2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-1999, 2001-2015 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 @cindex sequential execution
50 Evaluating forms in the order they appear is the most common way
51 control passes from one form to another. In some contexts, such as in a
52 function body, this happens automatically. Elsewhere you must use a
53 control structure construct to do this: @code{progn}, the simplest
54 control construct of Lisp.
56 A @code{progn} special form looks like this:
60 (progn @var{a} @var{b} @var{c} @dots{})
65 and it says to execute the forms @var{a}, @var{b}, @var{c}, and so on, in
66 that order. These forms are called the @dfn{body} of the @code{progn} form.
67 The value of the last form in the body becomes the value of the entire
68 @code{progn}. @code{(progn)} returns @code{nil}.
70 @cindex implicit @code{progn}
71 In the early days of Lisp, @code{progn} was the only way to execute
72 two or more forms in succession and use the value of the last of them.
73 But programmers found they often needed to use a @code{progn} in the
74 body of a function, where (at that time) only one form was allowed. So
75 the body of a function was made into an ``implicit @code{progn}'':
76 several forms are allowed just as in the body of an actual @code{progn}.
77 Many other control structures likewise contain an implicit @code{progn}.
78 As a result, @code{progn} is not used as much as it was many years ago.
79 It is needed now most often inside an @code{unwind-protect}, @code{and},
80 @code{or}, or in the @var{then}-part of an @code{if}.
82 @defspec progn forms@dots{}
83 This special form evaluates all of the @var{forms}, in textual
84 order, returning the result of the final form.
88 (progn (print "The first form")
89 (print "The second form")
90 (print "The third form"))
91 @print{} "The first form"
92 @print{} "The second form"
93 @print{} "The third form"
94 @result{} "The third form"
99 Two other constructs likewise evaluate a series of forms but return
102 @defspec prog1 form1 forms@dots{}
103 This special form evaluates @var{form1} and all of the @var{forms}, in
104 textual order, returning the result of @var{form1}.
108 (prog1 (print "The first form")
109 (print "The second form")
110 (print "The third form"))
111 @print{} "The first form"
112 @print{} "The second form"
113 @print{} "The third form"
114 @result{} "The first form"
118 Here is a way to remove the first element from a list in the variable
119 @code{x}, then return the value of that former element:
122 (prog1 (car x) (setq x (cdr x)))
126 @defspec prog2 form1 form2 forms@dots{}
127 This special form evaluates @var{form1}, @var{form2}, and all of the
128 following @var{forms}, in textual order, returning the result of
133 (prog2 (print "The first form")
134 (print "The second form")
135 (print "The third form"))
136 @print{} "The first form"
137 @print{} "The second form"
138 @print{} "The third form"
139 @result{} "The second form"
145 @section Conditionals
146 @cindex conditional evaluation
148 Conditional control structures choose among alternatives. Emacs Lisp
149 has four conditional forms: @code{if}, which is much the same as in
150 other languages; @code{when} and @code{unless}, which are variants of
151 @code{if}; and @code{cond}, which is a generalized case statement.
153 @defspec if condition then-form else-forms@dots{}
154 @code{if} chooses between the @var{then-form} and the @var{else-forms}
155 based on the value of @var{condition}. If the evaluated @var{condition} is
156 non-@code{nil}, @var{then-form} is evaluated and the result returned.
157 Otherwise, the @var{else-forms} are evaluated in textual order, and the
158 value of the last one is returned. (The @var{else} part of @code{if} is
159 an example of an implicit @code{progn}. @xref{Sequencing}.)
161 If @var{condition} has the value @code{nil}, and no @var{else-forms} are
162 given, @code{if} returns @code{nil}.
164 @code{if} is a special form because the branch that is not selected is
165 never evaluated---it is ignored. Thus, in this example,
166 @code{true} is not printed because @code{print} is never called:
178 @defmac when condition then-forms@dots{}
179 This is a variant of @code{if} where there are no @var{else-forms},
180 and possibly several @var{then-forms}. In particular,
183 (when @var{condition} @var{a} @var{b} @var{c})
187 is entirely equivalent to
190 (if @var{condition} (progn @var{a} @var{b} @var{c}) nil)
194 @defmac unless condition forms@dots{}
195 This is a variant of @code{if} where there is no @var{then-form}:
198 (unless @var{condition} @var{a} @var{b} @var{c})
202 is entirely equivalent to
205 (if @var{condition} nil
206 @var{a} @var{b} @var{c})
210 @defspec cond clause@dots{}
211 @code{cond} chooses among an arbitrary number of alternatives. Each
212 @var{clause} in the @code{cond} must be a list. The @sc{car} of this
213 list is the @var{condition}; the remaining elements, if any, the
214 @var{body-forms}. Thus, a clause looks like this:
217 (@var{condition} @var{body-forms}@dots{})
220 @code{cond} tries the clauses in textual order, by evaluating the
221 @var{condition} of each clause. If the value of @var{condition} is
222 non-@code{nil}, the clause ``succeeds''; then @code{cond} evaluates its
223 @var{body-forms}, and returns the value of the last of @var{body-forms}.
224 Any remaining clauses are ignored.
226 If the value of @var{condition} is @code{nil}, the clause ``fails'', so
227 the @code{cond} moves on to the following clause, trying its @var{condition}.
229 A clause may also look like this:
236 Then, if @var{condition} is non-@code{nil} when tested, the @code{cond}
237 form returns the value of @var{condition}.
239 If every @var{condition} evaluates to @code{nil}, so that every clause
240 fails, @code{cond} returns @code{nil}.
