2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2011, 2012
4 @c Free Software Foundation, Inc.
5 @c See the file guile.texi for copying conditions.
7 @node Control Mechanisms
8 @section Controlling the Flow of Program Execution
10 See @ref{Control Flow} for a discussion of how the more general control
11 flow of Scheme affects C code.
14 * begin:: Sequencing and splicing.
15 * Conditionals:: If, when, unless, case, and cond.
16 * and or:: Conditional evaluation of a sequence.
17 * while do:: Iteration mechanisms.
18 * Prompts:: Composable, delimited continuations.
19 * Continuations:: Non-composable continuations.
20 * Multiple Values:: Returning and accepting multiple values.
21 * Exceptions:: Throwing and catching exceptions.
22 * Error Reporting:: Procedures for signaling errors.
23 * Dynamic Wind:: Dealing with non-local entrance/exit.
24 * Handling Errors:: How to handle errors in C code.
25 * Continuation Barriers:: Protection from non-local control flow.
29 @subsection Sequencing and Splicing
33 @cindex expression sequencing
35 As an expression, the @code{begin} syntax is used to evaluate a sequence
36 of sub-expressions in order. Consider the conditional expression below:
40 (begin (display "greater") (newline)))
43 If the test is true, we want to display ``greater'' to the current
44 output port, then display a newline. We use @code{begin} to form a
45 compound expression out of this sequence of sub-expressions.
47 @deffn syntax begin expr1 expr2 @dots{}
48 The expression(s) are evaluated in left-to-right order and the value of
49 the last expression is returned as the value of the
50 @code{begin}-expression. This expression type is used when the
51 expressions before the last one are evaluated for their side effects.
55 @cindex definition splicing
57 The @code{begin} syntax has another role in definition context
58 (@pxref{Internal Definitions}). A @code{begin} form in a definition
59 context @dfn{splices} its subforms into its place. For example,
60 consider the following procedure:
64 (define-sealant seal open)
68 Let us assume the existence of a @code{define-sealant} macro that
69 expands out to some definitions wrapped in a @code{begin}, like so:
79 (and (pair? x) (eq? (car x) seal-tag)))
83 (error "Expected a sealed value:" x))))
87 Here, because the @code{begin} is in definition context, its subforms
88 are @dfn{spliced} into the place of the @code{begin}. This allows the
89 definitions created by the macro to be visible to the following
90 expression, the @code{values} form.
92 It is a fine point, but splicing and sequencing are different. It can
93 make sense to splice zero forms, because it can make sense to have zero
94 internal definitions before the expressions in a procedure or lexical
95 binding form. However it does not make sense to have a sequence of zero
96 expressions, because in that case it would not be clear what the value
97 of the sequence would be, because in a sequence of zero expressions,
98 there can be no last value. Sequencing zero expressions is an error.
100 It would be more elegant in some ways to eliminate splicing from the
101 Scheme language, and without macros (@pxref{Macros}), that would be a
102 good idea. But it is useful to be able to write macros that expand out
103 to multiple definitions, as in @code{define-sealant} above, so Scheme
104 abuses the @code{begin} form for these two tasks.
107 @subsection Simple Conditional Evaluation
109 @cindex conditional evaluation
116 Guile provides three syntactic constructs for conditional evaluation.
117 @code{if} is the normal if-then-else expression (with an optional else
118 branch), @code{cond} is a conditional expression with multiple branches
119 and @code{case} branches if an expression has one of a set of constant
122 @deffn syntax if test consequent [alternate]
123 All arguments may be arbitrary expressions. First, @var{test} is
124 evaluated. If it returns a true value, the expression @var{consequent}
125 is evaluated and @var{alternate} is ignored. If @var{test} evaluates to
126 @code{#f}, @var{alternate} is evaluated instead. The values of the
127 evaluated branch (@var{consequent} or @var{alternate}) are returned as
128 the values of the @code{if} expression.
130 When @var{alternate} is omitted and the @var{test} evaluates to
131 @code{#f}, the value of the expression is not specified.
134 When you go to write an @code{if} without an alternate (a @dfn{one-armed
135 @code{if}}), part of what you are expressing is that you don't care
136 about the return value (or values) of the expression. As such, you are
137 more interested in the @emph{effect} of evaluating the consequent
138 expression. (By convention, we use the word @dfn{statement} to refer to
139 an expression that is evaluated for effect, not for value).
141 In such a case, it is considered more clear to express these intentions
142 with these special forms, @code{when} and @code{unless}. As an added
143 bonus, these forms accept multiple statements to evaluate, which are
144 implicitly wrapped in a @code{begin}.
146 @deffn {Scheme Syntax} when test statement1 statement2 ...
147 @deffnx {Scheme Syntax} unless test statement1 statement2 ...
148 The actual definitions of these forms are in many ways their most clear
152 (define-syntax-rule (when test stmt stmt* ...)
153 (if test (begin stmt stmt* ...)))
155 (define-syntax-rule (unless condition stmt stmt* ...)
156 (if (not test) (begin stmt stmt* ...)))
159 That is to say, @code{when} evaluates its consequent statements in order
160 if @var{test} is true. @code{unless} is the opposite: it evaluates the
161 statements if @var{test} is false.
164 @deffn syntax cond clause1 clause2 @dots{}
165 Each @code{cond}-clause must look like this:
168 (@var{test} @var{expression} @dots{})
171 where @var{test} and @var{expression} are arbitrary expression, or like
175 (@var{test} => @var{expression})
178 where @var{expression} must evaluate to a procedure.
180 The @var{test}s of the clauses are evaluated in order and as soon as one
181 of them evaluates to a true values, the corresponding @var{expression}s
182 are evaluated in order and the last value is returned as the value of
183 the @code{cond}-expression. For the @code{=>} clause type,
184 @var{expression} is evaluated and the resulting procedure is applied to
185 the value of @var{test}. The result of this procedure application is
186 then the result of the @code{cond}-expression.
189 @cindex general cond clause
190 @cindex multiple values and cond
191 One additional @code{cond}-clause is available as an extension to
195 (@var{test} @var{guard} => @var{expression})
198 where @var{guard} and @var{expression} must evaluate to procedures.
199 For this clause type, @var{test} may return multiple values, and
200 @code{cond} ignores its boolean state; instead, @code{cond} evaluates
201 @var{guard} and applies the resulting procedure to the value(s) of
202 @var{test}, as if @var{guard} were the @var{consumer} argument of
203 @code{call-with-values}. Iff the result of that procedure call is a
204 true value, it evaluates @var{expression} and applies the resulting
205 procedure to the value(s) of @var{test}, in the same manner as the
206 @var{guard} was called.
208 The @var{test} of the last @var{clause} may be the symbol @code{else}.
209 Then, if none of the preceding @var{test}s is true, the
210 @var{expression}s following the @code{else} are evaluated to produce the
211 result of the @code{cond}-expression.
214 @deffn syntax case key clause1 clause2 @dots{}
215 @var{key} may be any expression, the @var{clause}s must have the form
218 ((@var{datum1} @dots{}) @var{expr1} @var{expr2} @dots{})
221 and the last @var{clause} may have the form
224 (else @var{expr1} @var{expr2} @dots{})
227 All @var{datum}s must be distinct. First, @var{key} is evaluated. The
228 result of this evaluation is compared against all @var{datum} values using
229 @code{eqv?}. When this comparison succeeds, the expression(s) following
230 the @var{datum} are evaluated from left to right, returning the value of
231 the last expression as the result of the @code{case} expression.
