2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010,
4 @c 2011, 2012, 2013, 2014 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 expr @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}. If 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, and the @var{clause}s must have the form
218 ((@var{datum1} @dots{}) @var{expr1} @var{expr2} @dots{})
224 ((@var{datum1} @dots{}) => @var{expression})
227 and the last @var{clause} may have the form
230 (else @var{expr1} @var{expr2} @dots{})
236 (else => @var{expression})
239 All @var{datum}s must be distinct. First, @var{key} is evaluated. The
240 result of this evaluation is compared against all @var{datum} values using
241 @code{eqv?}. When this comparison succeeds, the expression(s) following
242 the @var{datum} are evaluated from left to right, returning the value of
243 the last expression as the result of the @code{case} expression.
245 If the @var{key} matches no @var{datum} and there is an
246 @code{else}-clause, the expressions following the @code{else} are
247 evaluated. If there is no such clause, the result of the expression is
250 For the @code{=>} clause types, @var{expression} is evaluated and the
251 resulting procedure is applied to the value of @var{key}. The result of
252 this procedure application is then the result of the
253 @code{case}-expression.
258 @subsection Conditional Evaluation of a Sequence of Expressions
260 @code{and} and @code{or} evaluate all their arguments in order, similar
261 to @code{begin}, but evaluation stops as soon as one of the expressions
262 evaluates to false or true, respectively.
264 @deffn syntax and expr @dots{}
265 Evaluate the @var{expr}s from left to right and stop evaluation as soon
266 as one expression evaluates to @code{#f}; the remaining expressions are
267 not evaluated. The value of the last evaluated expression is returned.
268 If no expression evaluates to @code{#f}, the value of the last
269 expression is returned.
271 If used without expressions, @code{#t} is returned.
274 @deffn syntax or expr @dots{}
275 Evaluate the @var{expr}s from left to right and stop evaluation as soon
276 as one expression evaluates to a true value (that is, a value different
277 from @code{#f}); the remaining expressions are not evaluated. The value
278 of the last evaluated expression is returned. If all expressions
279 evaluate to @code{#f}, @code{#f} is returned.
281 If used without expressions, @code{#f} is returned.
286 @subsection Iteration mechanisms
292 Scheme has only few iteration mechanisms, mainly because iteration in
293 Scheme programs is normally expressed using recursion. Nevertheless,
294 R5RS defines a construct for programming loops, calling @code{do}. In
295 addition, Guile has an explicit looping syntax called @code{while}.
297 @deffn syntax do ((variable init [step]) @dots{}) (test expr @dots{}) body @dots{}
298 Bind @var{variable}s and evaluate @var{body} until @var{test} is true.
299 The return value is the last @var{expr} after @var{test}, if given. A
300 simple example will illustrate the basic form,
310 Or with two variables and a final return value,
317 (format #t "3**~s is ~s\n" i p))
327 The @var{variable} bindings are established like a @code{let}, in that
328 the expressions are all evaluated and then all bindings made. When
329 iterating, the optional @var{step} expressions are evaluated with the
330 previous bindings in scope, then new bindings all made.
332 The @var{test} expression is a termination condition. Looping stops
333 when the @var{test} is true. It's evaluated before running the
334 @var{body} each time, so if it's true the first time then @var{body}
337 The optional @var{expr}s after the @var{test} are evaluated at the end
338 of looping, with the final @var{variable} bindings available. The
339 last @var{expr} gives the return value, or if there are no @var{expr}s
340 the return value is unspecified.
342 Each iteration establishes bindings to fresh locations for the
343 @var{variable}s, like a new @code{let} for each iteration. This is
344 done for @var{variable}s without @var{step} expressions too. The
345 following illustrates this, showing how a new @code{i} is captured by
346 the @code{lambda} in each iteration (@pxref{About Closure,, The
347 Concept of Closure}).
353 (set! lst (cons (lambda () i) lst)))
354 (map (lambda (proc) (proc)) lst)
360 @deffn syntax while cond body @dots{}
361 Run a loop executing the @var{body} forms while @var{cond} is true.
362 @var{cond} is tested at the start of each iteration, so if it's
363 @code{#f} the first time then @var{body} is not executed at all.
365 Within @code{while}, two extra bindings are provided, they can be used
366 from both @var{cond} and @var{body}.
368 @deffn {Scheme Procedure} break break-arg @dots{}
369 Break out of the @code{while} form.
372 @deffn {Scheme Procedure} continue
373 Abandon the current iteration, go back to the start and test
374 @var{cond} again, etc.
377 If the loop terminates normally, by the @var{cond} evaluating to
378 @code{#f}, then the @code{while} expression as a whole evaluates to
379 @code{#f}. If it terminates by a call to @code{break} with some number
380 of arguments, those arguments are returned from the @code{while}
381 expression, as multiple values. Otherwise if it terminates by a call to
382 @code{break} with no arguments, then return value is @code{#t}.
385 (while #f (error "not reached")) @result{} #f
386 (while #t (break)) @result{} #t
387 (while #t (break 1 2 3)) @result{} 1 2 3
390 Each @code{while} form gets its own @code{break} and @code{continue}
391 procedures, operating on that @code{while}. This means when loops are
392 nested the outer @code{break} can be used to escape all the way out.
397 (let ((outer-break break))
404 Note that each @code{break} and @code{continue} procedure can only be
405 used within the dynamic extent of its @code{while}. Outside the
406 @code{while} their behaviour is unspecified.
410 Another very common way of expressing iteration in Scheme programs is
411 the use of the so-called @dfn{named let}.
413 Named let is a variant of @code{let} which creates a procedure and calls
414 it in one step. Because of the newly created procedure, named let is
415 more powerful than @code{do}--it can be used for iteration, but also
416 for arbitrary recursion.
418 @deffn syntax let variable bindings body
419 For the definition of @var{bindings} see the documentation about
420 @code{let} (@pxref{Local Bindings}).
422 Named @code{let} works as follows:
426 A new procedure which accepts as many arguments as are in @var{bindings}
427 is created and bound locally (using @code{let}) to @var{variable}. The
428 new procedure's formal argument names are the name of the
432 The @var{body} expressions are inserted into the newly created procedure.
435 The procedure is called with the @var{init} expressions as the formal
439 The next example implements a loop which iterates (by recursion) 1000
456 @cindex delimited continuations
457 @cindex composable continuations
458 @cindex non-local exit
460 Prompts are control-flow barriers between different parts of a program. In the
461 same way that a user sees a shell prompt (e.g., the Bash prompt) as a barrier
462 between the operating system and her programs, Scheme prompts allow the Scheme
463 programmer to treat parts of programs as if they were running in different
466 We use this roundabout explanation because, unless you're a functional
467 programming junkie, you probably haven't heard the term, ``delimited, composable
468 continuation''. That's OK; it's a relatively recent topic, but a very useful
472 * Prompt Primitives:: Call-with-prompt and abort-to-prompt.
473 * Shift and Reset:: The zoo of delimited control operators.
476 @node Prompt Primitives
477 @subsubsection Prompt Primitives
479 Guile's primitive delimited control operators are
480 @code{call-with-prompt} and @code{abort-to-prompt}.