242 The following example has four clauses, which test for the cases where
243 the value of @code{x} is a number, string, buffer and symbol,
248 (cond ((numberp x) x)
251 (setq temporary-hack x) ; @r{multiple body-forms}
252 (buffer-name x)) ; @r{in one clause}
253 ((symbolp x) (symbol-value x)))
257 Often we want to execute the last clause whenever none of the previous
258 clauses was successful. To do this, we use @code{t} as the
259 @var{condition} of the last clause, like this: @code{(t
260 @var{body-forms})}. The form @code{t} evaluates to @code{t}, which is
261 never @code{nil}, so this clause never fails, provided the @code{cond}
262 gets to it at all. For example:
267 (cond ((eq a 'hack) 'foo)
274 This @code{cond} expression returns @code{foo} if the value of @code{a}
275 is @code{hack}, and returns the string @code{"default"} otherwise.
278 Any conditional construct can be expressed with @code{cond} or with
279 @code{if}. Therefore, the choice between them is a matter of style.
284 (if @var{a} @var{b} @var{c})
286 (cond (@var{a} @var{b}) (t @var{c}))
291 * Pattern matching case statement::
294 @node Pattern matching case statement
295 @subsection Pattern matching case statement
297 @cindex pattern matching
299 To compare a particular value against various possible cases, the macro
300 @code{pcase} can come handy. It takes the following form:
303 (pcase @var{exp} @var{branch}1 @var{branch}2 @var{branch}3 @dots{})
306 where each @var{branch} takes the form @code{(@var{upattern}
307 @var{body-forms}@dots{})}.
309 It will first evaluate @var{exp} and then compare the value against each
310 @var{upattern} to see which @var{branch} to use, after which it will run the
311 corresponding @var{body-forms}. A common use case is to distinguish
312 between a few different constant values:
315 (pcase (get-return-code x)
316 (`success (message "Done!"))
317 (`would-block (message "Sorry, can't do it now"))
318 (`read-only (message "The shmliblick is read-only"))
319 (`access-denied (message "You do not have the needed rights"))
320 (code (message "Unknown return code %S" code)))
323 In the last clause, @code{code} is a variable that gets bound to the value that
324 was returned by @code{(get-return-code x)}.
326 To give a more complex example, a simple interpreter for a little
327 expression language could look like (note that this example requires
331 (defun evaluate (exp env)
333 (`(add ,x ,y) (+ (evaluate x env) (evaluate y env)))
334 (`(call ,fun ,arg) (funcall (evaluate fun env) (evaluate arg env)))
335 (`(fn ,arg ,body) (lambda (val)
336 (evaluate body (cons (cons arg val) env))))
338 ((pred symbolp) (cdr (assq exp env)))
339 (_ (error "Unknown expression %S" exp))))
342 Where @code{`(add ,x ,y)} is a pattern that checks that @code{exp} is a three
343 element list starting with the symbol @code{add}, then extracts the second and
344 third elements and binds them to the variables @code{x} and @code{y}.
345 @code{(pred numberp)} is a pattern that simply checks that @code{exp}
346 is a number, and @code{_} is the catch-all pattern that matches anything.
348 Here are some sample programs including their evaluation results:
351 (evaluate '(add 1 2) nil) ;=> 3
352 (evaluate '(add x y) '((x . 1) (y . 2))) ;=> 3
353 (evaluate '(call (fn x (add 1 x)) 2) nil) ;=> 3
354 (evaluate '(sub 1 2) nil) ;=> error
357 There are two kinds of patterns involved in @code{pcase}, called
358 @emph{U-patterns} and @emph{Q-patterns}. The @var{upattern} mentioned above
359 are U-patterns and can take the following forms:
362 @item `@var{qpattern}
363 This is one of the most common form of patterns. The intention is to mimic the
364 backquote macro: this pattern matches those values that could have been built
365 by such a backquote expression. Since we're pattern matching rather than
366 building a value, the unquote does not indicate where to plug an expression,
367 but instead it lets one specify a U-pattern that should match the value at
370 More specifically, a Q-pattern can take the following forms:
372 @item (@var{qpattern1} . @var{qpattern2})
373 This pattern matches any cons cell whose @code{car} matches @var{qpattern1} and
374 whose @code{cdr} matches @var{pattern2}.
376 This pattern matches any atom @code{equal} to @var{atom}.
377 @item ,@var{upattern}
378 This pattern matches any object that matches the @var{upattern}.
382 A mere symbol in a U-pattern matches anything, and additionally let-binds this
383 symbol to the value that it matched, so that you can later refer to it, either
384 in the @var{body-forms} or also later in the pattern.
386 This so-called @emph{don't care} pattern matches anything, like the previous
387 one, but unlike symbol patterns it does not bind any variable.
388 @item (pred @var{pred})
389 This pattern matches if the function @var{pred} returns non-@code{nil} when
390 called with the object being matched.
391 @item (or @var{upattern1} @var{upattern2}@dots{})
392 This pattern matches as soon as one of the argument patterns succeeds.
393 All argument patterns should let-bind the same variables.
394 @item (and @var{upattern1} @var{upattern2}@dots{})
395 This pattern matches only if all the argument patterns succeed.
396 @item (guard @var{exp})
397 This pattern ignores the object being examined and simply succeeds if @var{exp}
398 evaluates to non-@code{nil} and fails otherwise. It is typically used inside
399 an @code{and} pattern. For example, @code{(and x (guard (< x 10)))}
400 is a pattern which matches any number smaller than 10 and let-binds it to
401 the variable @code{x}.
404 @node Combining Conditions
405 @section Constructs for Combining Conditions
406 @cindex combining conditions
408 This section describes three constructs that are often used together
409 with @code{if} and @code{cond} to express complicated conditions. The
410 constructs @code{and} and @code{or} can also be used individually as
411 kinds of multiple conditional constructs.
414 This function tests for the falsehood of @var{condition}. It returns
415 @code{t} if @var{condition} is @code{nil}, and @code{nil} otherwise.