233 If the @var{key} matches no @var{datum} and there is an
234 @code{else}-clause, the expressions following the @code{else} are
235 evaluated. If there is no such clause, the result of the expression is
241 @subsection Conditional Evaluation of a Sequence of Expressions
243 @code{and} and @code{or} evaluate all their arguments in order, similar
244 to @code{begin}, but evaluation stops as soon as one of the expressions
245 evaluates to false or true, respectively.
247 @deffn syntax and expr @dots{}
248 Evaluate the @var{expr}s from left to right and stop evaluation as soon
249 as one expression evaluates to @code{#f}; the remaining expressions are
250 not evaluated. The value of the last evaluated expression is returned.
251 If no expression evaluates to @code{#f}, the value of the last
252 expression is returned.
254 If used without expressions, @code{#t} is returned.
257 @deffn syntax or expr @dots{}
258 Evaluate the @var{expr}s from left to right and stop evaluation as soon
259 as one expression evaluates to a true value (that is, a value different
260 from @code{#f}); the remaining expressions are not evaluated. The value
261 of the last evaluated expression is returned. If all expressions
262 evaluate to @code{#f}, @code{#f} is returned.
264 If used without expressions, @code{#f} is returned.
269 @subsection Iteration mechanisms
275 Scheme has only few iteration mechanisms, mainly because iteration in
276 Scheme programs is normally expressed using recursion. Nevertheless,
277 R5RS defines a construct for programming loops, calling @code{do}. In
278 addition, Guile has an explicit looping syntax called @code{while}.
280 @deffn syntax do ((variable init [step]) @dots{}) (test [expr @dots{}]) body @dots{}
281 Bind @var{variable}s and evaluate @var{body} until @var{test} is true.
282 The return value is the last @var{expr} after @var{test}, if given. A
283 simple example will illustrate the basic form,
293 Or with two variables and a final return value,
300 (format #t "3**~s is ~s\n" i p))
310 The @var{variable} bindings are established like a @code{let}, in that
311 the expressions are all evaluated and then all bindings made. When
312 iterating, the optional @var{step} expressions are evaluated with the
313 previous bindings in scope, then new bindings all made.
315 The @var{test} expression is a termination condition. Looping stops
316 when the @var{test} is true. It's evaluated before running the
317 @var{body} each time, so if it's true the first time then @var{body}
320 The optional @var{expr}s after the @var{test} are evaluated at the end
321 of looping, with the final @var{variable} bindings available. The
322 last @var{expr} gives the return value, or if there are no @var{expr}s
323 the return value is unspecified.
325 Each iteration establishes bindings to fresh locations for the
326 @var{variable}s, like a new @code{let} for each iteration. This is
327 done for @var{variable}s without @var{step} expressions too. The
328 following illustrates this, showing how a new @code{i} is captured by
329 the @code{lambda} in each iteration (@pxref{About Closure,, The
330 Concept of Closure}).
336 (set! lst (cons (lambda () i) lst)))
337 (map (lambda (proc) (proc)) lst)
343 @deffn syntax while cond body @dots{}
344 Run a loop executing the @var{body} forms while @var{cond} is true.
345 @var{cond} is tested at the start of each iteration, so if it's
346 @code{#f} the first time then @var{body} is not executed at all.
348 Within @code{while}, two extra bindings are provided, they can be used
349 from both @var{cond} and @var{body}.
351 @deffn {Scheme Procedure} break break-arg...
352 Break out of the @code{while} form.
355 @deffn {Scheme Procedure} continue
356 Abandon the current iteration, go back to the start and test
357 @var{cond} again, etc.
360 If the loop terminates normally, by the @var{cond} evaluating to
361 @code{#f}, then the @code{while} expression as a whole evaluates to
362 @code{#f}. If it terminates by a call to @code{break} with some number
363 of arguments, those arguments are returned from the @code{while}
364 expression, as multiple values. Otherwise if it terminates by a call to
365 @code{break} with no arguments, then return value is @code{#t}.
368 (while #f (error "not reached")) @result{} #f
369 (while #t (break)) @result{} #t
370 (while #t (break 1 2 3)) @result{} 1 2 3
373 Each @code{while} form gets its own @code{break} and @code{continue}
374 procedures, operating on that @code{while}. This means when loops are
375 nested the outer @code{break} can be used to escape all the way out.
380 (let ((outer-break break))
387 Note that each @code{break} and @code{continue} procedure can only be
388 used within the dynamic extent of its @code{while}. Outside the
389 @code{while} their behaviour is unspecified.
393 Another very common way of expressing iteration in Scheme programs is
394 the use of the so-called @dfn{named let}.
396 Named let is a variant of @code{let} which creates a procedure and calls
397 it in one step. Because of the newly created procedure, named let is
398 more powerful than @code{do}--it can be used for iteration, but also
399 for arbitrary recursion.
401 @deffn syntax let variable bindings body
402 For the definition of @var{bindings} see the documentation about
403 @code{let} (@pxref{Local Bindings}).
405 Named @code{let} works as follows:
409 A new procedure which accepts as many arguments as are in @var{bindings}
410 is created and bound locally (using @code{let}) to @var{variable}. The
411 new procedure's formal argument names are the name of the
415 The @var{body} expressions are inserted into the newly created procedure.
418 The procedure is called with the @var{init} expressions as the formal
422 The next example implements a loop which iterates (by recursion) 1000
439 @cindex delimited continuations
440 @cindex composable continuations
441 @cindex non-local exit
443 Prompts are control-flow barriers between different parts of a program. In the
444 same way that a user sees a shell prompt (e.g., the Bash prompt) as a barrier
445 between the operating system and her programs, Scheme prompts allow the Scheme
446 programmer to treat parts of programs as if they were running in different
449 We use this roundabout explanation because, unless you're a functional
450 programming junkie, you probably haven't heard the term, ``delimited, composable
451 continuation''. That's OK; it's a relatively recent topic, but a very useful
455 * Prompt Primitives:: Call-with-prompt and abort-to-prompt.
456 * Shift and Reset:: The zoo of delimited control operators.
459 @node Prompt Primitives
460 @subsubsection Prompt Primitives
462 Guile's primitive delimited control operators are
463 @code{call-with-prompt} and @code{abort-to-prompt}.
465 @deffn {Scheme Procedure} call-with-prompt tag thunk handler
466 Set up a prompt, and call @var{thunk} within that prompt.
468 During the dynamic extent of the call to @var{thunk}, a prompt named @var{tag}
469 will be present in the dynamic context, such that if a user calls
470 @code{abort-to-prompt} (see below) with that tag, control rewinds back to the
471 prompt, and the @var{handler} is run.
473 @var{handler} must be a procedure. The first argument to @var{handler} will be
474 the state of the computation begun when @var{thunk} was called, and ending with
475 the call to @code{abort-to-prompt}. The remaining arguments to @var{handler} are
476 those passed to @code{abort-to-prompt}.
479 @deffn {Scheme Procedure} make-prompt-tag [stem]
480 Make a new prompt tag. Currently prompt tags are generated symbols.
481 This may change in some future Guile version.
484 @deffn {Scheme Procedure} default-prompt-tag
485 Return the default prompt tag. Having a distinguished default prompt
486 tag allows some useful prompt and abort idioms, discussed in the next
490 @deffn {Scheme Procedure} abort-to-prompt tag val ...
491 Unwind the dynamic and control context to the nearest prompt named @var{tag},
492 also passing the given values.