482 @deffn {Scheme Procedure} call-with-prompt tag thunk handler
483 Set up a prompt, and call @var{thunk} within that prompt.
485 During the dynamic extent of the call to @var{thunk}, a prompt named @var{tag}
486 will be present in the dynamic context, such that if a user calls
487 @code{abort-to-prompt} (see below) with that tag, control rewinds back to the
488 prompt, and the @var{handler} is run.
490 @var{handler} must be a procedure. The first argument to @var{handler} will be
491 the state of the computation begun when @var{thunk} was called, and ending with
492 the call to @code{abort-to-prompt}. The remaining arguments to @var{handler} are
493 those passed to @code{abort-to-prompt}.
496 @deffn {Scheme Procedure} make-prompt-tag [stem]
497 Make a new prompt tag. Currently prompt tags are generated symbols.
498 This may change in some future Guile version.
501 @deffn {Scheme Procedure} default-prompt-tag
502 Return the default prompt tag. Having a distinguished default prompt
503 tag allows some useful prompt and abort idioms, discussed in the next
507 @deffn {Scheme Procedure} abort-to-prompt tag val1 val2 @dots{}
508 Unwind the dynamic and control context to the nearest prompt named @var{tag},
509 also passing the given values.
512 C programmers may recognize @code{call-with-prompt} and @code{abort-to-prompt}
513 as a fancy kind of @code{setjmp} and @code{longjmp}, respectively. Prompts are
514 indeed quite useful as non-local escape mechanisms. Guile's @code{catch} and
515 @code{throw} are implemented in terms of prompts. Prompts are more convenient
516 than @code{longjmp}, in that one has the opportunity to pass multiple values to
519 Also unlike @code{longjmp}, the prompt handler is given the full state of the
520 process that was aborted, as the first argument to the prompt's handler. That
521 state is the @dfn{continuation} of the computation wrapped by the prompt. It is
522 a @dfn{delimited continuation}, because it is not the whole continuation of the
523 program; rather, just the computation initiated by the call to
524 @code{call-with-prompt}.
526 The continuation is a procedure, and may be reinstated simply by invoking it,
527 with any number of values. Here's where things get interesting, and complicated
528 as well. Besides being described as delimited, continuations reified by prompts
529 are also @dfn{composable}, because invoking a prompt-saved continuation composes
530 that continuation with the current one.
532 Imagine you have saved a continuation via call-with-prompt:
541 (+ 34 (abort-to-prompt 'foo)))
546 The resulting continuation is the addition of 34. It's as if you had written:
554 So, if we call @code{cont} with one numeric value, we get that number,
564 The last example illustrates what we mean when we say, "composes with the
565 current continuation". We mean that there is a current continuation -- some
566 remaining things to compute, like @code{(lambda (x) (* x 2))} -- and that
567 calling the saved continuation doesn't wipe out the current continuation, it
568 composes the saved continuation with the current one.
570 We're belaboring the point here because traditional Scheme continuations, as
571 discussed in the next section, aren't composable, and are actually less
572 expressive than continuations captured by prompts. But there's a place for them
575 Before moving on, we should mention that if the handler of a prompt is a
576 @code{lambda} expression, and the first argument isn't referenced, an abort to
577 that prompt will not cause a continuation to be reified. This can be an
578 important efficiency consideration to keep in mind.
580 @cindex continuation, escape
581 One example where this optimization matters is @dfn{escape
582 continuations}. Escape continuations are delimited continuations whose
583 only use is to make a non-local exit---i.e., to escape from the current
584 continuation. Such continuations are invoked only once, and for this
585 reason they are sometimes called @dfn{one-shot continuations}. A common
586 use of escape continuations is when throwing an exception
587 (@pxref{Exceptions}).
589 The constructs below are syntactic sugar atop prompts to simplify the
590 use of escape continuations.
592 @deffn {Scheme Procedure} call-with-escape-continuation proc
593 @deffnx {Scheme Procedure} call/ec proc
594 Call @var{proc} with an escape continuation.
596 In the example below, the @var{return} continuation is used to escape
597 the continuation of the call to @code{fold}.
600 (use-modules (ice-9 control)
603 (define (prefix x lst)
604 ;; Return all the elements before the first occurrence
608 (fold (lambda (element prefix)
609 (if (equal? element x)
610 (return (reverse prefix)) ; escape `fold'
611 (cons element prefix)))
615 (prefix 'a '(0 1 2 a 3 4 5))
620 @deffn {Scheme Syntax} let-escape-continuation k body @dots{}
621 @deffnx {Scheme Syntax} let/ec k body @dots{}
622 Bind @var{k} within @var{body} to an escape continuation.
624 This is equivalent to
625 @code{(call/ec (lambda (@var{k}) @var{body} @dots{}))}.
629 @node Shift and Reset
630 @subsubsection Shift, Reset, and All That
632 There is a whole zoo of delimited control operators, and as it does not
633 seem to be a bounded set, Guile implements support for them in a
637 (use-modules (ice-9 control))
640 Firstly, we have a helpful abbreviation for the @code{call-with-prompt}
643 @deffn {Scheme Syntax} % expr
644 @deffnx {Scheme Syntax} % expr handler
645 @deffnx {Scheme Syntax} % tag expr handler
646 Evaluate @var{expr} in a prompt, optionally specifying a tag and a
647 handler. If no tag is given, the default prompt tag is used.
649 If no handler is given, a default handler is installed. The default
650 handler accepts a procedure of one argument, which will called on the
651 captured continuation, within a prompt.
653 Sometimes it's easier just to show code, as in this case:
656 (define (default-prompt-handler k proc)
657 (% (default-prompt-tag)
659 default-prompt-handler))
662 The @code{%} symbol is chosen because it looks like a prompt.
665 Likewise there is an abbreviation for @code{abort-to-prompt}, which
666 assumes the default prompt tag:
668 @deffn {Scheme Procedure} abort val1 val2 @dots{}
669 Abort to the default prompt tag, passing @var{val1} @var{val2} @dots{}
673 As mentioned before, @code{(ice-9 control)} also provides other
674 delimited control operators. This section is a bit technical, and
675 first-time users of delimited continuations should probably come back to
676 it after some practice with @code{%}.
678 Still here? So, when one implements a delimited control operator like
679 @code{call-with-prompt}, one needs to make two decisions. Firstly, does
680 the handler run within or outside the prompt? Having the handler run
681 within the prompt allows an abort inside the handler to return to the
682 same prompt handler, which is often useful. However it prevents tail
683 calls from the handler, so it is less general.
685 Similarly, does invoking a captured continuation reinstate a prompt?
686 Again we have the tradeoff of convenience versus proper tail calls.
688 These decisions are captured in the Felleisen @dfn{F} operator. If
689 neither the continuations nor the handlers implicitly add a prompt, the
690 operator is known as @dfn{--F--}. This is the case for Guile's
691 @code{call-with-prompt} and @code{abort-to-prompt}.