416 The function @code{not} is identical to @code{null}, and we recommend
417 using the name @code{null} if you are testing for an empty list.
420 @defspec and conditions@dots{}
421 The @code{and} special form tests whether all the @var{conditions} are
422 true. It works by evaluating the @var{conditions} one by one in the
425 If any of the @var{conditions} evaluates to @code{nil}, then the result
426 of the @code{and} must be @code{nil} regardless of the remaining
427 @var{conditions}; so @code{and} returns @code{nil} right away, ignoring
428 the remaining @var{conditions}.
430 If all the @var{conditions} turn out non-@code{nil}, then the value of
431 the last of them becomes the value of the @code{and} form. Just
432 @code{(and)}, with no @var{conditions}, returns @code{t}, appropriate
433 because all the @var{conditions} turned out non-@code{nil}. (Think
434 about it; which one did not?)
436 Here is an example. The first condition returns the integer 1, which is
437 not @code{nil}. Similarly, the second condition returns the integer 2,
438 which is not @code{nil}. The third condition is @code{nil}, so the
439 remaining condition is never evaluated.
443 (and (print 1) (print 2) nil (print 3))
450 Here is a more realistic example of using @code{and}:
454 (if (and (consp foo) (eq (car foo) 'x))
455 (message "foo is a list starting with x"))
460 Note that @code{(car foo)} is not executed if @code{(consp foo)} returns
461 @code{nil}, thus avoiding an error.
463 @code{and} expressions can also be written using either @code{if} or
464 @code{cond}. Here's how:
468 (and @var{arg1} @var{arg2} @var{arg3})
470 (if @var{arg1} (if @var{arg2} @var{arg3}))
472 (cond (@var{arg1} (cond (@var{arg2} @var{arg3}))))
477 @defspec or conditions@dots{}
478 The @code{or} special form tests whether at least one of the
479 @var{conditions} is true. It works by evaluating all the
480 @var{conditions} one by one in the order written.
482 If any of the @var{conditions} evaluates to a non-@code{nil} value, then
483 the result of the @code{or} must be non-@code{nil}; so @code{or} returns
484 right away, ignoring the remaining @var{conditions}. The value it
485 returns is the non-@code{nil} value of the condition just evaluated.
487 If all the @var{conditions} turn out @code{nil}, then the @code{or}
488 expression returns @code{nil}. Just @code{(or)}, with no
489 @var{conditions}, returns @code{nil}, appropriate because all the
490 @var{conditions} turned out @code{nil}. (Think about it; which one
493 For example, this expression tests whether @code{x} is either
494 @code{nil} or the integer zero:
497 (or (eq x nil) (eq x 0))
500 Like the @code{and} construct, @code{or} can be written in terms of
501 @code{cond}. For example:
505 (or @var{arg1} @var{arg2} @var{arg3})
513 You could almost write @code{or} in terms of @code{if}, but not quite:
517 (if @var{arg1} @var{arg1}
518 (if @var{arg2} @var{arg2}
524 This is not completely equivalent because it can evaluate @var{arg1} or
525 @var{arg2} twice. By contrast, @code{(or @var{arg1} @var{arg2}
526 @var{arg3})} never evaluates any argument more than once.
534 Iteration means executing part of a program repetitively. For
535 example, you might want to repeat some computation once for each element
536 of a list, or once for each integer from 0 to @var{n}. You can do this
537 in Emacs Lisp with the special form @code{while}:
539 @defspec while condition forms@dots{}
540 @code{while} first evaluates @var{condition}. If the result is
541 non-@code{nil}, it evaluates @var{forms} in textual order. Then it
542 reevaluates @var{condition}, and if the result is non-@code{nil}, it
543 evaluates @var{forms} again. This process repeats until @var{condition}
544 evaluates to @code{nil}.
546 There is no limit on the number of iterations that may occur. The loop
547 will continue until either @var{condition} evaluates to @code{nil} or
548 until an error or @code{throw} jumps out of it (@pxref{Nonlocal Exits}).
550 The value of a @code{while} form is always @code{nil}.
559 (princ (format "Iteration %d." num))
561 @print{} Iteration 0.
562 @print{} Iteration 1.
563 @print{} Iteration 2.
564 @print{} Iteration 3.
569 To write a ``repeat...until'' loop, which will execute something on each
570 iteration and then do the end-test, put the body followed by the
571 end-test in a @code{progn} as the first argument of @code{while}, as
578 (not (looking-at "^$"))))
583 This moves forward one line and continues moving by lines until it
584 reaches an empty line. It is peculiar in that the @code{while} has no
585 body, just the end test (which also does the real work of moving point).
588 The @code{dolist} and @code{dotimes} macros provide convenient ways to
589 write two common kinds of loops.
591 @defmac dolist (var list [result]) body@dots{}
592 This construct executes @var{body} once for each element of
593 @var{list}, binding the variable @var{var} locally to hold the current
594 element. Then it returns the value of evaluating @var{result}, or
595 @code{nil} if @var{result} is omitted. For example, here is how you
596 could use @code{dolist} to define the @code{reverse} function:
599 (defun reverse (list)
601 (dolist (elt list value)
602 (setq value (cons elt value)))))
606 @defmac dotimes (var count [result]) body@dots{}
607 This construct executes @var{body} once for each integer from 0
608 (inclusive) to @var{count} (exclusive), binding the variable @var{var}
609 to the integer for the current iteration. Then it returns the value
610 of evaluating @var{result}, or @code{nil} if @var{result} is omitted.
611 Here is an example of using @code{dotimes} to do something 100 times:
615 (insert "I will not obey absurd orders\n"))
620 @section Nonlocal Exits
621 @cindex nonlocal exits
623 A @dfn{nonlocal exit} is a transfer of control from one point in a
624 program to another remote point. Nonlocal exits can occur in Emacs Lisp
625 as a result of errors; you can also use them under explicit control.