495 C programmers may recognize @code{call-with-prompt} and @code{abort-to-prompt}
496 as a fancy kind of @code{setjmp} and @code{longjmp}, respectively. Prompts are
497 indeed quite useful as non-local escape mechanisms. Guile's @code{catch} and
498 @code{throw} are implemented in terms of prompts. Prompts are more convenient
499 than @code{longjmp}, in that one has the opportunity to pass multiple values to
502 Also unlike @code{longjmp}, the prompt handler is given the full state of the
503 process that was aborted, as the first argument to the prompt's handler. That
504 state is the @dfn{continuation} of the computation wrapped by the prompt. It is
505 a @dfn{delimited continuation}, because it is not the whole continuation of the
506 program; rather, just the computation initiated by the call to
507 @code{call-with-prompt}.
509 The continuation is a procedure, and may be reinstated simply by invoking it,
510 with any number of values. Here's where things get interesting, and complicated
511 as well. Besides being described as delimited, continuations reified by prompts
512 are also @dfn{composable}, because invoking a prompt-saved continuation composes
513 that continuation with the current one.
515 Imagine you have saved a continuation via call-with-prompt:
524 (+ 34 (abort-to-prompt 'foo)))
529 The resulting continuation is the addition of 34. It's as if you had written:
537 So, if we call @code{cont} with one numeric value, we get that number,
547 The last example illustrates what we mean when we say, "composes with the
548 current continuation". We mean that there is a current continuation -- some
549 remaining things to compute, like @code{(lambda (x) (* x 2))} -- and that
550 calling the saved continuation doesn't wipe out the current continuation, it
551 composes the saved continuation with the current one.
553 We're belaboring the point here because traditional Scheme continuations, as
554 discussed in the next section, aren't composable, and are actually less
555 expressive than continuations captured by prompts. But there's a place for them
558 Before moving on, we should mention that if the handler of a prompt is a
559 @code{lambda} expression, and the first argument isn't referenced, an abort to
560 that prompt will not cause a continuation to be reified. This can be an
561 important efficiency consideration to keep in mind.
563 @node Shift and Reset
564 @subsubsection Shift, Reset, and All That
566 There is a whole zoo of delimited control operators, and as it does not
567 seem to be a bounded set, Guile implements support for them in a
571 (use-modules (ice-9 control))
574 Firstly, we have a helpful abbreviation for the @code{call-with-prompt}
577 @deffn {Scheme Syntax} % expr
578 @deffnx {Scheme Syntax} % expr handler
579 @deffnx {Scheme Syntax} % tag expr handler
580 Evaluate @var{expr} in a prompt, optionally specifying a tag and a
581 handler. If no tag is given, the default prompt tag is used.
583 If no handler is given, a default handler is installed. The default
584 handler accepts a procedure of one argument, which will called on the
585 captured continuation, within a prompt.
587 Sometimes it's easier just to show code, as in this case:
590 (define (default-prompt-handler k proc)
591 (% (default-prompt-tag)
593 default-prompt-handler))
596 The @code{%} symbol is chosen because it looks like a prompt.
599 Likewise there is an abbreviation for @code{abort-to-prompt}, which
600 assumes the default prompt tag:
602 @deffn {Scheme Procedure} abort val...
603 Abort to the default prompt tag, passing @var{val...} to the handler.
606 As mentioned before, @code{(ice-9 control)} also provides other
607 delimited control operators. This section is a bit technical, and
608 first-time users of delimited continuations should probably come back to
609 it after some practice with @code{%}.
611 Still here? So, when one implements a delimited control operator like
612 @code{call-with-prompt}, one needs to make two decisions. Firstly, does
613 the handler run within or outside the prompt? Having the handler run
614 within the prompt allows an abort inside the handler to return to the
615 same prompt handler, which is often useful. However it prevents tail
616 calls from the handler, so it is less general.
618 Similarly, does invoking a captured continuation reinstate a prompt?
619 Again we have the tradeoff of convenience versus proper tail calls.
621 These decisions are captured in the Felleisen @dfn{F} operator. If
622 neither the continuations nor the handlers implicitly add a prompt, the
623 operator is known as @dfn{--F--}. This is the case for Guile's
624 @code{call-with-prompt} and @code{abort-to-prompt}.
626 If both continuation and handler implicitly add prompts, then the
627 operator is @dfn{+F+}. @code{shift} and @code{reset} are such
630 @deffn {Scheme Syntax} reset body...
631 Establish a prompt, and evaluate @var{body...} within that prompt.
633 The prompt handler is designed to work with @code{shift}, described
637 @deffn {Scheme Syntax} shift cont body...
638 Abort to the nearest @code{reset}, and evaluate @var{body...} in a
639 context in which the captured continuation is bound to @var{cont}.
641 As mentioned above, both the @var{body...} expression and invocations of
642 @var{cont} implicitly establish a prompt.
645 Interested readers are invited to explore Oleg Kiselyov's wonderful web
646 site at @uref{http://okmij.org/ftp/}, for more information on these
651 @subsection Continuations
652 @cindex continuations
654 A ``continuation'' is the code that will execute when a given function
655 or expression returns. For example, consider
660 (display (bar)) (newline)
664 The continuation from the call to @code{bar} comprises a
665 @code{display} of the value returned, a @code{newline} and an
666 @code{exit}. This can be expressed as a function of one argument.
670 (display r) (newline)
674 In Scheme, continuations are represented as special procedures just
675 like this. The special property is that when a continuation is called
676 it abandons the current program location and jumps directly to that
677 represented by the continuation.
679 A continuation is like a dynamic label, capturing at run-time a point
680 in program execution, including all the nested calls that have lead to
681 it (or rather the code that will execute when those calls return).
683 Continuations are created with the following functions.
685 @deffn {Scheme Procedure} call-with-current-continuation proc
686 @deffnx {Scheme Procedure} call/cc proc
687 @rnindex call-with-current-continuation
688 Capture the current continuation and call @code{(@var{proc}
689 @var{cont})} with it. The return value is the value returned by
690 @var{proc}, or when @code{(@var{cont} @var{value})} is later invoked,
691 the return is the @var{value} passed.
693 Normally @var{cont} should be called with one argument, but when the
694 location resumed is expecting multiple values (@pxref{Multiple
695 Values}) then they should be passed as multiple arguments, for
696 instance @code{(@var{cont} @var{x} @var{y} @var{z})}.
698 @var{cont} may only be used from the same side of a continuation
699 barrier as it was created (@pxref{Continuation Barriers}), and in a
700 multi-threaded program only from the thread in which it was created.
702 The call to @var{proc} is not part of the continuation captured, it runs
703 only when the continuation is created. Often a program will want to
704 store @var{cont} somewhere for later use; this can be done in
707 The @code{call} in the name @code{call-with-current-continuation}
708 refers to the way a call to @var{proc} gives the newly created
709 continuation. It's not related to the way a call is used later to
710 invoke that continuation.
712 @code{call/cc} is an alias for @code{call-with-current-continuation}.
713 This is in common use since the latter is rather long.
718 Here is a simple example,
722 (format #t "the return is ~a\n"
726 @result{} the return is 1
729 @result{} the return is 2
732 @code{call/cc} captures a continuation in which the value returned is
733 going to be displayed by @code{format}. The @code{lambda} stores this
734 in @code{kont} and gives an initial return @code{1} which is
735 displayed. The later invocation of @code{kont} resumes the captured
736 point, but this time returning @code{2}, which is displayed.
738 When Guile is run interactively, a call to @code{format} like this has
739 an implicit return back to the read-eval-print loop. @code{call/cc}
740 captures that like any other return, which is why interactively
741 @code{kont} will come back to read more input.
744 C programmers may note that @code{call/cc} is like @code{setjmp} in
745 the way it records at runtime a point in program execution. A call to
746 a continuation is like a @code{longjmp} in that it abandons the
747 present location and goes to the recorded one. Like @code{longjmp},
748 the value passed to the continuation is the value returned by
749 @code{call/cc} on resuming there. However @code{longjmp} can only go
750 up the program stack, but the continuation mechanism can go anywhere.
