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 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 @node Shift and Reset
581 @subsubsection Shift, Reset, and All That
583 There is a whole zoo of delimited control operators, and as it does not
584 seem to be a bounded set, Guile implements support for them in a
588 (use-modules (ice-9 control))
591 Firstly, we have a helpful abbreviation for the @code{call-with-prompt}
594 @deffn {Scheme Syntax} % expr
595 @deffnx {Scheme Syntax} % expr handler
596 @deffnx {Scheme Syntax} % tag expr handler
597 Evaluate @var{expr} in a prompt, optionally specifying a tag and a
598 handler. If no tag is given, the default prompt tag is used.
600 If no handler is given, a default handler is installed. The default
601 handler accepts a procedure of one argument, which will called on the
602 captured continuation, within a prompt.
604 Sometimes it's easier just to show code, as in this case:
607 (define (default-prompt-handler k proc)
608 (% (default-prompt-tag)
610 default-prompt-handler))
613 The @code{%} symbol is chosen because it looks like a prompt.
616 Likewise there is an abbreviation for @code{abort-to-prompt}, which
617 assumes the default prompt tag:
619 @deffn {Scheme Procedure} abort val1 val2 @dots{}
620 Abort to the default prompt tag, passing @var{val1} @var{val2} @dots{}
624 As mentioned before, @code{(ice-9 control)} also provides other
625 delimited control operators. This section is a bit technical, and
626 first-time users of delimited continuations should probably come back to
627 it after some practice with @code{%}.
629 Still here? So, when one implements a delimited control operator like
630 @code{call-with-prompt}, one needs to make two decisions. Firstly, does
631 the handler run within or outside the prompt? Having the handler run
632 within the prompt allows an abort inside the handler to return to the
633 same prompt handler, which is often useful. However it prevents tail
634 calls from the handler, so it is less general.
636 Similarly, does invoking a captured continuation reinstate a prompt?
637 Again we have the tradeoff of convenience versus proper tail calls.
639 These decisions are captured in the Felleisen @dfn{F} operator. If
640 neither the continuations nor the handlers implicitly add a prompt, the
641 operator is known as @dfn{--F--}. This is the case for Guile's
642 @code{call-with-prompt} and @code{abort-to-prompt}.
644 If both continuation and handler implicitly add prompts, then the
645 operator is @dfn{+F+}. @code{shift} and @code{reset} are such
648 @deffn {Scheme Syntax} reset body1 body2 @dots{}
649 Establish a prompt, and evaluate @var{body1} @var{body2} @dots{} within
652 The prompt handler is designed to work with @code{shift}, described
656 @deffn {Scheme Syntax} shift cont body1 body2 @dots{}
657 Abort to the nearest @code{reset}, and evaluate @var{body1} @var{body2}
658 @dots{} in a context in which the captured continuation is bound to
661 As mentioned above, taken together, the @var{body1} @var{body2} @dots{}
662 expressions and the invocations of @var{cont} implicitly establish a
666 Interested readers are invited to explore Oleg Kiselyov's wonderful web
667 site at @uref{http://okmij.org/ftp/}, for more information on these
672 @subsection Continuations
673 @cindex continuations
675 A ``continuation'' is the code that will execute when a given function
676 or expression returns. For example, consider
681 (display (bar)) (newline)
685 The continuation from the call to @code{bar} comprises a
686 @code{display} of the value returned, a @code{newline} and an
687 @code{exit}. This can be expressed as a function of one argument.
691 (display r) (newline)
695 In Scheme, continuations are represented as special procedures just
696 like this. The special property is that when a continuation is called
697 it abandons the current program location and jumps directly to that
698 represented by the continuation.
700 A continuation is like a dynamic label, capturing at run-time a point
701 in program execution, including all the nested calls that have lead to
702 it (or rather the code that will execute when those calls return).
704 Continuations are created with the following functions.
706 @deffn {Scheme Procedure} call-with-current-continuation proc
707 @deffnx {Scheme Procedure} call/cc proc
708 @rnindex call-with-current-continuation
709 Capture the current continuation and call @code{(@var{proc}
710 @var{cont})} with it. The return value is the value returned by
711 @var{proc}, or when @code{(@var{cont} @var{value})} is later invoked,
712 the return is the @var{value} passed.
714 Normally @var{cont} should be called with one argument, but when the
715 location resumed is expecting multiple values (@pxref{Multiple
716 Values}) then they should be passed as multiple arguments, for
717 instance @code{(@var{cont} @var{x} @var{y} @var{z})}.