693 If both continuation and handler implicitly add prompts, then the
694 operator is @dfn{+F+}. @code{shift} and @code{reset} are such
697 @deffn {Scheme Syntax} reset body1 body2 @dots{}
698 Establish a prompt, and evaluate @var{body1} @var{body2} @dots{} within
701 The prompt handler is designed to work with @code{shift}, described
705 @deffn {Scheme Syntax} shift cont body1 body2 @dots{}
706 Abort to the nearest @code{reset}, and evaluate @var{body1} @var{body2}
707 @dots{} in a context in which the captured continuation is bound to
710 As mentioned above, taken together, the @var{body1} @var{body2} @dots{}
711 expressions and the invocations of @var{cont} implicitly establish a
715 Interested readers are invited to explore Oleg Kiselyov's wonderful web
716 site at @uref{http://okmij.org/ftp/}, for more information on these
721 @subsection Continuations
722 @cindex continuations
724 A ``continuation'' is the code that will execute when a given function
725 or expression returns. For example, consider
730 (display (bar)) (newline)
734 The continuation from the call to @code{bar} comprises a
735 @code{display} of the value returned, a @code{newline} and an
736 @code{exit}. This can be expressed as a function of one argument.
740 (display r) (newline)
744 In Scheme, continuations are represented as special procedures just
745 like this. The special property is that when a continuation is called
746 it abandons the current program location and jumps directly to that
747 represented by the continuation.
749 A continuation is like a dynamic label, capturing at run-time a point
750 in program execution, including all the nested calls that have lead to
751 it (or rather the code that will execute when those calls return).
753 Continuations are created with the following functions.
755 @deffn {Scheme Procedure} call-with-current-continuation proc
756 @deffnx {Scheme Procedure} call/cc proc
757 @rnindex call-with-current-continuation
758 Capture the current continuation and call @code{(@var{proc}
759 @var{cont})} with it. The return value is the value returned by
760 @var{proc}, or when @code{(@var{cont} @var{value})} is later invoked,
761 the return is the @var{value} passed.
763 Normally @var{cont} should be called with one argument, but when the
764 location resumed is expecting multiple values (@pxref{Multiple
765 Values}) then they should be passed as multiple arguments, for
766 instance @code{(@var{cont} @var{x} @var{y} @var{z})}.
768 @var{cont} may only be used from the same side of a continuation
769 barrier as it was created (@pxref{Continuation Barriers}), and in a
770 multi-threaded program only from the thread in which it was created.
772 The call to @var{proc} is not part of the continuation captured, it runs
773 only when the continuation is created. Often a program will want to
774 store @var{cont} somewhere for later use; this can be done in
777 The @code{call} in the name @code{call-with-current-continuation}
778 refers to the way a call to @var{proc} gives the newly created
779 continuation. It's not related to the way a call is used later to
780 invoke that continuation.
782 @code{call/cc} is an alias for @code{call-with-current-continuation}.
783 This is in common use since the latter is rather long.
788 Here is a simple example,
792 (format #t "the return is ~a\n"
796 @result{} the return is 1
799 @result{} the return is 2
802 @code{call/cc} captures a continuation in which the value returned is
803 going to be displayed by @code{format}. The @code{lambda} stores this
804 in @code{kont} and gives an initial return @code{1} which is
805 displayed. The later invocation of @code{kont} resumes the captured
806 point, but this time returning @code{2}, which is displayed.
808 When Guile is run interactively, a call to @code{format} like this has
809 an implicit return back to the read-eval-print loop. @code{call/cc}
810 captures that like any other return, which is why interactively
811 @code{kont} will come back to read more input.
814 C programmers may note that @code{call/cc} is like @code{setjmp} in
815 the way it records at runtime a point in program execution. A call to
816 a continuation is like a @code{longjmp} in that it abandons the
817 present location and goes to the recorded one. Like @code{longjmp},
818 the value passed to the continuation is the value returned by
819 @code{call/cc} on resuming there. However @code{longjmp} can only go
820 up the program stack, but the continuation mechanism can go anywhere.
822 When a continuation is invoked, @code{call/cc} and subsequent code
823 effectively ``returns'' a second time. It can be confusing to imagine
824 a function returning more times than it was called. It may help
825 instead to think of it being stealthily re-entered and then program
826 flow going on as normal.
828 @code{dynamic-wind} (@pxref{Dynamic Wind}) can be used to ensure setup
829 and cleanup code is run when a program locus is resumed or abandoned
830 through the continuation mechanism.
833 Continuations are a powerful mechanism, and can be used to implement
834 almost any sort of control structure, such as loops, coroutines, or
837 However the implementation of continuations in Guile is not as
838 efficient as one might hope, because Guile is designed to cooperate
839 with programs written in other languages, such as C, which do not know
840 about continuations. Basically continuations are captured by a block
841 copy of the stack, and resumed by copying back.
843 For this reason, continuations captured by @code{call/cc} should be used only
844 when there is no other simple way to achieve the desired result, or when the
845 elegance of the continuation mechanism outweighs the need for performance.
847 Escapes upwards from loops or nested functions are generally best
848 handled with prompts (@pxref{Prompts}). Coroutines can be
849 efficiently implemented with cooperating threads (a thread holds a
850 full program stack but doesn't copy it around the way continuations
854 @node Multiple Values
855 @subsection Returning and Accepting Multiple Values
857 @cindex multiple values
860 Scheme allows a procedure to return more than one value to its caller.
861 This is quite different to other languages which only allow
862 single-value returns. Returning multiple values is different from
863 returning a list (or pair or vector) of values to the caller, because
864 conceptually not @emph{one} compound object is returned, but several
867 The primitive procedures for handling multiple values are @code{values}
868 and @code{call-with-values}. @code{values} is used for returning
869 multiple values from a procedure. This is done by placing a call to
870 @code{values} with zero or more arguments in tail position in a
871 procedure body. @code{call-with-values} combines a procedure returning
872 multiple values with a procedure which accepts these values as
876 @deffn {Scheme Procedure} values arg @dots{}
877 @deffnx {C Function} scm_values (args)
878 Delivers all of its arguments to its continuation. Except for
879 continuations created by the @code{call-with-values} procedure,
880 all continuations take exactly one value. The effect of
881 passing no value or more than one value to continuations that
882 were not created by @code{call-with-values} is unspecified.
884 For @code{scm_values}, @var{args} is a list of arguments and the
885 return is a multiple-values object which the caller can return. In
886 the current implementation that object shares structure with
887 @var{args}, so @var{args} should not be modified subsequently.
890 @deftypefn {C Function} SCM scm_c_values (SCM *base, size_t n)
891 @code{scm_c_values} is an alternative to @code{scm_values}. It creates
892 a new values object, and copies into it the @var{n} values starting from
895 Currently this creates a list and passes it to @code{scm_values}, but we
896 expect that in the future we will be able to use more a efficient
900 @deftypefn {C Function} size_t scm_c_nvalues (SCM obj)
901 If @var{obj} is a multiple-values object, returns the number of values
902 it contains. Otherwise returns 1.
905 @deftypefn {C Function} SCM scm_c_value_ref (SCM obj, size_t idx)
906 Returns the value at the position specified by @var{idx} in
907 @var{obj}. Note that @var{obj} will ordinarily be a
908 multiple-values object, but it need not be. Any other object
909 represents a single value (itself), and is handled appropriately.
