cause an error; in all cases, the
1371 @var{cleanup-forms} will be evaluated.
1373 If @var{body-form} finishes normally, @code{unwind-protect} returns the
1374 value of @var{body-form}, after it evaluates the @var{cleanup-forms}.
1375 If @var{body-form} does not finish, @code{unwind-protect} does not
1376 return any value in the normal sense.
1378 Only @var{body-form} is protected by the @code{unwind-protect}. If any
1379 of the @var{cleanup-forms} themselves exits nonlocally (via a
1380 @code{throw} or an error), @code{unwind-protect} is @emph{not}
1381 guaranteed to evaluate the rest of them. If the failure of one of the
1382 @var{cleanup-forms} has the potential to cause trouble, then protect
1383 it with another @code{unwind-protect} around that form.
1385 The number of currently active @code{unwind-protect} forms counts,
1386 together with the number of local variable bindings, against the limit
1387 @code{max-specpdl-size} (@pxref{Definition of max-specpdl-size,, Local
1391 For example, here we make an invisible buffer for temporary use, and
1392 make sure to kill it before finishing:
1396 (let ((buffer (get-buffer-create " *temp*")))
1397 (with-current-buffer buffer
1400 (kill-buffer buffer))))
1405 You might think that we could just as well write @code{(kill-buffer
1406 (current-buffer))} and dispense with the variable @code{buffer}.
1407 However, the way shown above is safer, if @var{body-form} happens to
1408 get an error after switching to a different buffer! (Alternatively,
1409 you could write a @code{save-current-buffer} around @var{body-form},
1410 to ensure that the temporary buffer becomes current again in time to
1413 Emacs includes a standard macro called @code{with-temp-buffer} which
1414 expands into more or less the code shown above (@pxref{Definition of
1415 with-temp-buffer,, Current Buffer}). Several of the macros defined in
1416 this manual use @code{unwind-protect} in this way.
1419 Here is an actual example derived from an FTP package. It creates a
1420 process (@pxref{Processes}) to try to establish a connection to a remote
1421 machine. As the function @code{ftp-login} is highly susceptible to
1422 numerous problems that the writer of the function cannot anticipate, it
1423 is protected with a form that guarantees deletion of the process in the
1424 event of failure. Otherwise, Emacs might fill up with useless
1432 (setq process (ftp-setup-buffer host file))
1433 (if (setq win (ftp-login process host user password))
1434 (message "Logged in")
1435 (error "Ftp login failed")))
1436 (or win (and process (delete-process process)))))
1440 This example has a small bug: if the user types @kbd{C-g} to
1441 quit, and the quit happens immediately after the function
1442 @code{ftp-setup-buffer} returns but before the variable @code{process} is
1443 set, the process will not be killed. There is no easy way to fix this bug,
1444 but at least it is very unlikely.