719 @var{cont} may only be used from the same side of a continuation
720 barrier as it was created (@pxref{Continuation Barriers}), and in a
721 multi-threaded program only from the thread in which it was created.
723 The call to @var{proc} is not part of the continuation captured, it runs
724 only when the continuation is created. Often a program will want to
725 store @var{cont} somewhere for later use; this can be done in
728 The @code{call} in the name @code{call-with-current-continuation}
729 refers to the way a call to @var{proc} gives the newly created
730 continuation. It's not related to the way a call is used later to
731 invoke that continuation.
733 @code{call/cc} is an alias for @code{call-with-current-continuation}.
734 This is in common use since the latter is rather long.
739 Here is a simple example,
743 (format #t "the return is ~a\n"
747 @result{} the return is 1
750 @result{} the return is 2
753 @code{call/cc} captures a continuation in which the value returned is
754 going to be displayed by @code{format}. The @code{lambda} stores this
755 in @code{kont} and gives an initial return @code{1} which is
756 displayed. The later invocation of @code{kont} resumes the captured
757 point, but this time returning @code{2}, which is displayed.
759 When Guile is run interactively, a call to @code{format} like this has
760 an implicit return back to the read-eval-print loop. @code{call/cc}
761 captures that like any other return, which is why interactively
762 @code{kont} will come back to read more input.
765 C programmers may note that @code{call/cc} is like @code{setjmp} in
766 the way it records at runtime a point in program execution. A call to
767 a continuation is like a @code{longjmp} in that it abandons the
768 present location and goes to the recorded one. Like @code{longjmp},
769 the value passed to the continuation is the value returned by
770 @code{call/cc} on resuming there. However @code{longjmp} can only go
771 up the program stack, but the continuation mechanism can go anywhere.
773 When a continuation is invoked, @code{call/cc} and subsequent code
774 effectively ``returns'' a second time. It can be confusing to imagine
775 a function returning more times than it was called. It may help
776 instead to think of it being stealthily re-entered and then program
777 flow going on as normal.
779 @code{dynamic-wind} (@pxref{Dynamic Wind}) can be used to ensure setup
780 and cleanup code is run when a program locus is resumed or abandoned
781 through the continuation mechanism.
784 Continuations are a powerful mechanism, and can be used to implement
785 almost any sort of control structure, such as loops, coroutines, or
788 However the implementation of continuations in Guile is not as
789 efficient as one might hope, because Guile is designed to cooperate
790 with programs written in other languages, such as C, which do not know
791 about continuations. Basically continuations are captured by a block
792 copy of the stack, and resumed by copying back.
794 For this reason, continuations captured by @code{call/cc} should be used only
795 when there is no other simple way to achieve the desired result, or when the
796 elegance of the continuation mechanism outweighs the need for performance.
798 Escapes upwards from loops or nested functions are generally best
799 handled with prompts (@pxref{Prompts}). Coroutines can be
800 efficiently implemented with cooperating threads (a thread holds a
801 full program stack but doesn't copy it around the way continuations
805 @node Multiple Values
806 @subsection Returning and Accepting Multiple Values
808 @cindex multiple values
811 Scheme allows a procedure to return more than one value to its caller.
812 This is quite different to other languages which only allow
813 single-value returns. Returning multiple values is different from
814 returning a list (or pair or vector) of values to the caller, because
815 conceptually not @emph{one} compound object is returned, but several
818 The primitive procedures for handling multiple values are @code{values}
819 and @code{call-with-values}. @code{values} is used for returning
820 multiple values from a procedure. This is done by placing a call to
821 @code{values} with zero or more arguments in tail position in a
822 procedure body. @code{call-with-values} combines a procedure returning
823 multiple values with a procedure which accepts these values as
827 @deffn {Scheme Procedure} values arg @dots{}
828 @deffnx {C Function} scm_values (args)
829 Delivers all of its arguments to its continuation. Except for
830 continuations created by the @code{call-with-values} procedure,
831 all continuations take exactly one value. The effect of
832 passing no value or more than one value to continuations that
833 were not created by @code{call-with-values} is unspecified.
835 For @code{scm_values}, @var{args} is a list of arguments and the
836 return is a multiple-values object which the caller can return. In
837 the current implementation that object shares structure with
838 @var{args}, so @var{args} should not be modified subsequently.