912 @rnindex call-with-values
913 @deffn {Scheme Procedure} call-with-values producer consumer
914 Calls its @var{producer} argument with no values and a
915 continuation that, when passed some values, calls the
916 @var{consumer} procedure with those values as arguments. The
917 continuation for the call to @var{consumer} is the continuation
918 of the call to @code{call-with-values}.
921 (call-with-values (lambda () (values 4 5))
927 (call-with-values * -)
932 In addition to the fundamental procedures described above, Guile has a
933 module which exports a syntax called @code{receive}, which is much
934 more convenient. This is in the @code{(ice-9 receive)} and is the
935 same as specified by SRFI-8 (@pxref{SRFI-8}).
938 (use-modules (ice-9 receive))
941 @deffn {library syntax} receive formals expr body @dots{}
942 Evaluate the expression @var{expr}, and bind the result values (zero
943 or more) to the formal arguments in @var{formals}. @var{formals} is a
944 list of symbols, like the argument list in a @code{lambda}
945 (@pxref{Lambda}). After binding the variables, the expressions in
946 @var{body} @dots{} are evaluated in order, the return value is the
947 result from the last expression.
949 For example getting results from @code{partition} in SRFI-1
953 (receive (odds evens)
954 (partition odd? '(7 4 2 8 3))
958 @print{} (7 3) and (4 2 8)
965 @subsection Exceptions
966 @cindex error handling
967 @cindex exception handling
969 A common requirement in applications is to want to jump
970 @dfn{non-locally} from the depths of a computation back to, say, the
971 application's main processing loop. Usually, the place that is the
972 target of the jump is somewhere in the calling stack of procedures that
973 called the procedure that wants to jump back. For example, typical
974 logic for a key press driven application might look something like this:
978 read the next key press and call dispatch-key
981 lookup the key in a keymap and call an appropriate procedure,
985 interactively read the required file name, then call
989 check whether file exists; if not, jump back to main-loop
993 The jump back to @code{main-loop} could be achieved by returning through
994 the stack one procedure at a time, using the return value of each
995 procedure to indicate the error condition, but Guile (like most modern
996 programming languages) provides an additional mechanism called
997 @dfn{exception handling} that can be used to implement such jumps much
1001 * Exception Terminology:: Different ways to say the same thing.
1002 * Catch:: Setting up to catch exceptions.
1003 * Throw Handlers:: Handling exceptions before unwinding the stack.
1004 * Throw:: Throwing an exception.
1005 * Exception Implementation:: How Guile implements exceptions.
1009 @node Exception Terminology
1010 @subsubsection Exception Terminology
1012 There are several variations on the terminology for dealing with
1013 non-local jumps. It is useful to be aware of them, and to realize
1014 that they all refer to the same basic mechanism.
1018 Actually making a non-local jump may be called @dfn{raising an
1019 exception}, @dfn{raising a signal}, @dfn{throwing an exception} or
1020 @dfn{doing a long jump}. When the jump indicates an error condition,
1021 people may talk about @dfn{signalling}, @dfn{raising} or @dfn{throwing}
1025 Handling the jump at its target may be referred to as @dfn{catching} or
1026 @dfn{handling} the @dfn{exception}, @dfn{signal} or, where an error
1027 condition is involved, @dfn{error}.
1030 Where @dfn{signal} and @dfn{signalling} are used, special care is needed
1031 to avoid the risk of confusion with POSIX signals.
1033 This manual prefers to speak of throwing and catching exceptions, since
1034 this terminology matches the corresponding Guile primitives.
1036 The exception mechanism described in this section has connections with
1037 @dfn{delimited continuations} (@pxref{Prompts}). In particular,
1038 throwing an exception is akin to invoking an @dfn{escape continuation}
1039 (@pxref{Prompt Primitives, @code{call/ec}}).
1043 @subsubsection Catching Exceptions
1045 @code{catch} is used to set up a target for a possible non-local jump.
1046 The arguments of a @code{catch} expression are a @dfn{key}, which
1047 restricts the set of exceptions to which this @code{catch} applies, a
1048 thunk that specifies the code to execute and one or two @dfn{handler}
1049 procedures that say what to do if an exception is thrown while executing
1050 the code. If the execution thunk executes @dfn{normally}, which means
1051 without throwing any exceptions, the handler procedures are not called
1054 When an exception is thrown using the @code{throw} function, the first
1055 argument of the @code{throw} is a symbol that indicates the type of the
1056 exception. For example, Guile throws an exception using the symbol
1057 @code{numerical-overflow} to indicate numerical overflow errors such as
1063 ABORT: (numerical-overflow)
1066 The @var{key} argument in a @code{catch} expression corresponds to this
1067 symbol. @var{key} may be a specific symbol, such as
1068 @code{numerical-overflow}, in which case the @code{catch} applies
1069 specifically to exceptions of that type; or it may be @code{#t}, which
1070 means that the @code{catch} applies to all exceptions, irrespective of
1073 The second argument of a @code{catch} expression should be a thunk
1074 (i.e.@: a procedure that accepts no arguments) that specifies the normal
1075 case code. The @code{catch} is active for the execution of this thunk,
1076 including any code called directly or indirectly by the thunk's body.
1077 Evaluation of the @code{catch} expression activates the catch and then
1080 The third argument of a @code{catch} expression is a handler procedure.
1081 If an exception is thrown, this procedure is called with exactly the
1082 arguments specified by the @code{throw}. Therefore, the handler
1083 procedure must be designed to accept a number of arguments that
1084 corresponds to the number of arguments in all @code{throw} expressions
1085 that can be caught by this @code{catch}.
1087 The fourth, optional argument of a @code{catch} expression is another
1088 handler procedure, called the @dfn{pre-unwind} handler. It differs from
1089 the third argument in that if an exception is thrown, it is called,
1090 @emph{before} the third argument handler, in exactly the dynamic context
1091 of the @code{throw} expression that threw the exception. This means
1092 that it is useful for capturing or displaying the stack at the point of
1093 the @code{throw}, or for examining other aspects of the dynamic context,
1094 such as fluid values, before the context is unwound back to that of the
1095 prevailing @code{catch}.
1097 @deffn {Scheme Procedure} catch key thunk handler [pre-unwind-handler]
1098 @deffnx {C Function} scm_catch_with_pre_unwind_handler (key, thunk, handler, pre_unwind_handler)
1099 @deffnx {C Function} scm_catch (key, thunk, handler)
1100 Invoke @var{thunk} in the dynamic context of @var{handler} for
1101 exceptions matching @var{key}. If thunk throws to the symbol
1102 @var{key}, then @var{handler} is invoked this way:
1104 (handler key args ...)
1107 @var{key} is a symbol or @code{#t}.
1109 @var{thunk} takes no arguments. If @var{thunk} returns
1110 normally, that is the return value of @code{catch}.
1112 Handler is invoked outside the scope of its own @code{catch}.
1113 If @var{handler} again throws to the same key, a new handler
1114 from further up the call chain is invoked.