752 When a continuation is invoked, @code{call/cc} and subsequent code
753 effectively ``returns'' a second time. It can be confusing to imagine
754 a function returning more times than it was called. It may help
755 instead to think of it being stealthily re-entered and then program
756 flow going on as normal.
758 @code{dynamic-wind} (@pxref{Dynamic Wind}) can be used to ensure setup
759 and cleanup code is run when a program locus is resumed or abandoned
760 through the continuation mechanism.
763 Continuations are a powerful mechanism, and can be used to implement
764 almost any sort of control structure, such as loops, coroutines, or
767 However the implementation of continuations in Guile is not as
768 efficient as one might hope, because Guile is designed to cooperate
769 with programs written in other languages, such as C, which do not know
770 about continuations. Basically continuations are captured by a block
771 copy of the stack, and resumed by copying back.
773 For this reason, continuations captured by @code{call/cc} should be used only
774 when there is no other simple way to achieve the desired result, or when the
775 elegance of the continuation mechanism outweighs the need for performance.
777 Escapes upwards from loops or nested functions are generally best
778 handled with prompts (@pxref{Prompts}). Coroutines can be
779 efficiently implemented with cooperating threads (a thread holds a
780 full program stack but doesn't copy it around the way continuations
784 @node Multiple Values
785 @subsection Returning and Accepting Multiple Values
787 @cindex multiple values
790 Scheme allows a procedure to return more than one value to its caller.
791 This is quite different to other languages which only allow
792 single-value returns. Returning multiple values is different from
793 returning a list (or pair or vector) of values to the caller, because
794 conceptually not @emph{one} compound object is returned, but several
797 The primitive procedures for handling multiple values are @code{values}
798 and @code{call-with-values}. @code{values} is used for returning
799 multiple values from a procedure. This is done by placing a call to
800 @code{values} with zero or more arguments in tail position in a
801 procedure body. @code{call-with-values} combines a procedure returning
802 multiple values with a procedure which accepts these values as
806 @deffn {Scheme Procedure} values arg1 @dots{} argN
807 @deffnx {C Function} scm_values (args)
808 Delivers all of its arguments to its continuation. Except for
809 continuations created by the @code{call-with-values} procedure,
810 all continuations take exactly one value. The effect of
811 passing no value or more than one value to continuations that
812 were not created by @code{call-with-values} is unspecified.
814 For @code{scm_values}, @var{args} is a list of arguments and the
815 return is a multiple-values object which the caller can return. In
816 the current implementation that object shares structure with
817 @var{args}, so @var{args} should not be modified subsequently.
820 @deffn {C Function} scm_c_value_ref (values, idx)
821 Returns the value at the position specified by @var{idx} in
822 @var{values}. Note that @var{values} will ordinarily be a
823 multiple-values object, but it need not be. Any other object
824 represents a single value (itself), and is handled appropriately.
827 @rnindex call-with-values
828 @deffn {Scheme Procedure} call-with-values producer consumer
829 Calls its @var{producer} argument with no values and a
830 continuation that, when passed some values, calls the
831 @var{consumer} procedure with those values as arguments. The
832 continuation for the call to @var{consumer} is the continuation
833 of the call to @code{call-with-values}.
836 (call-with-values (lambda () (values 4 5))
842 (call-with-values * -)
847 In addition to the fundamental procedures described above, Guile has a
848 module which exports a syntax called @code{receive}, which is much
849 more convenient. This is in the @code{(ice-9 receive)} and is the
850 same as specified by SRFI-8 (@pxref{SRFI-8}).
853 (use-modules (ice-9 receive))
856 @deffn {library syntax} receive formals expr body @dots{}
857 Evaluate the expression @var{expr}, and bind the result values (zero
858 or more) to the formal arguments in @var{formals}. @var{formals} is a
859 list of symbols, like the argument list in a @code{lambda}
860 (@pxref{Lambda}). After binding the variables, the expressions in
861 @var{body} @dots{} are evaluated in order, the return value is the
862 result from the last expression.
864 For example getting results from @code{partition} in SRFI-1
868 (receive (odds evens)
869 (partition odd? '(7 4 2 8 3))
873 @print{} (7 3) and (4 2 8)
880 @subsection Exceptions
881 @cindex error handling
882 @cindex exception handling
884 A common requirement in applications is to want to jump
885 @dfn{non-locally} from the depths of a computation back to, say, the
886 application's main processing loop. Usually, the place that is the
887 target of the jump is somewhere in the calling stack of procedures that
888 called the procedure that wants to jump back. For example, typical
889 logic for a key press driven application might look something like this:
893 read the next key press and call dispatch-key
896 lookup the key in a keymap and call an appropriate procedure,
900 interactively read the required file name, then call
904 check whether file exists; if not, jump back to main-loop
908 The jump back to @code{main-loop} could be achieved by returning through
909 the stack one procedure at a time, using the return value of each
910 procedure to indicate the error condition, but Guile (like most modern
911 programming languages) provides an additional mechanism called
912 @dfn{exception handling} that can be used to implement such jumps much
916 * Exception Terminology:: Different ways to say the same thing.
917 * Catch:: Setting up to catch exceptions.
918 * Throw Handlers:: Handling exceptions before unwinding the stack.
919 * Throw:: Throwing an exception.
920 * Exception Implementation:: How Guile implements exceptions.
924 @node Exception Terminology
925 @subsubsection Exception Terminology
927 There are several variations on the terminology for dealing with
928 non-local jumps. It is useful to be aware of them, and to realize
929 that they all refer to the same basic mechanism.
933 Actually making a non-local jump may be called @dfn{raising an
934 exception}, @dfn{raising a signal}, @dfn{throwing an exception} or
935 @dfn{doing a long jump}. When the jump indicates an error condition,
936 people may talk about @dfn{signalling}, @dfn{raising} or @dfn{throwing}
940 Handling the jump at its target may be referred to as @dfn{catching} or
941 @dfn{handling} the @dfn{exception}, @dfn{signal} or, where an error
942 condition is involved, @dfn{error}.
945 Where @dfn{signal} and @dfn{signalling} are used, special care is needed
946 to avoid the risk of confusion with POSIX signals.
948 This manual prefers to speak of throwing and catching exceptions, since
949 this terminology matches the corresponding Guile primitives.
953 @subsubsection Catching Exceptions
955 @code{catch} is used to set up a target for a possible non-local jump.
956 The arguments of a @code{catch} expression are a @dfn{key}, which
957 restricts the set of exceptions to which this @code{catch} applies, a
958 thunk that specifies the code to execute and one or two @dfn{handler}
959 procedures that say what to do if an exception is thrown while executing
960 the code. If the execution thunk executes @dfn{normally}, which means
961 without throwing any exceptions, the handler procedures are not called
964 When an exception is thrown using the @code{throw} function, the first
965 argument of the @code{throw} is a symbol that indicates the type of the
966 exception. For example, Guile throws an exception using the symbol
967 @code{numerical-overflow} to indicate numerical overflow errors such as
973 ABORT: (numerical-overflow)
976 The @var{key} argument in a @code{catch} expression corresponds to this
977 symbol. @var{key} may be a specific symbol, such as
978 @code{numerical-overflow}, in which case the @code{catch} applies
979 specifically to exceptions of that type; or it may be @code{#t}, which
980 means that the @code{catch} applies to all exceptions, irrespective of
983 The second argument of a @code{catch} expression should be a thunk
984 (i.e.@: a procedure that accepts no arguments) that specifies the normal
985 case code. The @code{catch} is active for the execution of this thunk,
986 including any code called directly or indirectly by the thunk's body.