841 @deftypefn {C Function} SCM scm_c_values (SCM *base, size_t n)
842 @code{scm_c_values} is an alternative to @code{scm_values}. It creates
843 a new values object, and copies into it the @var{n} values starting from
846 Currently this creates a list and passes it to @code{scm_values}, but we
847 expect that in the future we will be able to use more a efficient
851 @deftypefn {C Function} size_t scm_c_nvalues (SCM obj)
852 If @var{obj} is a multiple-values object, returns the number of values
853 it contains. Otherwise returns 1.
856 @deftypefn {C Function} SCM scm_c_value_ref (SCM obj, size_t idx)
857 Returns the value at the position specified by @var{idx} in
858 @var{obj}. Note that @var{obj} will ordinarily be a
859 multiple-values object, but it need not be. Any other object
860 represents a single value (itself), and is handled appropriately.
863 @rnindex call-with-values
864 @deffn {Scheme Procedure} call-with-values producer consumer
865 Calls its @var{producer} argument with no values and a
866 continuation that, when passed some values, calls the
867 @var{consumer} procedure with those values as arguments. The
868 continuation for the call to @var{consumer} is the continuation
869 of the call to @code{call-with-values}.
872 (call-with-values (lambda () (values 4 5))
878 (call-with-values * -)
883 In addition to the fundamental procedures described above, Guile has a
884 module which exports a syntax called @code{receive}, which is much
885 more convenient. This is in the @code{(ice-9 receive)} and is the
886 same as specified by SRFI-8 (@pxref{SRFI-8}).
889 (use-modules (ice-9 receive))
892 @deffn {library syntax} receive formals expr body @dots{}
893 Evaluate the expression @var{expr}, and bind the result values (zero
894 or more) to the formal arguments in @var{formals}. @var{formals} is a
895 list of symbols, like the argument list in a @code{lambda}
896 (@pxref{Lambda}). After binding the variables, the expressions in
897 @var{body} @dots{} are evaluated in order, the return value is the
898 result from the last expression.
900 For example getting results from @code{partition} in SRFI-1
904 (receive (odds evens)
905 (partition odd? '(7 4 2 8 3))
909 @print{} (7 3) and (4 2 8)
916 @subsection Exceptions
917 @cindex error handling
918 @cindex exception handling
920 A common requirement in applications is to want to jump
921 @dfn{non-locally} from the depths of a computation back to, say, the
922 application's main processing loop. Usually, the place that is the
923 target of the jump is somewhere in the calling stack of procedures that
924 called the procedure that wants to jump back. For example, typical
925 logic for a key press driven application might look something like this:
929 read the next key press and call dispatch-key
932 lookup the key in a keymap and call an appropriate procedure,
936 interactively read the required file name, then call
940 check whether file exists; if not, jump back to main-loop
944 The jump back to @code{main-loop} could be achieved by returning through
945 the stack one procedure at a time, using the return value of each
946 procedure to indicate the error condition, but Guile (like most modern
947 programming languages) provides an additional mechanism called
948 @dfn{exception handling} that can be used to implement such jumps much
952 * Exception Terminology:: Different ways to say the same thing.
953 * Catch:: Setting up to catch exceptions.
954 * Throw Handlers:: Handling exceptions before unwinding the stack.
955 * Throw:: Throwing an exception.
956 * Exception Implementation:: How Guile implements exceptions.
960 @node Exception Terminology
961 @subsubsection Exception Terminology
963 There are several variations on the terminology for dealing with
964 non-local jumps. It is useful to be aware of them, and to realize
965 that they all refer to the same basic mechanism.
969 Actually making a non-local jump may be called @dfn{raising an
970 exception}, @dfn{raising a signal}, @dfn{throwing an exception} or
971 @dfn{doing a long jump}. When the jump indicates an error condition,
972 people may talk about @dfn{signalling}, @dfn{raising} or @dfn{throwing}
976 Handling the jump at its target may be referred to as @dfn{catching} or
977 @dfn{handling} the @dfn{exception}, @dfn{signal} or, where an error
978 condition is involved, @dfn{error}.
981 Where @dfn{signal} and @dfn{signalling} are used, special care is needed
982 to avoid the risk of confusion with POSIX signals.
984 This manual prefers to speak of throwing and catching exceptions, since
985 this terminology matches the corresponding Guile primitives.
989 @subsubsection Catching Exceptions
991 @code{catch} is used to set up a target for a possible non-local jump.