626 Nonlocal exits unbind all variable bindings made by the constructs being
630 * Catch and Throw:: Nonlocal exits for the program's own purposes.
631 * Examples of Catch:: Showing how such nonlocal exits can be written.
632 * Errors:: How errors are signaled and handled.
633 * Cleanups:: Arranging to run a cleanup form if an error happens.
636 @node Catch and Throw
637 @subsection Explicit Nonlocal Exits: @code{catch} and @code{throw}
639 Most control constructs affect only the flow of control within the
640 construct itself. The function @code{throw} is the exception to this
641 rule of normal program execution: it performs a nonlocal exit on
642 request. (There are other exceptions, but they are for error handling
643 only.) @code{throw} is used inside a @code{catch}, and jumps back to
644 that @code{catch}. For example:
661 The @code{throw} form, if executed, transfers control straight back to
662 the corresponding @code{catch}, which returns immediately. The code
663 following the @code{throw} is not executed. The second argument of
664 @code{throw} is used as the return value of the @code{catch}.
666 The function @code{throw} finds the matching @code{catch} based on the
667 first argument: it searches for a @code{catch} whose first argument is
668 @code{eq} to the one specified in the @code{throw}. If there is more
669 than one applicable @code{catch}, the innermost one takes precedence.
670 Thus, in the above example, the @code{throw} specifies @code{foo}, and
671 the @code{catch} in @code{foo-outer} specifies the same symbol, so that
672 @code{catch} is the applicable one (assuming there is no other matching
673 @code{catch} in between).
675 Executing @code{throw} exits all Lisp constructs up to the matching
676 @code{catch}, including function calls. When binding constructs such
677 as @code{let} or function calls are exited in this way, the bindings
678 are unbound, just as they are when these constructs exit normally
679 (@pxref{Local Variables}). Likewise, @code{throw} restores the buffer
680 and position saved by @code{save-excursion} (@pxref{Excursions}), and
681 the narrowing status saved by @code{save-restriction}. It also runs
682 any cleanups established with the @code{unwind-protect} special form
683 when it exits that form (@pxref{Cleanups}).
685 The @code{throw} need not appear lexically within the @code{catch}
686 that it jumps to. It can equally well be called from another function
687 called within the @code{catch}. As long as the @code{throw} takes place
688 chronologically after entry to the @code{catch}, and chronologically
689 before exit from it, it has access to that @code{catch}. This is why
690 @code{throw} can be used in commands such as @code{exit-recursive-edit}
691 that throw back to the editor command loop (@pxref{Recursive Editing}).
693 @cindex CL note---only @code{throw} in Emacs
695 @b{Common Lisp note:} Most other versions of Lisp, including Common Lisp,
696 have several ways of transferring control nonsequentially: @code{return},
697 @code{return-from}, and @code{go}, for example. Emacs Lisp has only
698 @code{throw}. The @file{cl-lib} library provides versions of some of
699 these. @xref{Blocks and Exits,,,cl,Common Lisp Extensions}.
702 @defspec catch tag body@dots{}
703 @cindex tag on run time stack
704 @code{catch} establishes a return point for the @code{throw} function.
705 The return point is distinguished from other such return points by
706 @var{tag}, which may be any Lisp object except @code{nil}. The argument
707 @var{tag} is evaluated normally before the return point is established.
709 With the return point in effect, @code{catch} evaluates the forms of the
710 @var{body} in textual order. If the forms execute normally (without
711 error or nonlocal exit) the value of the last body form is returned from
714 If a @code{throw} is executed during the execution of @var{body},
715 specifying the same value @var{tag}, the @code{catch} form exits
716 immediately; the value it returns is whatever was specified as the
717 second argument of @code{throw}.
720 @defun throw tag value
721 The purpose of @code{throw} is to return from a return point previously
722 established with @code{catch}. The argument @var{tag} is used to choose
723 among the various existing return points; it must be @code{eq} to the value
724 specified in the @code{catch}. If multiple return points match @var{tag},
725 the innermost one is used.
727 The argument @var{value} is used as the value to return from that
731 If no return point is in effect with tag @var{tag}, then a @code{no-catch}
732 error is signaled with data @code{(@var{tag} @var{value})}.
735 @node Examples of Catch
736 @subsection Examples of @code{catch} and @code{throw}
738 One way to use @code{catch} and @code{throw} is to exit from a doubly
739 nested loop. (In most languages, this would be done with a ``goto''.)
740 Here we compute @code{(foo @var{i} @var{j})} for @var{i} and @var{j}
752 (throw 'loop (list i j)))
759 If @code{foo} ever returns non-@code{nil}, we stop immediately and return a
760 list of @var{i} and @var{j}. If @code{foo} always returns @code{nil}, the
761 @code{catch} returns normally, and the value is @code{nil}, since that
762 is the result of the @code{while}.
764 Here are two tricky examples, slightly different, showing two
765 return points at once. First, two return points with the same tag,
778 (print (catch2 'hack))
786 Since both return points have tags that match the @code{throw}, it goes to
787 the inner one, the one established in @code{catch2}. Therefore,
788 @code{catch2} returns normally with value @code{yes}, and this value is
789 printed. Finally the second body form in the outer @code{catch}, which is
790 @code{'no}, is evaluated and returned from the outer @code{catch}.
792 Now let's change the argument given to @code{catch2}:
797 (print (catch2 'quux))
804 We still have two return points, but this time only the outer one has
805 the tag @code{hack}; the inner one has the tag @code{quux} instead.
806 Therefore, @code{throw} makes the outer @code{catch} return the value
807 @code{yes}. The function @code{print} is never called, and the
808 body-form @code{'no} is never evaluated.
814 When Emacs Lisp attempts to evaluate a form that, for some reason,
815 cannot be evaluated, it @dfn{signals} an @dfn{error}.