987 Evaluation of the @code{catch} expression activates the catch and then
990 The third argument of a @code{catch} expression is a handler procedure.
991 If an exception is thrown, this procedure is called with exactly the
992 arguments specified by the @code{throw}. Therefore, the handler
993 procedure must be designed to accept a number of arguments that
994 corresponds to the number of arguments in all @code{throw} expressions
995 that can be caught by this @code{catch}.
997 The fourth, optional argument of a @code{catch} expression is another
998 handler procedure, called the @dfn{pre-unwind} handler. It differs from
999 the third argument in that if an exception is thrown, it is called,
1000 @emph{before} the third argument handler, in exactly the dynamic context
1001 of the @code{throw} expression that threw the exception. This means
1002 that it is useful for capturing or displaying the stack at the point of
1003 the @code{throw}, or for examining other aspects of the dynamic context,
1004 such as fluid values, before the context is unwound back to that of the
1005 prevailing @code{catch}.
1007 @deffn {Scheme Procedure} catch key thunk handler [pre-unwind-handler]
1008 @deffnx {C Function} scm_catch_with_pre_unwind_handler (key, thunk, handler, pre_unwind_handler)
1009 @deffnx {C Function} scm_catch (key, thunk, handler)
1010 Invoke @var{thunk} in the dynamic context of @var{handler} for
1011 exceptions matching @var{key}. If thunk throws to the symbol
1012 @var{key}, then @var{handler} is invoked this way:
1014 (handler key args ...)
1017 @var{key} is a symbol or @code{#t}.
1019 @var{thunk} takes no arguments. If @var{thunk} returns
1020 normally, that is the return value of @code{catch}.
1022 Handler is invoked outside the scope of its own @code{catch}.
1023 If @var{handler} again throws to the same key, a new handler
1024 from further up the call chain is invoked.
1026 If the key is @code{#t}, then a throw to @emph{any} symbol will
1027 match this call to @code{catch}.
1029 If a @var{pre-unwind-handler} is given and @var{thunk} throws
1030 an exception that matches @var{key}, Guile calls the
1031 @var{pre-unwind-handler} before unwinding the dynamic state and
1032 invoking the main @var{handler}. @var{pre-unwind-handler} should
1033 be a procedure with the same signature as @var{handler}, that
1034 is @code{(lambda (key . args))}. It is typically used to save
1035 the stack at the point where the exception occurred, but can also
1036 query other parts of the dynamic state at that point, such as
1039 A @var{pre-unwind-handler} can exit either normally or non-locally.
1040 If it exits normally, Guile unwinds the stack and dynamic context
1041 and then calls the normal (third argument) handler. If it exits
1042 non-locally, that exit determines the continuation.
1045 If a handler procedure needs to match a variety of @code{throw}
1046 expressions with varying numbers of arguments, you should write it like
1050 (lambda (key . args)
1055 The @var{key} argument is guaranteed always to be present, because a
1056 @code{throw} without a @var{key} is not valid. The number and
1057 interpretation of the @var{args} varies from one type of exception to
1058 another, but should be specified by the documentation for each exception
1061 Note that, once the normal (post-unwind) handler procedure is invoked,
1062 the catch that led to the handler procedure being called is no longer
1063 active. Therefore, if the handler procedure itself throws an exception,
1064 that exception can only be caught by another active catch higher up the
1065 call stack, if there is one.
1068 @deftypefn {C Function} SCM scm_c_catch (SCM tag, scm_t_catch_body body, void *body_data, scm_t_catch_handler handler, void *handler_data, scm_t_catch_handler pre_unwind_handler, void *pre_unwind_handler_data)
1069 @deftypefnx {C Function} SCM scm_internal_catch (SCM tag, scm_t_catch_body body, void *body_data, scm_t_catch_handler handler, void *handler_data)
1070 The above @code{scm_catch_with_pre_unwind_handler} and @code{scm_catch}
1071 take Scheme procedures as body and handler arguments.
1072 @code{scm_c_catch} and @code{scm_internal_catch} are equivalents taking
1075 @var{body} is called as @code{@var{body} (@var{body_data})} with a catch
1076 on exceptions of the given @var{tag} type. If an exception is caught,
1077 @var{pre_unwind_handler} and @var{handler} are called as
1078 @code{@var{handler} (@var{handler_data}, @var{key}, @var{args})}.
1079 @var{key} and @var{args} are the @code{SCM} key and argument list from
1082 @tpindex scm_t_catch_body
1083 @tpindex scm_t_catch_handler
1084 @var{body} and @var{handler} should have the following prototypes.
1085 @code{scm_t_catch_body} and @code{scm_t_catch_handler} are pointer
1089 SCM body (void *data);
1090 SCM handler (void *data, SCM key, SCM args);
1093 The @var{body_data} and @var{handler_data} parameters are passed to
1094 the respective calls so an application can communicate extra
1095 information to those functions.
1097 If the data consists of an @code{SCM} object, care should be taken
1098 that it isn't garbage collected while still required. If the
1099 @code{SCM} is a local C variable, one way to protect it is to pass a
1100 pointer to that variable as the data parameter, since the C compiler
1101 will then know the value must be held on the stack. Another way is to
1102 use @code{scm_remember_upto_here_1} (@pxref{Remembering During
1107 @node Throw Handlers
1108 @subsubsection Throw Handlers
1110 It's sometimes useful to be able to intercept an exception that is being
1111 thrown before the stack is unwound. This could be to clean up some
1112 related state, to print a backtrace, or to pass information about the
1113 exception to a debugger, for example. The @code{with-throw-handler}
1114 procedure provides a way to do this.
1116 @deffn {Scheme Procedure} with-throw-handler key thunk handler
1117 @deffnx {C Function} scm_with_throw_handler (key, thunk, handler)
1118 Add @var{handler} to the dynamic context as a throw handler
1119 for key @var{key}, then invoke @var{thunk}.
1121 This behaves exactly like @code{catch}, except that it does not unwind
1122 the stack before invoking @var{handler}. If the @var{handler} procedure
1123 returns normally, Guile rethrows the same exception again to the next
1124 innermost catch or throw handler. @var{handler} may exit nonlocally, of
1125 course, via an explicit throw or via invoking a continuation.
1128 Typically @var{handler} is used to display a backtrace of the stack at
1129 the point where the corresponding @code{throw} occurred, or to save off
1130 this information for possible display later.
1132 Not unwinding the stack means that throwing an exception that is handled
1133 via a throw handler is equivalent to calling the throw handler handler
1134 inline instead of each @code{throw}, and then omitting the surrounding
1135 @code{with-throw-handler}. In other words,
1138 (with-throw-handler 'key
1139 (lambda () @dots{} (throw 'key args @dots{}) @dots{})
1144 is mostly equivalent to
1147 ((lambda () @dots{} (handler 'key args @dots{}) @dots{}))
1150 In particular, the dynamic context when @var{handler} is invoked is that
1151 of the site where @code{throw} is called. The examples are not quite
1152 equivalent, because the body of a @code{with-throw-handler} is not in
1153 tail position with respect to the @code{with-throw-handler}, and if
1154 @var{handler} exits normally, Guile arranges to rethrow the error, but
1155 hopefully the intention is clear. (For an introduction to what is meant
1156 by dynamic context, @xref{Dynamic Wind}.)
1158 @deftypefn {C Function} SCM scm_c_with_throw_handler (SCM tag, scm_t_catch_body body, void *body_data, scm_t_catch_handler handler, void *handler_data, int lazy_catch_p)
1159 The above @code{scm_with_throw_handler} takes Scheme procedures as body
1160 (thunk) and handler arguments. @code{scm_c_with_throw_handler} is an
1161 equivalent taking C functions. See @code{scm_c_catch} (@pxref{Catch})
1162 for a description of the parameters, the behaviour however of course
1163 follows @code{with-throw-handler}.