992 The arguments of a @code{catch} expression are a @dfn{key}, which
993 restricts the set of exceptions to which this @code{catch} applies, a
994 thunk that specifies the code to execute and one or two @dfn{handler}
995 procedures that say what to do if an exception is thrown while executing
996 the code. If the execution thunk executes @dfn{normally}, which means
997 without throwing any exceptions, the handler procedures are not called
1000 When an exception is thrown using the @code{throw} function, the first
1001 argument of the @code{throw} is a symbol that indicates the type of the
1002 exception. For example, Guile throws an exception using the symbol
1003 @code{numerical-overflow} to indicate numerical overflow errors such as
1009 ABORT: (numerical-overflow)
1012 The @var{key} argument in a @code{catch} expression corresponds to this
1013 symbol. @var{key} may be a specific symbol, such as
1014 @code{numerical-overflow}, in which case the @code{catch} applies
1015 specifically to exceptions of that type; or it may be @code{#t}, which
1016 means that the @code{catch} applies to all exceptions, irrespective of
1019 The second argument of a @code{catch} expression should be a thunk
1020 (i.e.@: a procedure that accepts no arguments) that specifies the normal
1021 case code. The @code{catch} is active for the execution of this thunk,
1022 including any code called directly or indirectly by the thunk's body.
1023 Evaluation of the @code{catch} expression activates the catch and then
1026 The third argument of a @code{catch} expression is a handler procedure.
1027 If an exception is thrown, this procedure is called with exactly the
1028 arguments specified by the @code{throw}. Therefore, the handler
1029 procedure must be designed to accept a number of arguments that
1030 corresponds to the number of arguments in all @code{throw} expressions
1031 that can be caught by this @code{catch}.
1033 The fourth, optional argument of a @code{catch} expression is another
1034 handler procedure, called the @dfn{pre-unwind} handler. It differs from
1035 the third argument in that if an exception is thrown, it is called,
1036 @emph{before} the third argument handler, in exactly the dynamic context
1037 of the @code{throw} expression that threw the exception. This means
1038 that it is useful for capturing or displaying the stack at the point of
1039 the @code{throw}, or for examining other aspects of the dynamic context,
1040 such as fluid values, before the context is unwound back to that of the
1041 prevailing @code{catch}.
1043 @deffn {Scheme Procedure} catch key thunk handler [pre-unwind-handler]
1044 @deffnx {C Function} scm_catch_with_pre_unwind_handler (key, thunk, handler, pre_unwind_handler)
1045 @deffnx {C Function} scm_catch (key, thunk, handler)
1046 Invoke @var{thunk} in the dynamic context of @var{handler} for
1047 exceptions matching @var{key}. If thunk throws to the symbol
1048 @var{key}, then @var{handler} is invoked this way:
1050 (handler key args ...)
1053 @var{key} is a symbol or @code{#t}.
1055 @var{thunk} takes no arguments. If @var{thunk} returns
1056 normally, that is the return value of @code{catch}.
1058 Handler is invoked outside the scope of its own @code{catch}.
1059 If @var{handler} again throws to the same key, a new handler
1060 from further up the call chain is invoked.
1062 If the key is @code{#t}, then a throw to @emph{any} symbol will
1063 match this call to @code{catch}.
1065 If a @var{pre-unwind-handler} is given and @var{thunk} throws
1066 an exception that matches @var{key}, Guile calls the
1067 @var{pre-unwind-handler} before unwinding the dynamic state and
1068 invoking the main @var{handler}. @var{pre-unwind-handler} should
1069 be a procedure with the same signature as @var{handler}, that
1070 is @code{(lambda (key . args))}. It is typically used to save
1071 the stack at the point where the exception occurred, but can also
1072 query other parts of the dynamic state at that point, such as
1075 A @var{pre-unwind-handler} can exit either normally or non-locally.
1076 If it exits normally, Guile unwinds the stack and dynamic context
1077 and then calls the normal (third argument) handler. If it exits
1078 non-locally, that exit determines the continuation.
1081 If a handler procedure needs to match a variety of @code{throw}
1082 expressions with varying numbers of arguments, you should write it like
1086 (lambda (key . args)
1091 The @var{key} argument is guaranteed always to be present, because a
1092 @code{throw} without a @var{key} is not valid. The number and
1093 interpretation of the @var{args} varies from one type of exception to
1094 another, but should be specified by the documentation for each exception
1097 Note that, once the normal (post-unwind) handler procedure is invoked,
1098 the catch that led to the handler procedure being called is no longer
1099 active. Therefore, if the handler procedure itself throws an exception,
1100 that exception can only be caught by another active catch higher up the
1101 call stack, if there is one.