817 When an error is signaled, Emacs's default reaction is to print an
818 error message and terminate execution of the current command. This is
819 the right thing to do in most cases, such as if you type @kbd{C-f} at
820 the end of the buffer.
822 In complicated programs, simple termination may not be what you want.
823 For example, the program may have made temporary changes in data
824 structures, or created temporary buffers that should be deleted before
825 the program is finished. In such cases, you would use
826 @code{unwind-protect} to establish @dfn{cleanup expressions} to be
827 evaluated in case of error. (@xref{Cleanups}.) Occasionally, you may
828 wish the program to continue execution despite an error in a subroutine.
829 In these cases, you would use @code{condition-case} to establish
830 @dfn{error handlers} to recover control in case of error.
832 Resist the temptation to use error handling to transfer control from
833 one part of the program to another; use @code{catch} and @code{throw}
834 instead. @xref{Catch and Throw}.
837 * Signaling Errors:: How to report an error.
838 * Processing of Errors:: What Emacs does when you report an error.
839 * Handling Errors:: How you can trap errors and continue execution.
840 * Error Symbols:: How errors are classified for trapping them.
843 @node Signaling Errors
844 @subsubsection How to Signal an Error
845 @cindex signaling errors
847 @dfn{Signaling} an error means beginning error processing. Error
848 processing normally aborts all or part of the running program and
849 returns to a point that is set up to handle the error
850 (@pxref{Processing of Errors}). Here we describe how to signal an
853 Most errors are signaled ``automatically'' within Lisp primitives
854 which you call for other purposes, such as if you try to take the
855 @sc{car} of an integer or move forward a character at the end of the
856 buffer. You can also signal errors explicitly with the functions
857 @code{error} and @code{signal}.
859 Quitting, which happens when the user types @kbd{C-g}, is not
860 considered an error, but it is handled almost like an error.
863 Every error specifies an error message, one way or another. The
864 message should state what is wrong (``File does not exist''), not how
865 things ought to be (``File must exist''). The convention in Emacs
866 Lisp is that error messages should start with a capital letter, but
867 should not end with any sort of punctuation.
869 @defun error format-string &rest args
870 This function signals an error with an error message constructed by
871 applying @code{format} (@pxref{Formatting Strings}) to
872 @var{format-string} and @var{args}.
874 These examples show typical uses of @code{error}:
878 (error "That is an error -- try something else")
879 @error{} That is an error -- try something else
883 (error "You have committed %d errors" 10)
884 @error{} You have committed 10 errors
888 @code{error} works by calling @code{signal} with two arguments: the
889 error symbol @code{error}, and a list containing the string returned by
892 @strong{Warning:} If you want to use your own string as an error message
893 verbatim, don't just write @code{(error @var{string})}. If @var{string}
894 contains @samp{%}, it will be interpreted as a format specifier, with
895 undesirable results. Instead, use @code{(error "%s" @var{string})}.
898 @defun signal error-symbol data
899 @anchor{Definition of signal}
900 This function signals an error named by @var{error-symbol}. The
901 argument @var{data} is a list of additional Lisp objects relevant to
902 the circumstances of the error.
904 The argument @var{error-symbol} must be an @dfn{error symbol}---a symbol
905 defined with @code{define-error}. This is how Emacs Lisp classifies different
906 sorts of errors. @xref{Error Symbols}, for a description of error symbols,
907 error conditions and condition names.
909 If the error is not handled, the two arguments are used in printing
910 the error message. Normally, this error message is provided by the
911 @code{error-message} property of @var{error-symbol}. If @var{data} is
912 non-@code{nil}, this is followed by a colon and a comma separated list
913 of the unevaluated elements of @var{data}. For @code{error}, the
914 error message is the @sc{car} of @var{data} (that must be a string).
915 Subcategories of @code{file-error} are handled specially.
917 The number and significance of the objects in @var{data} depends on
918 @var{error-symbol}. For example, with a @code{wrong-type-argument} error,
919 there should be two objects in the list: a predicate that describes the type
920 that was expected, and the object that failed to fit that type.
922 Both @var{error-symbol} and @var{data} are available to any error
923 handlers that handle the error: @code{condition-case} binds a local
924 variable to a list of the form @code{(@var{error-symbol} .@:
925 @var{data})} (@pxref{Handling Errors}).
927 The function @code{signal} never returns.
928 @c (though in older Emacs versions it sometimes could).
932 (signal 'wrong-number-of-arguments '(x y))
933 @error{} Wrong number of arguments: x, y
937 (signal 'no-such-error '("My unknown error condition"))
938 @error{} peculiar error: "My unknown error condition"
943 @cindex user errors, signaling
944 @defun user-error format-string &rest args
945 This function behaves exactly like @code{error}, except that it uses
946 the error symbol @code{user-error} rather than @code{error}. As the
947 name suggests, this is intended to report errors on the part of the
948 user, rather than errors in the code itself. For example,
949 if you try to use the command @code{Info-history-back} (@kbd{l}) to
950 move back beyond the start of your Info browsing history, Emacs
951 signals a @code{user-error}. Such errors do not cause entry to the
952 debugger, even when @code{debug-on-error} is non-@code{nil}.
953 @xref{Error Debugging}.
956 @cindex CL note---no continuable errors
958 @b{Common Lisp note:} Emacs Lisp has nothing like the Common Lisp
959 concept of continuable errors.
962 @node Processing of Errors
963 @subsubsection How Emacs Processes Errors
964 @cindex processing of errors
966 When an error is signaled, @code{signal} searches for an active
967 @dfn{handler} for the error. A handler is a sequence of Lisp
968 expressions designated to be executed if an error happens in part of the
969 Lisp program. If the error has an applicable handler, the handler is
970 executed, and control resumes following the handler. The handler
971 executes in the environment of the @code{condition-case} that
972 established it; all functions called within that @code{condition-case}
973 have already been exited, and the handler cannot return to them.