1166 If @var{thunk} throws an exception, Guile handles that exception by
1167 invoking the innermost @code{catch} or throw handler whose key matches
1168 that of the exception. When the innermost thing is a throw handler,
1169 Guile calls the specified handler procedure using @code{(apply
1170 @var{handler} key args)}. The handler procedure may either return
1171 normally or exit non-locally. If it returns normally, Guile passes the
1172 exception on to the next innermost @code{catch} or throw handler. If it
1173 exits non-locally, that exit determines the continuation.
1175 The behaviour of a throw handler is very similar to that of a
1176 @code{catch} expression's optional pre-unwind handler. In particular, a
1177 throw handler's handler procedure is invoked in the exact dynamic
1178 context of the @code{throw} expression, just as a pre-unwind handler is.
1179 @code{with-throw-handler} may be seen as a half-@code{catch}: it does
1180 everything that a @code{catch} would do until the point where
1181 @code{catch} would start unwinding the stack and dynamic context, but
1182 then it rethrows to the next innermost @code{catch} or throw handler
1185 Note also that since the dynamic context is not unwound, if a
1186 @code{with-throw-handler} handler throws to a key that does not match
1187 the @code{with-throw-handler} expression's @var{key}, the new throw may
1188 be handled by a @code{catch} or throw handler that is @emph{closer} to
1189 the throw than the first @code{with-throw-handler}.
1191 Here is an example to illustrate this behavior:
1196 (with-throw-handler 'b
1202 (lambda (key . args)
1208 This code will call @code{inner-handler} and then continue with the
1209 continuation of the inner @code{catch}.
1213 @subsubsection Throwing Exceptions
1215 The @code{throw} primitive is used to throw an exception. One argument,
1216 the @var{key}, is mandatory, and must be a symbol; it indicates the type
1217 of exception that is being thrown. Following the @var{key},
1218 @code{throw} accepts any number of additional arguments, whose meaning
1219 depends on the exception type. The documentation for each possible type
1220 of exception should specify the additional arguments that are expected
1221 for that kind of exception.
1223 @deffn {Scheme Procedure} throw key . args
1224 @deffnx {C Function} scm_throw (key, args)
1225 Invoke the catch form matching @var{key}, passing @var{args} to the
1228 @var{key} is a symbol. It will match catches of the same symbol or of
1231 If there is no handler at all, Guile prints an error and then exits.
1234 When an exception is thrown, it will be caught by the innermost
1235 @code{catch} or throw handler that applies to the type of the thrown
1236 exception; in other words, whose @var{key} is either @code{#t} or the
1237 same symbol as that used in the @code{throw} expression. Once Guile has
1238 identified the appropriate @code{catch} or throw handler, it handles the
1239 exception by applying the relevant handler procedure(s) to the arguments
1240 of the @code{throw}.
1242 If there is no appropriate @code{catch} or throw handler for a thrown
1243 exception, Guile prints an error to the current error port indicating an
1244 uncaught exception, and then exits. In practice, it is quite difficult
1245 to observe this behaviour, because Guile when used interactively
1246 installs a top level @code{catch} handler that will catch all exceptions
1247 and print an appropriate error message @emph{without} exiting. For
1248 example, this is what happens if you try to throw an unhandled exception
1249 in the standard Guile REPL; note that Guile's command loop continues
1250 after the error message:
1253 guile> (throw 'badex)
1254 <unnamed port>:3:1: In procedure gsubr-apply @dots{}
1255 <unnamed port>:3:1: unhandled-exception: badex
1260 The default uncaught exception behaviour can be observed by evaluating a
1261 @code{throw} expression from the shell command line:
1264 $ guile -c "(begin (throw 'badex) (display \"here\\n\"))"
1265 guile: uncaught throw to badex: ()
1270 That Guile exits immediately following the uncaught exception
1271 is shown by the absence of any output from the @code{display}
1272 expression, because Guile never gets to the point of evaluating that
1276 @node Exception Implementation
1277 @subsubsection How Guile Implements Exceptions
1279 It is traditional in Scheme to implement exception systems using
1280 @code{call-with-current-continuation}. Continuations
1281 (@pxref{Continuations}) are such a powerful concept that any other
1282 control mechanism --- including @code{catch} and @code{throw} --- can be
1283 implemented in terms of them.
1285 Guile does not implement @code{catch} and @code{throw} like this,
1286 though. Why not? Because Guile is specifically designed to be easy to
1287 integrate with applications written in C. In a mixed Scheme/C
1288 environment, the concept of @dfn{continuation} must logically include
1289 ``what happens next'' in the C parts of the application as well as the
1290 Scheme parts, and it turns out that the only reasonable way of
1291 implementing continuations like this is to save and restore the complete
1294 So Guile's implementation of @code{call-with-current-continuation} is a
1295 stack copying one. This allows it to interact well with ordinary C
1296 code, but means that creating and calling a continuation is slowed down
1297 by the time that it takes to copy the C stack.
1299 The more targeted mechanism provided by @code{catch} and @code{throw}
1300 does not need to save and restore the C stack because the @code{throw}
1301 always jumps to a location higher up the stack of the code that executes
1302 the @code{throw}. Therefore Guile implements the @code{catch} and
1303 @code{throw} primitives independently of
1304 @code{call-with-current-continuation}, in a way that takes advantage of
1305 this @emph{upwards only} nature of exceptions.
1308 @node Error Reporting
1309 @subsection Procedures for Signaling Errors
1311 Guile provides a set of convenience procedures for signaling error
1312 conditions that are implemented on top of the exception primitives just
1315 @deffn {Scheme Procedure} error msg args @dots{}
1316 Raise an error with key @code{misc-error} and a message constructed by
1317 displaying @var{msg} and writing @var{args}.
1320 @deffn {Scheme Procedure} scm-error key subr message args data
1321 @deffnx {C Function} scm_error_scm (key, subr, message, args, data)
1322 Raise an error with key @var{key}. @var{subr} can be a string
1323 naming the procedure associated with the error, or @code{#f}.
1324 @var{message} is the error message string, possibly containing
1325 @code{~S} and @code{~A} escapes. When an error is reported,
1326 these are replaced by formatting the corresponding members of
1327 @var{args}: @code{~A} (was @code{%s} in older versions of
1328 Guile) formats using @code{display} and @code{~S} (was
1329 @code{%S}) formats using @code{write}. @var{data} is a list or
1330 @code{#f} depending on @var{key}: if @var{key} is
1331 @code{system-error} then it should be a list containing the
1332 Unix @code{errno} value; If @var{key} is @code{signal} then it
1333 should be a list containing the Unix signal number; If
1334 @var{key} is @code{out-of-range} or @code{wrong-type-arg},
1335 it is a list containing the bad value; otherwise
1336 it will usually be @code{#f}.
1339 @deffn {Scheme Procedure} strerror err
1340 @deffnx {C Function} scm_strerror (err)
1341 Return the Unix error message corresponding to @var{err}, an integer
1344 When @code{setlocale} has been called (@pxref{Locales}), the message
1345 is in the language and charset of @code{LC_MESSAGES}. (This is done
1349 @c begin (scm-doc-string "boot-9.scm" "false-if-exception")
1350 @deffn syntax false-if-exception expr
1351 Returns the result of evaluating its argument; however
1352 if an exception occurs then @code{#f} is returned instead.