1104 @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)
1105 @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)
1106 The above @code{scm_catch_with_pre_unwind_handler} and @code{scm_catch}
1107 take Scheme procedures as body and handler arguments.
1108 @code{scm_c_catch} and @code{scm_internal_catch} are equivalents taking
1111 @var{body} is called as @code{@var{body} (@var{body_data})} with a catch
1112 on exceptions of the given @var{tag} type. If an exception is caught,
1113 @var{pre_unwind_handler} and @var{handler} are called as
1114 @code{@var{handler} (@var{handler_data}, @var{key}, @var{args})}.
1115 @var{key} and @var{args} are the @code{SCM} key and argument list from
1118 @tpindex scm_t_catch_body
1119 @tpindex scm_t_catch_handler
1120 @var{body} and @var{handler} should have the following prototypes.
1121 @code{scm_t_catch_body} and @code{scm_t_catch_handler} are pointer
1125 SCM body (void *data);
1126 SCM handler (void *data, SCM key, SCM args);
1129 The @var{body_data} and @var{handler_data} parameters are passed to
1130 the respective calls so an application can communicate extra
1131 information to those functions.
1133 If the data consists of an @code{SCM} object, care should be taken
1134 that it isn't garbage collected while still required. If the
1135 @code{SCM} is a local C variable, one way to protect it is to pass a
1136 pointer to that variable as the data parameter, since the C compiler
1137 will then know the value must be held on the stack. Another way is to
1138 use @code{scm_remember_upto_here_1} (@pxref{Remembering During
1143 @node Throw Handlers
1144 @subsubsection Throw Handlers
1146 It's sometimes useful to be able to intercept an exception that is being
1147 thrown before the stack is unwound. This could be to clean up some
1148 related state, to print a backtrace, or to pass information about the
1149 exception to a debugger, for example. The @code{with-throw-handler}
1150 procedure provides a way to do this.
1152 @deffn {Scheme Procedure} with-throw-handler key thunk handler
1153 @deffnx {C Function} scm_with_throw_handler (key, thunk, handler)
1154 Add @var{handler} to the dynamic context as a throw handler
1155 for key @var{key}, then invoke @var{thunk}.
1157 This behaves exactly like @code{catch}, except that it does not unwind
1158 the stack before invoking @var{handler}. If the @var{handler} procedure
1159 returns normally, Guile rethrows the same exception again to the next
1160 innermost catch or throw handler. @var{handler} may exit nonlocally, of
1161 course, via an explicit throw or via invoking a continuation.
1164 Typically @var{handler} is used to display a backtrace of the stack at
1165 the point where the corresponding @code{throw} occurred, or to save off
1166 this information for possible display later.
1168 Not unwinding the stack means that throwing an exception that is handled
1169 via a throw handler is equivalent to calling the throw handler handler
1170 inline instead of each @code{throw}, and then omitting the surrounding
1171 @code{with-throw-handler}. In other words,
1174 (with-throw-handler 'key
1175 (lambda () @dots{} (throw 'key args @dots{}) @dots{})
1180 is mostly equivalent to
1183 ((lambda () @dots{} (handler 'key args @dots{}) @dots{}))
1186 In particular, the dynamic context when @var{handler} is invoked is that
1187 of the site where @code{throw} is called. The examples are not quite
1188 equivalent, because the body of a @code{with-throw-handler} is not in
1189 tail position with respect to the @code{with-throw-handler}, and if
1190 @var{handler} exits normally, Guile arranges to rethrow the error, but
1191 hopefully the intention is clear. (For an introduction to what is meant
1192 by dynamic context, @xref{Dynamic Wind}.)
1194 @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)
1195 The above @code{scm_with_throw_handler} takes Scheme procedures as body
1196 (thunk) and handler arguments. @code{scm_c_with_throw_handler} is an
1197 equivalent taking C functions. See @code{scm_c_catch} (@pxref{Catch})
1198 for a description of the parameters, the behaviour however of course
1199 follows @code{with-throw-handler}.