975 If there is no applicable handler for the error, it terminates the
976 current command and returns control to the editor command loop. (The
977 command loop has an implicit handler for all kinds of errors.) The
978 command loop's handler uses the error symbol and associated data to
979 print an error message. You can use the variable
980 @code{command-error-function} to control how this is done:
982 @defvar command-error-function
983 This variable, if non-@code{nil}, specifies a function to use to
984 handle errors that return control to the Emacs command loop. The
985 function should take three arguments: @var{data}, a list of the same
986 form that @code{condition-case} would bind to its variable;
987 @var{context}, a string describing the situation in which the error
988 occurred, or (more often) @code{nil}; and @var{caller}, the Lisp
989 function which called the primitive that signaled the error.
992 @cindex @code{debug-on-error} use
993 An error that has no explicit handler may call the Lisp debugger. The
994 debugger is enabled if the variable @code{debug-on-error} (@pxref{Error
995 Debugging}) is non-@code{nil}. Unlike error handlers, the debugger runs
996 in the environment of the error, so that you can examine values of
997 variables precisely as they were at the time of the error.
999 @node Handling Errors
1000 @subsubsection Writing Code to Handle Errors
1001 @cindex error handler
1002 @cindex handling errors
1004 The usual effect of signaling an error is to terminate the command
1005 that is running and return immediately to the Emacs editor command loop.
1006 You can arrange to trap errors occurring in a part of your program by
1007 establishing an error handler, with the special form
1008 @code{condition-case}. A simple example looks like this:
1013 (delete-file filename)
1019 This deletes the file named @var{filename}, catching any error and
1020 returning @code{nil} if an error occurs. (You can use the macro
1021 @code{ignore-errors} for a simple case like this; see below.)
1023 The @code{condition-case} construct is often used to trap errors that
1024 are predictable, such as failure to open a file in a call to
1025 @code{insert-file-contents}. It is also used to trap errors that are
1026 totally unpredictable, such as when the program evaluates an expression
1029 The second argument of @code{condition-case} is called the
1030 @dfn{protected form}. (In the example above, the protected form is a
1031 call to @code{delete-file}.) The error handlers go into effect when
1032 this form begins execution and are deactivated when this form returns.
1033 They remain in effect for all the intervening time. In particular, they
1034 are in effect during the execution of functions called by this form, in
1035 their subroutines, and so on. This is a good thing, since, strictly
1036 speaking, errors can be signaled only by Lisp primitives (including
1037 @code{signal} and @code{error}) called by the protected form, not by the
1038 protected form itself.
1040 The arguments after the protected form are handlers. Each handler
1041 lists one or more @dfn{condition names} (which are symbols) to specify
1042 which errors it will handle. The error symbol specified when an error
1043 is signaled also defines a list of condition names. A handler applies
1044 to an error if they have any condition names in common. In the example
1045 above, there is one handler, and it specifies one condition name,
1046 @code{error}, which covers all errors.
1048 The search for an applicable handler checks all the established handlers
1049 starting with the most recently established one. Thus, if two nested
1050 @code{condition-case} forms offer to handle the same error, the inner of
1051 the two gets to handle it.
1053 If an error is handled by some @code{condition-case} form, this
1054 ordinarily prevents the debugger from being run, even if
1055 @code{debug-on-error} says this error should invoke the debugger.
1057 If you want to be able to debug errors that are caught by a
1058 @code{condition-case}, set the variable @code{debug-on-signal} to a
1059 non-@code{nil} value. You can also specify that a particular handler
1060 should let the debugger run first, by writing @code{debug} among the
1061 conditions, like this:
1066 (delete-file filename)
1067 ((debug error) nil))
1072 The effect of @code{debug} here is only to prevent
1073 @code{condition-case} from suppressing the call to the debugger. Any
1074 given error will invoke the debugger only if @code{debug-on-error} and
1075 the other usual filtering mechanisms say it should. @xref{Error Debugging}.
1077 @defmac condition-case-unless-debug var protected-form handlers@dots{}
1078 The macro @code{condition-case-unless-debug} provides another way to
1079 handle debugging of such forms. It behaves exactly like
1080 @code{condition-case}, unless the variable @code{debug-on-error} is
1081 non-@code{nil}, in which case it does not handle any errors at all.
1084 Once Emacs decides that a certain handler handles the error, it
1085 returns control to that handler. To do so, Emacs unbinds all variable
1086 bindings made by binding constructs that are being exited, and
1087 executes the cleanups of all @code{unwind-protect} forms that are
1088 being exited. Once control arrives at the handler, the body of the
1089 handler executes normally.
1091 After execution of the handler body, execution returns from the
1092 @code{condition-case} form. Because the protected form is exited
1093 completely before execution of the handler, the handler cannot resume
1094 execution at the point of the error, nor can it examine variable
1095 bindings that were made within the protected form. All it can do is
1096 clean up and proceed.
1098 Error signaling and handling have some resemblance to @code{throw} and
1099 @code{catch} (@pxref{Catch and Throw}), but they are entirely separate
1100 facilities. An error cannot be caught by a @code{catch}, and a
1101 @code{throw} cannot be handled by an error handler (though using
1102 @code{throw} when there is no suitable @code{catch} signals an error
1103 that can be handled).
1105 @defspec condition-case var protected-form handlers@dots{}
1106 This special form establishes the error handlers @var{handlers} around
1107 the execution of @var{protected-form}. If @var{protected-form} executes
1108 without error, the value it returns becomes the value of the
1109 @code{condition-case} form; in this case, the @code{condition-case} has
1110 no effect. The @code{condition-case} form makes a difference when an
1111 error occurs during @var{protected-form}.