1358 @subsection Dynamic Wind
1360 For Scheme code, the fundamental procedure to react to non-local entry
1361 and exits of dynamic contexts is @code{dynamic-wind}. C code could
1362 use @code{scm_internal_dynamic_wind}, but since C does not allow the
1363 convenient construction of anonymous procedures that close over
1364 lexical variables, this will be, well, inconvenient.
1366 Therefore, Guile offers the functions @code{scm_dynwind_begin} and
1367 @code{scm_dynwind_end} to delimit a dynamic extent. Within this
1368 dynamic extent, which is called a @dfn{dynwind context}, you can
1369 perform various @dfn{dynwind actions} that control what happens when
1370 the dynwind context is entered or left. For example, you can register
1371 a cleanup routine with @code{scm_dynwind_unwind_handler} that is
1372 executed when the context is left. There are several other more
1373 specialized dynwind actions as well, for example to temporarily block
1374 the execution of asyncs or to temporarily change the current output
1375 port. They are described elsewhere in this manual.
1377 Here is an example that shows how to prevent memory leaks.
1381 /* Suppose there is a function called FOO in some library that you
1382 would like to make available to Scheme code (or to C code that
1383 follows the Scheme conventions).
1385 FOO takes two C strings and returns a new string. When an error has
1386 occurred in FOO, it returns NULL.
1389 char *foo (char *s1, char *s2);
1391 /* SCM_FOO interfaces the C function FOO to the Scheme way of life.
1392 It takes care to free up all temporary strings in the case of
1397 scm_foo (SCM s1, SCM s2)
1399 char *c_s1, *c_s2, *c_res;
1401 scm_dynwind_begin (0);
1403 c_s1 = scm_to_locale_string (s1);
1405 /* Call 'free (c_s1)' when the dynwind context is left.
1407 scm_dynwind_unwind_handler (free, c_s1, SCM_F_WIND_EXPLICITLY);
1409 c_s2 = scm_to_locale_string (s2);
1411 /* Same as above, but more concisely.
1413 scm_dynwind_free (c_s2);
1415 c_res = foo (c_s1, c_s2);
1417 scm_memory_error ("foo");
1421 return scm_take_locale_string (res);
1425 @rnindex dynamic-wind
1426 @deffn {Scheme Procedure} dynamic-wind in_guard thunk out_guard
1427 @deffnx {C Function} scm_dynamic_wind (in_guard, thunk, out_guard)
1428 All three arguments must be 0-argument procedures.
1429 @var{in_guard} is called, then @var{thunk}, then
1432 If, any time during the execution of @var{thunk}, the
1433 dynamic extent of the @code{dynamic-wind} expression is escaped
1434 non-locally, @var{out_guard} is called. If the dynamic extent of
1435 the dynamic-wind is re-entered, @var{in_guard} is called. Thus
1436 @var{in_guard} and @var{out_guard} may be called any number of
1440 (define x 'normal-binding)
1443 (call-with-current-continuation
1449 (lambda () (set! x 'special-binding))
1453 (lambda () (display x) (newline)
1454 (call-with-current-continuation escape)
1455 (display x) (newline)
1460 (lambda () (set! x old-x)))))))
1466 @result{} normal-binding
1471 @result{} a-cont ;; the value of the (define a-cont...)
1473 @result{} normal-binding
1475 @result{} special-binding
1479 @deftp {C Type} scm_t_dynwind_flags
1480 This is an enumeration of several flags that modify the behavior of
1481 @code{scm_dynwind_begin}. The flags are listed in the following
1485 @item SCM_F_DYNWIND_REWINDABLE
1486 The dynamic context is @dfn{rewindable}. This means that it can be
1487 reentered non-locally (via the invocation of a continuation). The
1488 default is that a dynwind context can not be reentered non-locally.
1493 @deftypefn {C Function} void scm_dynwind_begin (scm_t_dynwind_flags flags)
1494 The function @code{scm_dynwind_begin} starts a new dynamic context and
1495 makes it the `current' one.
1497 The @var{flags} argument determines the default behavior of the
1498 context. Normally, use 0. This will result in a context that can not
1499 be reentered with a captured continuation. When you are prepared to
1500 handle reentries, include @code{SCM_F_DYNWIND_REWINDABLE} in
1503 Being prepared for reentry means that the effects of unwind handlers
1504 can be undone on reentry. In the example above, we want to prevent a
1505 memory leak on non-local exit and thus register an unwind handler that
1506 frees the memory. But once the memory is freed, we can not get it
1507 back on reentry. Thus reentry can not be allowed.
1509 The consequence is that continuations become less useful when
1510 non-reentrant contexts are captured, but you don't need to worry
1511 about that too much.
1513 The context is ended either implicitly when a non-local exit happens,
1514 or explicitly with @code{scm_dynwind_end}. You must make sure that a
1515 dynwind context is indeed ended properly. If you fail to call
1516 @code{scm_dynwind_end} for each @code{scm_dynwind_begin}, the behavior
1520 @deftypefn {C Function} void scm_dynwind_end ()
1521 End the current dynamic context explicitly and make the previous one
1525 @deftp {C Type} scm_t_wind_flags
1526 This is an enumeration of several flags that modify the behavior of
1527 @code{scm_dynwind_unwind_handler} and
1528 @code{scm_dynwind_rewind_handler}. The flags are listed in the
1532 @item SCM_F_WIND_EXPLICITLY
1533 @vindex SCM_F_WIND_EXPLICITLY
1534 The registered action is also carried out when the dynwind context is
1535 entered or left locally.
1539 @deftypefn {C Function} void scm_dynwind_unwind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
1540 @deftypefnx {C Function} void scm_dynwind_unwind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
1541 Arranges for @var{func} to be called with @var{data} as its arguments
1542 when the current context ends implicitly. If @var{flags} contains
1543 @code{SCM_F_WIND_EXPLICITLY}, @var{func} is also called when the
1544 context ends explicitly with @code{scm_dynwind_end}.
1546 The function @code{scm_dynwind_unwind_handler_with_scm} takes care that
1547 @var{data} is protected from garbage collection.
1550 @deftypefn {C Function} void scm_dynwind_rewind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
1551 @deftypefnx {C Function} void scm_dynwind_rewind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
1552 Arrange for @var{func} to be called with @var{data} as its argument when
1553 the current context is restarted by rewinding the stack. When @var{flags}
1554 contains @code{SCM_F_WIND_EXPLICITLY}, @var{func} is called immediately
1557 The function @code{scm_dynwind_rewind_handler_with_scm} takes care that
1558 @var{data} is protected from garbage collection.
1561 @deftypefn {C Function} void scm_dynwind_free (void *mem)
1562 Arrange for @var{mem} to be freed automatically whenever the current
1563 context is exited, whether normally or non-locally.
1564 @code{scm_dynwind_free (mem)} is an equivalent shorthand for
1565 @code{scm_dynwind_unwind_handler (free, mem, SCM_F_WIND_EXPLICITLY)}.
1569 @node Handling Errors
1570 @subsection How to Handle Errors
1572 Error handling is based on @code{catch} and @code{throw}. Errors are
1573 always thrown with a @var{key} and four arguments:
1577 @var{key}: a symbol which indicates the type of error. The symbols used
1578 by libguile are listed below.
1581 @var{subr}: the name of the procedure from which the error is thrown, or
1585 @var{message}: a string (possibly language and system dependent)
1586 describing the error. The tokens @code{~A} and @code{~S} can be
1587 embedded within the message: they will be replaced with members of the
1588 @var{args} list when the message is printed. @code{~A} indicates an
1589 argument printed using @code{display}, while @code{~S} indicates an
1590 argument printed using @code{write}. @var{message} can also be
1591 @code{#f}, to allow it to be derived from the @var{key} by the error
1592 handler (may be useful if the @var{key} is to be thrown from both C and
1596 @var{args}: a list of arguments to be used to expand @code{~A} and
1597 @code{~S} tokens in @var{message}. Can also be @code{#f} if no
1598 arguments are required.