1202 If @var{thunk} throws an exception, Guile handles that exception by
1203 invoking the innermost @code{catch} or throw handler whose key matches
1204 that of the exception. When the innermost thing is a throw handler,
1205 Guile calls the specified handler procedure using @code{(apply
1206 @var{handler} key args)}. The handler procedure may either return
1207 normally or exit non-locally. If it returns normally, Guile passes the
1208 exception on to the next innermost @code{catch} or throw handler. If it
1209 exits non-locally, that exit determines the continuation.
1211 The behaviour of a throw handler is very similar to that of a
1212 @code{catch} expression's optional pre-unwind handler. In particular, a
1213 throw handler's handler procedure is invoked in the exact dynamic
1214 context of the @code{throw} expression, just as a pre-unwind handler is.
1215 @code{with-throw-handler} may be seen as a half-@code{catch}: it does
1216 everything that a @code{catch} would do until the point where
1217 @code{catch} would start unwinding the stack and dynamic context, but
1218 then it rethrows to the next innermost @code{catch} or throw handler
1221 Note also that since the dynamic context is not unwound, if a
1222 @code{with-throw-handler} handler throws to a key that does not match
1223 the @code{with-throw-handler} expression's @var{key}, the new throw may
1224 be handled by a @code{catch} or throw handler that is @emph{closer} to
1225 the throw than the first @code{with-throw-handler}.
1227 Here is an example to illustrate this behavior:
1232 (with-throw-handler 'b
1238 (lambda (key . args)
1244 This code will call @code{inner-handler} and then continue with the
1245 continuation of the inner @code{catch}.
1249 @subsubsection Throwing Exceptions
1251 The @code{throw} primitive is used to throw an exception. One argument,
1252 the @var{key}, is mandatory, and must be a symbol; it indicates the type
1253 of exception that is being thrown. Following the @var{key},
1254 @code{throw} accepts any number of additional arguments, whose meaning
1255 depends on the exception type. The documentation for each possible type
1256 of exception should specify the additional arguments that are expected
1257 for that kind of exception.
1259 @deffn {Scheme Procedure} throw key arg @dots{}
1260 @deffnx {C Function} scm_throw (key, args)
1261 Invoke the catch form matching @var{key}, passing @var{arg} @dots{} to
1264 @var{key} is a symbol. It will match catches of the same symbol or of
1267 If there is no handler at all, Guile prints an error and then exits.
1270 When an exception is thrown, it will be caught by the innermost
1271 @code{catch} or throw handler that applies to the type of the thrown
1272 exception; in other words, whose @var{key} is either @code{#t} or the
1273 same symbol as that used in the @code{throw} expression. Once Guile has
1274 identified the appropriate @code{catch} or throw handler, it handles the
1275 exception by applying the relevant handler procedure(s) to the arguments
1276 of the @code{throw}.
1278 If there is no appropriate @code{catch} or throw handler for a thrown
1279 exception, Guile prints an error to the current error port indicating an
1280 uncaught exception, and then exits. In practice, it is quite difficult
1281 to observe this behaviour, because Guile when used interactively
1282 installs a top level @code{catch} handler that will catch all exceptions
1283 and print an appropriate error message @emph{without} exiting. For
1284 example, this is what happens if you try to throw an unhandled exception
1285 in the standard Guile REPL; note that Guile's command loop continues
1286 after the error message:
1289 guile> (throw 'badex)
1290 <unnamed port>:3:1: In procedure gsubr-apply @dots{}
1291 <unnamed port>:3:1: unhandled-exception: badex
1296 The default uncaught exception behaviour can be observed by evaluating a
1297 @code{throw} expression from the shell command line:
1300 $ guile -c "(begin (throw 'badex) (display \"here\\n\"))"
1301 guile: uncaught throw to badex: ()
1306 That Guile exits immediately following the uncaught exception
1307 is shown by the absence of any output from the @code{display}
1308 expression, because Guile never gets to the point of evaluating that
1312 @node Exception Implementation
1313 @subsubsection How Guile Implements Exceptions
1315 It is traditional in Scheme to implement exception systems using
1316 @code{call-with-current-continuation}. Continuations
1317 (@pxref{Continuations}) are such a powerful concept that any other
1318 control mechanism --- including @code{catch} and @code{throw} --- can be
1319 implemented in terms of them.