1116 If the key is @code{#t}, then a throw to @emph{any} symbol will
1117 match this call to @code{catch}.
1119 If a @var{pre-unwind-handler} is given and @var{thunk} throws
1120 an exception that matches @var{key}, Guile calls the
1121 @var{pre-unwind-handler} before unwinding the dynamic state and
1122 invoking the main @var{handler}. @var{pre-unwind-handler} should
1123 be a procedure with the same signature as @var{handler}, that
1124 is @code{(lambda (key . args))}. It is typically used to save
1125 the stack at the point where the exception occurred, but can also
1126 query other parts of the dynamic state at that point, such as
1129 A @var{pre-unwind-handler} can exit either normally or non-locally.
1130 If it exits normally, Guile unwinds the stack and dynamic context
1131 and then calls the normal (third argument) handler. If it exits
1132 non-locally, that exit determines the continuation.
1135 If a handler procedure needs to match a variety of @code{throw}
1136 expressions with varying numbers of arguments, you should write it like
1140 (lambda (key . args)
1145 The @var{key} argument is guaranteed always to be present, because a
1146 @code{throw} without a @var{key} is not valid. The number and
1147 interpretation of the @var{args} varies from one type of exception to
1148 another, but should be specified by the documentation for each exception
1151 Note that, once the normal (post-unwind) handler procedure is invoked,
1152 the catch that led to the handler procedure being called is no longer
1153 active. Therefore, if the handler procedure itself throws an exception,
1154 that exception can only be caught by another active catch higher up the
1155 call stack, if there is one.
1158 @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)
1159 @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)
1160 The above @code{scm_catch_with_pre_unwind_handler} and @code{scm_catch}
1161 take Scheme procedures as body and handler arguments.
1162 @code{scm_c_catch} and @code{scm_internal_catch} are equivalents taking
1165 @var{body} is called as @code{@var{body} (@var{body_data})} with a catch
1166 on exceptions of the given @var{tag} type. If an exception is caught,
1167 @var{pre_unwind_handler} and @var{handler} are called as
1168 @code{@var{handler} (@var{handler_data}, @var{key}, @var{args})}.
1169 @var{key} and @var{args} are the @code{SCM} key and argument list from
1172 @tpindex scm_t_catch_body
1173 @tpindex scm_t_catch_handler
1174 @var{body} and @var{handler} should have the following prototypes.
1175 @code{scm_t_catch_body} and @code{scm_t_catch_handler} are pointer
1179 SCM body (void *data);
1180 SCM handler (void *data, SCM key, SCM args);
1183 The @var{body_data} and @var{handler_data} parameters are passed to
1184 the respective calls so an application can communicate extra
1185 information to those functions.
1187 If the data consists of an @code{SCM} object, care should be taken that
1188 it isn't garbage collected while still required. If the @code{SCM} is a
1189 local C variable, one way to protect it is to pass a pointer to that
1190 variable as the data parameter, since the C compiler will then know the
1191 value must be held on the stack. Another way is to use
1192 @code{scm_remember_upto_here_1} (@pxref{Foreign Object Memory
1197 @node Throw Handlers
1198 @subsubsection Throw Handlers
1200 It's sometimes useful to be able to intercept an exception that is being
1201 thrown before the stack is unwound. This could be to clean up some
1202 related state, to print a backtrace, or to pass information about the
1203 exception to a debugger, for example. The @code{with-throw-handler}
1204 procedure provides a way to do this.
1206 @deffn {Scheme Procedure} with-throw-handler key thunk handler
1207 @deffnx {C Function} scm_with_throw_handler (key, thunk, handler)
1208 Add @var{handler} to the dynamic context as a throw handler
1209 for key @var{key}, then invoke @var{thunk}.
1211 This behaves exactly like @code{catch}, except that it does not unwind
1212 the stack before invoking @var{handler}. If the @var{handler} procedure
1213 returns normally, Guile rethrows the same exception again to the next
1214 innermost catch or throw handler. @var{handler} may exit nonlocally, of
1215 course, via an explicit throw or via invoking a continuation.
1218 Typically @var{handler} is used to display a backtrace of the stack at
1219 the point where the corresponding @code{throw} occurred, or to save off
1220 this information for possible display later.
1222 Not unwinding the stack means that throwing an exception that is handled
1223 via a throw handler is equivalent to calling the throw handler handler
1224 inline instead of each @code{throw}, and then omitting the surrounding
1225 @code{with-throw-handler}. In other words,
1228 (with-throw-handler 'key
1229 (lambda () @dots{} (throw 'key args @dots{}) @dots{})
1234 is mostly equivalent to
1237 ((lambda () @dots{} (handler 'key args @dots{}) @dots{}))
1240 In particular, the dynamic context when @var{handler} is invoked is that
1241 of the site where @code{throw} is called. The examples are not quite
1242 equivalent, because the body of a @code{with-throw-handler} is not in
1243 tail position with respect to the @code{with-throw-handler}, and if
1244 @var{handler} exits normally, Guile arranges to rethrow the error, but
1245 hopefully the intention is clear. (For an introduction to what is meant
1246 by dynamic context, @xref{Dynamic Wind}.)
1248 @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)
1249 The above @code{scm_with_throw_handler} takes Scheme procedures as body
1250 (thunk) and handler arguments. @code{scm_c_with_throw_handler} is an
1251 equivalent taking C functions. See @code{scm_c_catch} (@pxref{Catch})
1252 for a description of the parameters, the behaviour however of course
1253 follows @code{with-throw-handler}.
1256 If @var{thunk} throws an exception, Guile handles that exception by
1257 invoking the innermost @code{catch} or throw handler whose key matches
1258 that of the exception. When the innermost thing is a throw handler,
1259 Guile calls the specified handler procedure using @code{(apply
1260 @var{handler} key args)}. The handler procedure may either return
1261 normally or exit non-locally. If it returns normally, Guile passes the
1262 exception on to the next innermost @code{catch} or throw handler. If it
1263 exits non-locally, that exit determines the continuation.
1265 The behaviour of a throw handler is very similar to that of a
1266 @code{catch} expression's optional pre-unwind handler. In particular, a
1267 throw handler's handler procedure is invoked in the exact dynamic
1268 context of the @code{throw} expression, just as a pre-unwind handler is.
1269 @code{with-throw-handler} may be seen as a half-@code{catch}: it does
1270 everything that a @code{catch} would do until the point where
1271 @code{catch} would start unwinding the stack and dynamic context, but
1272 then it rethrows to the next innermost @code{catch} or throw handler
1275 Note also that since the dynamic context is not unwound, if a
1276 @code{with-throw-handler} handler throws to a key that does not match
1277 the @code{with-throw-handler} expression's @var{key}, the new throw may
1278 be handled by a @code{catch} or throw handler that is @emph{closer} to
1279 the throw than the first @code{with-throw-handler}.
1281 Here is an example to illustrate this behavior:
1286 (with-throw-handler 'b
1292 (lambda (key . args)
1298 This code will call @code{inner-handler} and then continue with the
1299 continuation of the inner @code{catch}.