1601 @var{rest}: a list of any additional objects required. e.g., when the
1602 key is @code{'system-error}, this contains the C errno value. Can also
1603 be @code{#f} if no additional objects are required.
1606 In addition to @code{catch} and @code{throw}, the following Scheme
1607 facilities are available:
1609 @deffn {Scheme Procedure} display-error frame port subr message args rest
1610 @deffnx {C Function} scm_display_error (frame, port, subr, message, args, rest)
1611 Display an error message to the output port @var{port}.
1612 @var{frame} is the frame in which the error occurred, @var{subr} is
1613 the name of the procedure in which the error occurred and
1614 @var{message} is the actual error message, which may contain
1615 formatting instructions. These will format the arguments in
1616 the list @var{args} accordingly. @var{rest} is currently
1620 The following are the error keys defined by libguile and the situations
1621 in which they are used:
1625 @cindex @code{error-signal}
1626 @code{error-signal}: thrown after receiving an unhandled fatal signal
1627 such as SIGSEGV, SIGBUS, SIGFPE etc. The @var{rest} argument in the throw
1628 contains the coded signal number (at present this is not the same as the
1629 usual Unix signal number).
1632 @cindex @code{system-error}
1633 @code{system-error}: thrown after the operating system indicates an
1634 error condition. The @var{rest} argument in the throw contains the
1638 @cindex @code{numerical-overflow}
1639 @code{numerical-overflow}: numerical overflow.
1642 @cindex @code{out-of-range}
1643 @code{out-of-range}: the arguments to a procedure do not fall within the
1647 @cindex @code{wrong-type-arg}
1648 @code{wrong-type-arg}: an argument to a procedure has the wrong type.
1651 @cindex @code{wrong-number-of-args}
1652 @code{wrong-number-of-args}: a procedure was called with the wrong number
1656 @cindex @code{memory-allocation-error}
1657 @code{memory-allocation-error}: memory allocation error.
1660 @cindex @code{stack-overflow}
1661 @code{stack-overflow}: stack overflow error.
1664 @cindex @code{regular-expression-syntax}
1665 @code{regular-expression-syntax}: errors generated by the regular
1669 @cindex @code{misc-error}
1670 @code{misc-error}: other errors.
1674 @subsubsection C Support
1676 In the following C functions, @var{SUBR} and @var{MESSAGE} parameters
1677 can be @code{NULL} to give the effect of @code{#f} described above.
1679 @deftypefn {C Function} SCM scm_error (SCM @var{key}, char *@var{subr}, char *@var{message}, SCM @var{args}, SCM @var{rest})
1680 Throw an error, as per @code{scm-error} (@pxref{Error Reporting}).
1683 @deftypefn {C Function} void scm_syserror (char *@var{subr})
1684 @deftypefnx {C Function} void scm_syserror_msg (char *@var{subr}, char *@var{message}, SCM @var{args})
1685 Throw an error with key @code{system-error} and supply @code{errno} in
1686 the @var{rest} argument. For @code{scm_syserror} the message is
1687 generated using @code{strerror}.
1689 Care should be taken that any code in between the failing operation
1690 and the call to these routines doesn't change @code{errno}.
1693 @deftypefn {C Function} void scm_num_overflow (char *@var{subr})
1694 @deftypefnx {C Function} void scm_out_of_range (char *@var{subr}, SCM @var{bad_value})
1695 @deftypefnx {C Function} void scm_wrong_num_args (SCM @var{proc})
1696 @deftypefnx {C Function} void scm_wrong_type_arg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value})
1697 @deftypefnx {C Function} void scm_wrong_type_arg_msg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value}, const char *@var{expected})
1698 @deftypefnx {C Function} void scm_memory_error (char *@var{subr})
1699 Throw an error with the various keys described above.
1700 @deftypefnx {C Function} void scm_misc_error (const char *@var{subr}, const char *@var{message}, SCM @var{args})
1702 In @code{scm_wrong_num_args}, @var{proc} should be a Scheme symbol
1703 which is the name of the procedure incorrectly invoked. The other
1704 routines take the name of the invoked procedure as a C string.
1706 In @code{scm_wrong_type_arg_msg}, @var{expected} is a C string
1707 describing the type of argument that was expected.
1709 In @code{scm_misc_error}, @var{message} is the error message string,
1710 possibly containing @code{simple-format} escapes (@pxref{Writing}), and
1711 the corresponding arguments in the @var{args} list.
1715 @subsubsection Signalling Type Errors
1717 Every function visible at the Scheme level should aggressively check the
1718 types of its arguments, to avoid misinterpreting a value, and perhaps
1719 causing a segmentation fault. Guile provides some macros to make this
1722 @deftypefn Macro void SCM_ASSERT (int @var{test}, SCM @var{obj}, unsigned int @var{position}, const char *@var{subr})
1723 @deftypefnx Macro void SCM_ASSERT_TYPE (int @var{test}, SCM @var{obj}, unsigned int @var{position}, const char *@var{subr}, const char *@var{expected})
1724 If @var{test} is zero, signal a ``wrong type argument'' error,
1725 attributed to the subroutine named @var{subr}, operating on the value
1726 @var{obj}, which is the @var{position}'th argument of @var{subr}.
1728 In @code{SCM_ASSERT_TYPE}, @var{expected} is a C string describing the
1729 type of argument that was expected.
1732 @deftypefn Macro int SCM_ARG1
1733 @deftypefnx Macro int SCM_ARG2
1734 @deftypefnx Macro int SCM_ARG3
1735 @deftypefnx Macro int SCM_ARG4
1736 @deftypefnx Macro int SCM_ARG5
1737 @deftypefnx Macro int SCM_ARG6
1738 @deftypefnx Macro int SCM_ARG7
1739 One of the above values can be used for @var{position} to indicate the
1740 number of the argument of @var{subr} which is being checked.
1741 Alternatively, a positive integer number can be used, which allows to
1742 check arguments after the seventh. However, for parameter numbers up to
1743 seven it is preferable to use @code{SCM_ARGN} instead of the
1744 corresponding raw number, since it will make the code easier to
1748 @deftypefn Macro int SCM_ARGn
1749 Passing a value of zero or @code{SCM_ARGn} for @var{position} allows to
1750 leave it unspecified which argument's type is incorrect. Again,
1751 @code{SCM_ARGn} should be preferred over a raw zero constant.
1754 @node Continuation Barriers
1755 @subsection Continuation Barriers
1757 The non-local flow of control caused by continuations might sometimes
1758 not be wanted. You can use @code{with-continuation-barrier} to erect
1759 fences that continuations can not pass.
1761 @deffn {Scheme Procedure} with-continuation-barrier proc
1762 @deffnx {C Function} scm_with_continuation_barrier (proc)
1763 Call @var{proc} and return its result. Do not allow the invocation of
1764 continuations that would leave or enter the dynamic extent of the call
1765 to @code{with-continuation-barrier}. Such an attempt causes an error
1768 Throws (such as errors) that are not caught from within @var{proc} are
1769 caught by @code{with-continuation-barrier}. In that case, a short
1770 message is printed to the current error port and @code{#f} is returned.
1772 Thus, @code{with-continuation-barrier} returns exactly once.
1775 @deftypefn {C Function} {void *} scm_c_with_continuation_barrier (void *(*func) (void *), void *data)
1776 Like @code{scm_with_continuation_barrier} but call @var{func} on
1777 @var{data}. When an error is caught, @code{NULL} is returned.
1782 @c TeX-master: "guile.texi"