1321 Guile does not implement @code{catch} and @code{throw} like this,
1322 though. Why not? Because Guile is specifically designed to be easy to
1323 integrate with applications written in C. In a mixed Scheme/C
1324 environment, the concept of @dfn{continuation} must logically include
1325 ``what happens next'' in the C parts of the application as well as the
1326 Scheme parts, and it turns out that the only reasonable way of
1327 implementing continuations like this is to save and restore the complete
1330 So Guile's implementation of @code{call-with-current-continuation} is a
1331 stack copying one. This allows it to interact well with ordinary C
1332 code, but means that creating and calling a continuation is slowed down
1333 by the time that it takes to copy the C stack.
1335 The more targeted mechanism provided by @code{catch} and @code{throw}
1336 does not need to save and restore the C stack because the @code{throw}
1337 always jumps to a location higher up the stack of the code that executes
1338 the @code{throw}. Therefore Guile implements the @code{catch} and
1339 @code{throw} primitives independently of
1340 @code{call-with-current-continuation}, in a way that takes advantage of
1341 this @emph{upwards only} nature of exceptions.
1344 @node Error Reporting
1345 @subsection Procedures for Signaling Errors
1347 Guile provides a set of convenience procedures for signaling error
1348 conditions that are implemented on top of the exception primitives just
1351 @deffn {Scheme Procedure} error msg arg @dots{}
1352 Raise an error with key @code{misc-error} and a message constructed by
1353 displaying @var{msg} and writing @var{arg} @enddots{}.
1356 @deffn {Scheme Procedure} scm-error key subr message args data
1357 @deffnx {C Function} scm_error_scm (key, subr, message, args, data)
1358 Raise an error with key @var{key}. @var{subr} can be a string
1359 naming the procedure associated with the error, or @code{#f}.
1360 @var{message} is the error message string, possibly containing
1361 @code{~S} and @code{~A} escapes. When an error is reported,
1362 these are replaced by formatting the corresponding members of
1363 @var{args}: @code{~A} (was @code{%s} in older versions of
1364 Guile) formats using @code{display} and @code{~S} (was
1365 @code{%S}) formats using @code{write}. @var{data} is a list or
1366 @code{#f} depending on @var{key}: if @var{key} is
1367 @code{system-error} then it should be a list containing the
1368 Unix @code{errno} value; If @var{key} is @code{signal} then it
1369 should be a list containing the Unix signal number; If
1370 @var{key} is @code{out-of-range} or @code{wrong-type-arg},
1371 it is a list containing the bad value; otherwise
1372 it will usually be @code{#f}.
1375 @deffn {Scheme Procedure} strerror err
1376 @deffnx {C Function} scm_strerror (err)
1377 Return the Unix error message corresponding to @var{err}, an integer
1380 When @code{setlocale} has been called (@pxref{Locales}), the message
1381 is in the language and charset of @code{LC_MESSAGES}. (This is done
1385 @c begin (scm-doc-string "boot-9.scm" "false-if-exception")
1386 @deffn syntax false-if-exception expr
1387 Returns the result of evaluating its argument; however
1388 if an exception occurs then @code{#f} is returned instead.
1394 @subsection Dynamic Wind
1396 For Scheme code, the fundamental procedure to react to non-local entry
1397 and exits of dynamic contexts is @code{dynamic-wind}. C code could
1398 use @code{scm_internal_dynamic_wind}, but since C does not allow the
1399 convenient construction of anonymous procedures that close over
1400 lexical variables, this will be, well, inconvenient.
1402 Therefore, Guile offers the functions @code{scm_dynwind_begin} and
1403 @code{scm_dynwind_end} to delimit a dynamic extent. Within this
1404 dynamic extent, which is called a @dfn{dynwind context}, you can
1405 perform various @dfn{dynwind actions} that control what happens when
1406 the dynwind context is entered or left. For example, you can register
1407 a cleanup routine with @code{scm_dynwind_unwind_handler} that is
1408 executed when the context is left. There are several other more
1409 specialized dynwind actions as well, for example to temporarily block
1410 the execution of asyncs or to temporarily change the current output
1411 port. They are described elsewhere in this manual.
1413 Here is an example that shows how to prevent memory leaks.
1417 /* Suppose there is a function called FOO in some library that you
1418 would like to make available to Scheme code (or to C code that
1419 follows the Scheme conventions).
1421 FOO takes two C strings and returns a new string. When an error has
1422 occurred in FOO, it returns NULL.
1425 char *foo (char *s1, char *s2);
1427 /* SCM_FOO interfaces the C function FOO to the Scheme way of life.