1303 @subsubsection Throwing Exceptions
1305 The @code{throw} primitive is used to throw an exception. One argument,
1306 the @var{key}, is mandatory, and must be a symbol; it indicates the type
1307 of exception that is being thrown. Following the @var{key},
1308 @code{throw} accepts any number of additional arguments, whose meaning
1309 depends on the exception type. The documentation for each possible type
1310 of exception should specify the additional arguments that are expected
1311 for that kind of exception.
1313 @deffn {Scheme Procedure} throw key arg @dots{}
1314 @deffnx {C Function} scm_throw (key, args)
1315 Invoke the catch form matching @var{key}, passing @var{arg} @dots{} to
1318 @var{key} is a symbol. It will match catches of the same symbol or of
1321 If there is no handler at all, Guile prints an error and then exits.
1324 When an exception is thrown, it will be caught by the innermost
1325 @code{catch} or throw handler that applies to the type of the thrown
1326 exception; in other words, whose @var{key} is either @code{#t} or the
1327 same symbol as that used in the @code{throw} expression. Once Guile has
1328 identified the appropriate @code{catch} or throw handler, it handles the
1329 exception by applying the relevant handler procedure(s) to the arguments
1330 of the @code{throw}.
1332 If there is no appropriate @code{catch} or throw handler for a thrown
1333 exception, Guile prints an error to the current error port indicating an
1334 uncaught exception, and then exits. In practice, it is quite difficult
1335 to observe this behaviour, because Guile when used interactively
1336 installs a top level @code{catch} handler that will catch all exceptions
1337 and print an appropriate error message @emph{without} exiting. For
1338 example, this is what happens if you try to throw an unhandled exception
1339 in the standard Guile REPL; note that Guile's command loop continues
1340 after the error message:
1343 guile> (throw 'badex)
1344 <unnamed port>:3:1: In procedure gsubr-apply @dots{}
1345 <unnamed port>:3:1: unhandled-exception: badex
1350 The default uncaught exception behaviour can be observed by evaluating a
1351 @code{throw} expression from the shell command line:
1354 $ guile -c "(begin (throw 'badex) (display \"here\\n\"))"
1355 guile: uncaught throw to badex: ()
1360 That Guile exits immediately following the uncaught exception
1361 is shown by the absence of any output from the @code{display}
1362 expression, because Guile never gets to the point of evaluating that
1366 @node Exception Implementation
1367 @subsubsection How Guile Implements Exceptions
1369 It is traditional in Scheme to implement exception systems using
1370 @code{call-with-current-continuation}. Continuations
1371 (@pxref{Continuations}) are such a powerful concept that any other
1372 control mechanism --- including @code{catch} and @code{throw} --- can be
1373 implemented in terms of them.
1375 Guile does not implement @code{catch} and @code{throw} like this,
1376 though. Why not? Because Guile is specifically designed to be easy to
1377 integrate with applications written in C. In a mixed Scheme/C
1378 environment, the concept of @dfn{continuation} must logically include
1379 ``what happens next'' in the C parts of the application as well as the
1380 Scheme parts, and it turns out that the only reasonable way of
1381 implementing continuations like this is to save and restore the complete
1384 So Guile's implementation of @code{call-with-current-continuation} is a
1385 stack copying one. This allows it to interact well with ordinary C
1386 code, but means that creating and calling a continuation is slowed down
1387 by the time that it takes to copy the C stack.
1389 The more targeted mechanism provided by @code{catch} and @code{throw}
1390 does not need to save and restore the C stack because the @code{throw}
1391 always jumps to a location higher up the stack of the code that executes
1392 the @code{throw}. Therefore Guile implements the @code{catch} and
1393 @code{throw} primitives independently of
1394 @code{call-with-current-continuation}, in a way that takes advantage of
1395 this @emph{upwards only} nature of exceptions.
1398 @node Error Reporting
1399 @subsection Procedures for Signaling Errors
1401 Guile provides a set of convenience procedures for signaling error
1402 conditions that are implemented on top of the exception primitives just
1405 @deffn {Scheme Procedure} error msg arg @dots{}
1406 Raise an error with key @code{misc-error} and a message constructed by
1407 displaying @var{msg} and writing @var{arg} @enddots{}.
1410 @deffn {Scheme Procedure} scm-error key subr message args data
1411 @deffnx {C Function} scm_error_scm (key, subr, message, args, data)
1412 Raise an error with key @var{key}. @var{subr} can be a string
1413 naming the procedure associated with the error, or @code{#f}.
1414 @var{message} is the error message string, possibly containing
1415 @code{~S} and @code{~A} escapes. When an error is reported,
1416 these are replaced by formatting the corresponding members of
1417 @var{args}: @code{~A} (was @code{%s} in older versions of
1418 Guile) formats using @code{display} and @code{~S} (was
1419 @code{%S}) formats using @code{write}. @var{data} is a list or
1420 @code{#f} depending on @var{key}: if @var{key} is
1421 @code{system-error} then it should be a list containing the
1422 Unix @code{errno} value; If @var{key} is @code{signal} then it
1423 should be a list containing the Unix signal number; If
1424 @var{key} is @code{out-of-range}, @code{wrong-type-arg},
1425 or @code{keyword-argument-error},
1426 it is a list containing the bad value; otherwise
1427 it will usually be @code{#f}.
1430 @deffn {Scheme Procedure} strerror err
1431 @deffnx {C Function} scm_strerror (err)
1432 Return the Unix error message corresponding to @var{err}, an integer
1435 When @code{setlocale} has been called (@pxref{Locales}), the message
1436 is in the language and charset of @code{LC_MESSAGES}. (This is done
1440 @c begin (scm-doc-string "boot-9.scm" "false-if-exception")
1441 @deffn syntax false-if-exception expr
1442 Returns the result of evaluating its argument; however
1443 if an exception occurs then @code{#f} is returned instead.
1449 @subsection Dynamic Wind
1451 For Scheme code, the fundamental procedure to react to non-local entry
1452 and exits of dynamic contexts is @code{dynamic-wind}. C code could
1453 use @code{scm_internal_dynamic_wind}, but since C does not allow the
1454 convenient construction of anonymous procedures that close over
1455 lexical variables, this will be, well, inconvenient.
1457 Therefore, Guile offers the functions @code{scm_dynwind_begin} and
1458 @code{scm_dynwind_end} to delimit a dynamic extent. Within this
1459 dynamic extent, which is called a @dfn{dynwind context}, you can
1460 perform various @dfn{dynwind actions} that control what happens when
1461 the dynwind context is entered or left. For example, you can register
1462 a cleanup routine with @code{scm_dynwind_unwind_handler} that is
1463 executed when the context is left. There are several other more
1464 specialized dynwind actions as well, for example to temporarily block
1465 the execution of asyncs or to temporarily change the current output
1466 port. They are described elsewhere in this manual.
1468 Here is an example that shows how to prevent memory leaks.
1472 /* Suppose there is a function called FOO in some library that you
1473 would like to make available to Scheme code (or to C code that
1474 follows the Scheme conventions).
1476 FOO takes two C strings and returns a new string. When an error has
1477 occurred in FOO, it returns NULL.