1113 Each of the @var{handlers} is a list of the form @code{(@var{conditions}
1114 @var{body}@dots{})}. Here @var{conditions} is an error condition name
1115 to be handled, or a list of condition names (which can include @code{debug}
1116 to allow the debugger to run before the handler); @var{body} is one or more
1117 Lisp expressions to be executed when this handler handles an error.
1118 Here are examples of handlers:
1124 (arith-error (message "Division by zero"))
1126 ((arith-error file-error)
1128 "Either division by zero or failure to open a file"))
1132 Each error that occurs has an @dfn{error symbol} that describes what
1133 kind of error it is, and which describes also a list of condition names
1134 (@pxref{Error Symbols}). Emacs
1135 searches all the active @code{condition-case} forms for a handler that
1136 specifies one or more of these condition names; the innermost matching
1137 @code{condition-case} handles the error. Within this
1138 @code{condition-case}, the first applicable handler handles the error.
1140 After executing the body of the handler, the @code{condition-case}
1141 returns normally, using the value of the last form in the handler body
1142 as the overall value.
1144 @cindex error description
1145 The argument @var{var} is a variable. @code{condition-case} does not
1146 bind this variable when executing the @var{protected-form}, only when it
1147 handles an error. At that time, it binds @var{var} locally to an
1148 @dfn{error description}, which is a list giving the particulars of the
1149 error. The error description has the form @code{(@var{error-symbol}
1150 . @var{data})}. The handler can refer to this list to decide what to
1151 do. For example, if the error is for failure opening a file, the file
1152 name is the second element of @var{data}---the third element of the
1155 If @var{var} is @code{nil}, that means no variable is bound. Then the
1156 error symbol and associated data are not available to the handler.
1158 @cindex rethrow a signal
1159 Sometimes it is necessary to re-throw a signal caught by
1160 @code{condition-case}, for some outer-level handler to catch. Here's
1164 (signal (car err) (cdr err))
1168 where @code{err} is the error description variable, the first argument
1169 to @code{condition-case} whose error condition you want to re-throw.
1170 @xref{Definition of signal}.
1173 @defun error-message-string error-descriptor
1174 This function returns the error message string for a given error
1175 descriptor. It is useful if you want to handle an error by printing the
1176 usual error message for that error. @xref{Definition of signal}.
1179 @cindex @code{arith-error} example
1180 Here is an example of using @code{condition-case} to handle the error
1181 that results from dividing by zero. The handler displays the error
1182 message (but without a beep), then returns a very large number.
1186 (defun safe-divide (dividend divisor)
1188 ;; @r{Protected form.}
1189 (/ dividend divisor)
1193 (arith-error ; @r{Condition.}
1194 ;; @r{Display the usual message for this error.}
1195 (message "%s" (error-message-string err))
1197 @result{} safe-divide
1202 @print{} Arithmetic error: (arith-error)
1208 The handler specifies condition name @code{arith-error} so that it
1209 will handle only division-by-zero errors. Other kinds of errors will
1210 not be handled (by this @code{condition-case}). Thus:
1215 @error{} Wrong type argument: number-or-marker-p, nil
1219 Here is a @code{condition-case} that catches all kinds of errors,
1220 including those from @code{error}:
1232 ;; @r{This is a call to the function @code{error}.}
1233 (error "Rats! The variable %s was %s, not 35" 'baz baz))
1234 ;; @r{This is the handler; it is not a form.}
1235 (error (princ (format "The error was: %s" err))
1237 @print{} The error was: (error "Rats! The variable baz was 34, not 35")
1242 @defmac ignore-errors body@dots{}
1243 This construct executes @var{body}, ignoring any errors that occur
1244 during its execution. If the execution is without error,
1245 @code{ignore-errors} returns the value of the last form in @var{body};
1246 otherwise, it returns @code{nil}.
1248 Here's the example at the beginning of this subsection rewritten using
1249 @code{ignore-errors}:
1254 (delete-file filename))
1259 @defmac with-demoted-errors format body@dots{}
1260 This macro is like a milder version of @code{ignore-errors}. Rather
1261 than suppressing errors altogether, it converts them into messages.
1262 It uses the string @var{format} to format the message.
1263 @var{format} should contain a single @samp{%}-sequence; e.g.,
1264 @code{"Error: %S"}. Use @code{with-demoted-errors} around code
1265 that is not expected to signal errors, but
1266 should be robust if one does occur. Note that this macro uses
1267 @code{condition-case-unless-debug} rather than @code{condition-case}.
1271 @subsubsection Error Symbols and Condition Names
1272 @cindex error symbol
1274 @cindex condition name
1275 @cindex user-defined error
1276 @kindex error-conditions
1277 @kindex define-error
1279 When you signal an error, you specify an @dfn{error symbol} to specify
1280 the kind of error you have in mind. Each error has one and only one
1281 error symbol to categorize it. This is the finest classification of
1282 errors defined by the Emacs Lisp language.
1284 These narrow classifications are grouped into a hierarchy of wider
1285 classes called @dfn{error conditions}, identified by @dfn{condition
1286 names}. The narrowest such classes belong to the error symbols
1287 themselves: each error symbol is also a condition name. There are also
1288 condition names for more extensive classes, up to the condition name
1289 @code{error} which takes in all kinds of errors (but not @code{quit}).
1290 Thus, each error has one or more condition names: @code{error}, the
1291 error symbol if that is distinct from @code{error}, and perhaps some
1292 intermediate classifications.
1294 @defun define-error name message &optional parent
1295 In order for a symbol to be an error symbol, it must be defined with
1296 @code{define-error} which takes a parent condition (defaults to @code{error}).