1480 char *foo (char *s1, char *s2);
1482 /* SCM_FOO interfaces the C function FOO to the Scheme way of life.
1483 It takes care to free up all temporary strings in the case of
1488 scm_foo (SCM s1, SCM s2)
1490 char *c_s1, *c_s2, *c_res;
1492 scm_dynwind_begin (0);
1494 c_s1 = scm_to_locale_string (s1);
1496 /* Call 'free (c_s1)' when the dynwind context is left.
1498 scm_dynwind_unwind_handler (free, c_s1, SCM_F_WIND_EXPLICITLY);
1500 c_s2 = scm_to_locale_string (s2);
1502 /* Same as above, but more concisely.
1504 scm_dynwind_free (c_s2);
1506 c_res = foo (c_s1, c_s2);
1508 scm_memory_error ("foo");
1512 return scm_take_locale_string (res);
1516 @rnindex dynamic-wind
1517 @deffn {Scheme Procedure} dynamic-wind in_guard thunk out_guard
1518 @deffnx {C Function} scm_dynamic_wind (in_guard, thunk, out_guard)
1519 All three arguments must be 0-argument procedures.
1520 @var{in_guard} is called, then @var{thunk}, then
1523 If, any time during the execution of @var{thunk}, the
1524 dynamic extent of the @code{dynamic-wind} expression is escaped
1525 non-locally, @var{out_guard} is called. If the dynamic extent of
1526 the dynamic-wind is re-entered, @var{in_guard} is called. Thus
1527 @var{in_guard} and @var{out_guard} may be called any number of
1531 (define x 'normal-binding)
1534 (call-with-current-continuation
1540 (lambda () (set! x 'special-binding))
1544 (lambda () (display x) (newline)
1545 (call-with-current-continuation escape)
1546 (display x) (newline)
1551 (lambda () (set! x old-x)))))))
1557 @result{} normal-binding
1562 @result{} a-cont ;; the value of the (define a-cont...)
1564 @result{} normal-binding
1566 @result{} special-binding
1570 @deftp {C Type} scm_t_dynwind_flags
1571 This is an enumeration of several flags that modify the behavior of
1572 @code{scm_dynwind_begin}. The flags are listed in the following
1576 @item SCM_F_DYNWIND_REWINDABLE
1577 The dynamic context is @dfn{rewindable}. This means that it can be
1578 reentered non-locally (via the invocation of a continuation). The
1579 default is that a dynwind context can not be reentered non-locally.
1584 @deftypefn {C Function} void scm_dynwind_begin (scm_t_dynwind_flags flags)
1585 The function @code{scm_dynwind_begin} starts a new dynamic context and
1586 makes it the `current' one.
1588 The @var{flags} argument determines the default behavior of the
1589 context. Normally, use 0. This will result in a context that can not
1590 be reentered with a captured continuation. When you are prepared to
1591 handle reentries, include @code{SCM_F_DYNWIND_REWINDABLE} in
1594 Being prepared for reentry means that the effects of unwind handlers
1595 can be undone on reentry. In the example above, we want to prevent a
1596 memory leak on non-local exit and thus register an unwind handler that
1597 frees the memory. But once the memory is freed, we can not get it
1598 back on reentry. Thus reentry can not be allowed.
1600 The consequence is that continuations become less useful when
1601 non-reentrant contexts are captured, but you don't need to worry
1602 about that too much.
1604 The context is ended either implicitly when a non-local exit happens,
1605 or explicitly with @code{scm_dynwind_end}. You must make sure that a
1606 dynwind context is indeed ended properly. If you fail to call
1607 @code{scm_dynwind_end} for each @code{scm_dynwind_begin}, the behavior
1611 @deftypefn {C Function} void scm_dynwind_end ()
1612 End the current dynamic context explicitly and make the previous one
1616 @deftp {C Type} scm_t_wind_flags
1617 This is an enumeration of several flags that modify the behavior of
1618 @code{scm_dynwind_unwind_handler} and
1619 @code{scm_dynwind_rewind_handler}. The flags are listed in the
1623 @item SCM_F_WIND_EXPLICITLY
1624 @vindex SCM_F_WIND_EXPLICITLY
1625 The registered action is also carried out when the dynwind context is
1626 entered or left locally.
1630 @deftypefn {C Function} void scm_dynwind_unwind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
1631 @deftypefnx {C Function} void scm_dynwind_unwind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
1632 Arranges for @var{func} to be called with @var{data} as its arguments
1633 when the current context ends implicitly. If @var{flags} contains
1634 @code{SCM_F_WIND_EXPLICITLY}, @var{func} is also called when the
1635 context ends explicitly with @code{scm_dynwind_end}.
1637 The function @code{scm_dynwind_unwind_handler_with_scm} takes care that
1638 @var{data} is protected from garbage collection.
1641 @deftypefn {C Function} void scm_dynwind_rewind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
1642 @deftypefnx {C Function} void scm_dynwind_rewind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
1643 Arrange for @var{func} to be called with @var{data} as its argument when
1644 the current context is restarted by rewinding the stack. When @var{flags}
1645 contains @code{SCM_F_WIND_EXPLICITLY}, @var{func} is called immediately
1648 The function @code{scm_dynwind_rewind_handler_with_scm} takes care that
1649 @var{data} is protected from garbage collection.
1652 @deftypefn {C Function} void scm_dynwind_free (void *mem)
1653 Arrange for @var{mem} to be freed automatically whenever the current
1654 context is exited, whether normally or non-locally.
1655 @code{scm_dynwind_free (mem)} is an equivalent shorthand for
1656 @code{scm_dynwind_unwind_handler (free, mem, SCM_F_WIND_EXPLICITLY)}.
1660 @node Handling Errors
1661 @subsection How to Handle Errors
1663 Error handling is based on @code{catch} and @code{throw}. Errors are
1664 always thrown with a @var{key} and four arguments:
1668 @var{key}: a symbol which indicates the type of error. The symbols used
1669 by libguile are listed below.
1672 @var{subr}: the name of the procedure from which the error is thrown, or
1676 @var{message}: a string (possibly language and system dependent)
1677 describing the error. The tokens @code{~A} and @code{~S} can be
1678 embedded within the message: they will be replaced with members of the
1679 @var{args} list when the message is printed. @code{~A} indicates an
1680 argument printed using @code{display}, while @code{~S} indicates an
1681 argument printed using @code{write}. @var{message} can also be
1682 @code{#f}, to allow it to be derived from the @var{key} by the error
1683 handler (may be useful if the @var{key} is to be thrown from both C and
1687 @var{args}: a list of arguments to be used to expand @code{~A} and
1688 @code{~S} tokens in @var{message}. Can also be @code{#f} if no
1689 arguments are required.
1692 @var{rest}: a list of any additional objects required. e.g., when the
1693 key is @code{'system-error}, this contains the C errno value. Can also
1694 be @code{#f} if no additional objects are required.
1697 In addition to @code{catch} and @code{throw}, the following Scheme
1698 facilities are available:
1700 @deffn {Scheme Procedure} display-error frame port subr message args rest
1701 @deffnx {C Function} scm_display_error (frame, port, subr, message, args, rest)
1702 Display an error message to the output port @var{port}.
1703 @var{frame} is the frame in which the error occurred, @var{subr} is
1704 the name of the procedure in which the error occurred and
1705 @var{message} is the actual error message, which may contain
1706 formatting instructions. These will format the arguments in
1707 the list @var{args} accordingly. @var{rest} is currently
1711 The following are the error keys defined by libguile and the situations
1712 in which they are used:
1716 @cindex @code{error-signal}
1717 @code{error-signal}: thrown after receiving an unhandled fatal signal
1718 such as SIGSEGV, SIGBUS, SIGFPE etc. The @var{rest} argument in the throw
1719 contains the coded signal number (at present this is not the same as the
1720 usual Unix signal number).
1723 @cindex @code{system-error}
1724 @code{system-error}: thrown after the operating system indicates an
1725 error condition. The @var{rest} argument in the throw contains the
1729 @cindex @code{numerical-overflow}
1730 @code{numerical-overflow}: numerical overflow.
1733 @cindex @code{out-of-range}
1734 @code{out-of-range}: the arguments to a procedure do not fall within the
1738 @cindex @code{wrong-type-arg}
1739 @code{wrong-type-arg}: an argument to a procedure has the wrong type.
1742 @cindex @code{wrong-number-of-args}
1743 @code{wrong-number-of-args}: a procedure was called with the wrong number
1747 @cindex @code{memory-allocation-error}
1748 @code{memory-allocation-error}: memory allocation error.
1751 @cindex @code{stack-overflow}
1752 @code{stack-overflow}: stack overflow error.
1755 @cindex @code{regular-expression-syntax}
1756 @code{regular-expression-syntax}: errors generated by the regular
1760 @cindex @code{misc-error}
1761 @code{misc-error}: other errors.
1765 @subsubsection C Support
1767 In the following C functions, @var{SUBR} and @var{MESSAGE} parameters
1768 can be @code{NULL} to give the effect of @code{#f} described above.
1770 @deftypefn {C Function} SCM scm_error (SCM @var{key}, char *@var{subr}, char *@var{message}, SCM @var{args}, SCM @var{rest})
1771 Throw an error, as per @code{scm-error} (@pxref{Error Reporting}).
1774 @deftypefn {C Function} void scm_syserror (char *@var{subr})
1775 @deftypefnx {C Function} void scm_syserror_msg (char *@var{subr}, char *@var{message}, SCM @var{args})
1776 Throw an error with key @code{system-error} and supply @code{errno} in
1777 the @var{rest} argument. For @code{scm_syserror} the message is
1778 generated using @code{strerror}.
1780 Care should be taken that any code in between the failing operation
1781 and the call to these routines doesn't change @code{errno}.
1784 @deftypefn {C Function} void scm_num_overflow (char *@var{subr})
1785 @deftypefnx {C Function} void scm_out_of_range (char *@var{subr}, SCM @var{bad_value})
1786 @deftypefnx {C Function} void scm_wrong_num_args (SCM @var{proc})
1787 @deftypefnx {C Function} void scm_wrong_type_arg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value})
1788 @deftypefnx {C Function} void scm_wrong_type_arg_msg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value}, const char *@var{expected})
1789 @deftypefnx {C Function} void scm_memory_error (char *@var{subr})
1790 @deftypefnx {C Function} void scm_misc_error (const char *@var{subr}, const char *@var{message}, SCM @var{args})
1791 Throw an error with the various keys described above.
1793 In @code{scm_wrong_num_args}, @var{proc} should be a Scheme symbol
1794 which is the name of the procedure incorrectly invoked. The other
1795 routines take the name of the invoked procedure as a C string.
1797 In @code{scm_wrong_type_arg_msg}, @var{expected} is a C string
1798 describing the type of argument that was expected.
1800 In @code{scm_misc_error}, @var{message} is the error message string,
1801 possibly containing @code{simple-format} escapes (@pxref{Writing}), and
1802 the corresponding arguments in the @var{args} list.
1806 @subsubsection Signalling Type Errors
1808 Every function visible at the Scheme level should aggressively check the
1809 types of its arguments, to avoid misinterpreting a value, and perhaps
1810 causing a segmentation fault. Guile provides some macros to make this
1813 @deftypefn Macro void SCM_ASSERT (int @var{test}, SCM @var{obj}, unsigned int @var{position}, const char *@var{subr})
1814 @deftypefnx Macro void SCM_ASSERT_TYPE (int @var{test}, SCM @var{obj}, unsigned int @var{position}, const char *@var{subr}, const char *@var{expected})
1815 If @var{test} is zero, signal a ``wrong type argument'' error,
1816 attributed to the subroutine named @var{subr}, operating on the value
1817 @var{obj}, which is the @var{position}'th argument of @var{subr}.
1819 In @code{SCM_ASSERT_TYPE}, @var{expected} is a C string describing the
1820 type of argument that was expected.
1823 @deftypefn Macro int SCM_ARG1
1824 @deftypefnx Macro int SCM_ARG2
1825 @deftypefnx Macro int SCM_ARG3
1826 @deftypefnx Macro int SCM_ARG4
1827 @deftypefnx Macro int SCM_ARG5
1828 @deftypefnx Macro int SCM_ARG6
1829 @deftypefnx Macro int SCM_ARG7
1830 One of the above values can be used for @var{position} to indicate the
1831 number of the argument of @var{subr} which is being checked.
1832 Alternatively, a positive integer number can be used, which allows to
1833 check arguments after the seventh. However, for parameter numbers up to
1834 seven it is preferable to use @code{SCM_ARGN} instead of the
1835 corresponding raw number, since it will make the code easier to
1839 @deftypefn Macro int SCM_ARGn
1840 Passing a value of zero or @code{SCM_ARGn} for @var{position} allows to
1841 leave it unspecified which argument's type is incorrect. Again,
1842 @code{SCM_ARGn} should be preferred over a raw zero constant.
1845 @node Continuation Barriers
1846 @subsection Continuation Barriers
1848 The non-local flow of control caused by continuations might sometimes
1849 not be wanted. You can use @code{with-continuation-barrier} to erect
1850 fences that continuations can not pass.
1852 @deffn {Scheme Procedure} with-continuation-barrier proc
1853 @deffnx {C Function} scm_with_continuation_barrier (proc)
1854 Call @var{proc} and return its result. Do not allow the invocation of
1855 continuations that would leave or enter the dynamic extent of the call
1856 to @code{with-continuation-barrier}. Such an attempt causes an error
1859 Throws (such as errors) that are not caught from within @var{proc} are
1860 caught by @code{with-continuation-barrier}. In that case, a short
1861 message is printed to the current error port and @code{#f} is returned.
1863 Thus, @code{with-continuation-barrier} returns exactly once.
1866 @deftypefn {C Function} {void *} scm_c_with_continuation_barrier (void *(*func) (void *), void *data)
1867 Like @code{scm_with_continuation_barrier} but call @var{func} on
1868 @var{data}. When an error is caught, @code{NULL} is returned.
1873 @c TeX-master: "guile.texi"