1297 This parent defines the conditions that this kind of error belongs to.
1298 The transitive set of parents always includes the error symbol itself, and the
1299 symbol @code{error}. Because quitting is not considered an error, the set of
1300 parents of @code{quit} is just @code{(quit)}.
1303 @cindex peculiar error
1304 In addition to its parents, the error symbol has a @var{message} which
1305 is a string to be printed when that error is signaled but not handled. If that
1306 message is not valid, the error message @samp{peculiar error} is used.
1307 @xref{Definition of signal}.
1309 Internally, the set of parents is stored in the @code{error-conditions}
1310 property of the error symbol and the message is stored in the
1311 @code{error-message} property of the error symbol.
1313 Here is how we define a new error symbol, @code{new-error}:
1317 (define-error 'new-error "A new error" 'my-own-errors)
1322 This error has several condition names: @code{new-error}, the narrowest
1323 classification; @code{my-own-errors}, which we imagine is a wider
1324 classification; and all the conditions of @code{my-own-errors} which should
1325 include @code{error}, which is the widest of all.
1327 The error string should start with a capital letter but it should
1328 not end with a period. This is for consistency with the rest of Emacs.
1330 Naturally, Emacs will never signal @code{new-error} on its own; only
1331 an explicit call to @code{signal} (@pxref{Definition of signal}) in
1332 your code can do this:
1336 (signal 'new-error '(x y))
1337 @error{} A new error: x, y
1341 This error can be handled through any of its condition names.
1342 This example handles @code{new-error} and any other errors in the class
1343 @code{my-own-errors}:
1349 (my-own-errors nil))
1353 The significant way that errors are classified is by their condition
1354 names---the names used to match errors with handlers. An error symbol
1355 serves only as a convenient way to specify the intended error message
1356 and list of condition names. It would be cumbersome to give
1357 @code{signal} a list of condition names rather than one error symbol.
1359 By contrast, using only error symbols without condition names would
1360 seriously decrease the power of @code{condition-case}. Condition names
1361 make it possible to categorize errors at various levels of generality
1362 when you write an error handler. Using error symbols alone would
1363 eliminate all but the narrowest level of classification.
1365 @xref{Standard Errors}, for a list of the main error symbols
1366 and their conditions.
1369 @subsection Cleaning Up from Nonlocal Exits
1370 @cindex nonlocal exits, cleaning up
1372 The @code{unwind-protect} construct is essential whenever you
1373 temporarily put a data structure in an inconsistent state; it permits
1374 you to make the data consistent again in the event of an error or
1375 throw. (Another more specific cleanup construct that is used only for
1376 changes in buffer contents is the atomic change group; @ref{Atomic
1379 @defspec unwind-protect body-form cleanup-forms@dots{}
1380 @cindex cleanup forms
1381 @cindex protected forms
1382 @cindex error cleanup
1384 @code{unwind-protect} executes @var{body-form} with a guarantee that
1385 the @var{cleanup-forms} will be evaluated if control leaves
1386 @var{body-form}, no matter how that happens. @var{body-form} may
1387 complete normally, or execute a @code{throw} out of the
1388 @code{unwind-protect}, or cause an error; in all cases, the
1389 @var{cleanup-forms} will be evaluated.
1391 If @var{body-form} finishes normally, @code{unwind-protect} returns the
1392 value of @var{body-form}, after it evaluates the @var{cleanup-forms}.
1393 If @var{body-form} does not finish, @code{unwind-protect} does not
1394 return any value in the normal sense.
1396 Only @var{body-form} is protected by the @code{unwind-protect}. If any
1397 of the @var{cleanup-forms} themselves exits nonlocally (via a
1398 @code{throw} or an error), @code{unwind-protect} is @emph{not}
1399 guaranteed to evaluate the rest of them. If the failure of one of the
1400 @var{cleanup-forms} has the potential to cause trouble, then protect
1401 it with another @code{unwind-protect} around that form.
1403 The number of currently active @code{unwind-protect} forms counts,
1404 together with the number of local variable bindings, against the limit
1405 @code{max-specpdl-size} (@pxref{Definition of max-specpdl-size,, Local
1409 For example, here we make an invisible buffer for temporary use, and
1410 make sure to kill it before finishing:
1414 (let ((buffer (get-buffer-create " *temp*")))
1415 (with-current-buffer buffer
1418 (kill-buffer buffer))))
1423 You might think that we could just as well write @code{(kill-buffer
1424 (current-buffer))} and dispense with the variable @code{buffer}.
1425 However, the way shown above is safer, if @var{body-form} happens to
1426 get an error after switching to a different buffer! (Alternatively,
1427 you could write a @code{save-current-buffer} around @var{body-form},
1428 to ensure that the temporary buffer becomes current again in time to
1431 Emacs includes a standard macro called @code{with-temp-buffer} which
1432 expands into more or less the code shown above (@pxref{Definition of
1433 with-temp-buffer,, Current Buffer}). Several of the macros defined in
1434 this manual use @code{unwind-protect} in this way.
1437 Here is an actual example derived from an FTP package. It creates a
1438 process (@pxref{Processes}) to try to establish a connection to a remote
1439 machine. As the function @code{ftp-login} is highly susceptible to
1440 numerous problems that the writer of the function cannot anticipate, it
1441 is protected with a form that guarantees deletion of the process in the
1442 event of failure. Otherwise, Emacs might fill up with useless
1450 (setq process (ftp-setup-buffer host file))
1451 (if (setq win (ftp-login process host user password))
1452 (message "Logged in")
1453 (error "Ftp login failed")))
1454 (or win (and process (delete-process process)))))
1458 This example has a small bug: if the user types @kbd{C-g} to
1459 quit, and the quit happens immediately after the function
1460 @code{ftp-setup-buffer} returns but before the variable @code{process} is
1461 set, the process will not be killed. There is no easy way to fix this bug,
1462 but at least it is very unlikely.