1428 It takes care to free up all temporary strings in the case of
1433 scm_foo (SCM s1, SCM s2)
1435 char *c_s1, *c_s2, *c_res;
1437 scm_dynwind_begin (0);
1439 c_s1 = scm_to_locale_string (s1);
1441 /* Call 'free (c_s1)' when the dynwind context is left.
1443 scm_dynwind_unwind_handler (free, c_s1, SCM_F_WIND_EXPLICITLY);
1445 c_s2 = scm_to_locale_string (s2);
1447 /* Same as above, but more concisely.
1449 scm_dynwind_free (c_s2);
1451 c_res = foo (c_s1, c_s2);
1453 scm_memory_error ("foo");
1457 return scm_take_locale_string (res);
1461 @rnindex dynamic-wind
1462 @deffn {Scheme Procedure} dynamic-wind in_guard thunk out_guard
1463 @deffnx {C Function} scm_dynamic_wind (in_guard, thunk, out_guard)
1464 All three arguments must be 0-argument procedures.
1465 @var{in_guard} is called, then @var{thunk}, then
1468 If, any time during the execution of @var{thunk}, the
1469 dynamic extent of the @code{dynamic-wind} expression is escaped
1470 non-locally, @var{out_guard} is called. If the dynamic extent of
1471 the dynamic-wind is re-entered, @var{in_guard} is called. Thus
1472 @var{in_guard} and @var{out_guard} may be called any number of
1476 (define x 'normal-binding)
1479 (call-with-current-continuation
1485 (lambda () (set! x 'special-binding))
1489 (lambda () (display x) (newline)
1490 (call-with-current-continuation escape)
1491 (display x) (newline)
1496 (lambda () (set! x old-x)))))))
1502 @result{} normal-binding
1507 @result{} a-cont ;; the value of the (define a-cont...)
1509 @result{} normal-binding
1511 @result{} special-binding
1515 @deftp {C Type} scm_t_dynwind_flags
1516 This is an enumeration of several flags that modify the behavior of
1517 @code{scm_dynwind_begin}. The flags are listed in the following
1521 @item SCM_F_DYNWIND_REWINDABLE
1522 The dynamic context is @dfn{rewindable}. This means that it can be
1523 reentered non-locally (via the invocation of a continuation). The
1524 default is that a dynwind context can not be reentered non-locally.
1529 @deftypefn {C Function} void scm_dynwind_begin (scm_t_dynwind_flags flags)
1530 The function @code{scm_dynwind_begin} starts a new dynamic context and
1531 makes it the `current' one.
1533 The @var{flags} argument determines the default behavior of the
1534 context. Normally, use 0. This will result in a context that can not
1535 be reentered with a captured continuation. When you are prepared to
1536 handle reentries, include @code{SCM_F_DYNWIND_REWINDABLE} in
1539 Being prepared for reentry means that the effects of unwind handlers
1540 can be undone on reentry. In the example above, we want to prevent a
1541 memory leak on non-local exit and thus register an unwind handler that
1542 frees the memory. But once the memory is freed, we can not get it
1543 back on reentry. Thus reentry can not be allowed.
1545 The consequence is that continuations become less useful when
1546 non-reentrant contexts are captured, but you don't need to worry
1547 about that too much.
1549 The context is ended either implicitly when a non-local exit happens,
1550 or explicitly with @code{scm_dynwind_end}. You must make sure that a
1551 dynwind context is indeed ended properly. If you fail to call
1552 @code{scm_dynwind_end} for each @code{scm_dynwind_begin}, the behavior
1556 @deftypefn {C Function} void scm_dynwind_end ()
1557 End the current dynamic context explicitly and make the previous one
1561 @deftp {C Type} scm_t_wind_flags
1562 This is an enumeration of several flags that modify the behavior of
1563 @code{scm_dynwind_unwind_handler} and
1564 @code{scm_dynwind_rewind_handler}. The flags are listed in the
1568 @item SCM_F_WIND_EXPLICITLY
1569 @vindex SCM_F_WIND_EXPLICITLY
1570 The registered action is also carried out when the dynwind context is
1571 entered or left locally.
1575 @deftypefn {C Function} void scm_dynwind_unwind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
1576 @deftypefnx {C Function} void scm_dynwind_unwind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
1577 Arranges for @var{func} to be called with @var{data} as its arguments
1578 when the current context ends implicitly. If @var{flags} contains
1579 @code{SCM_F_WIND_EXPLICITLY}, @var{func} is also called when the
1580 context ends explicitly with @code{scm_dynwind_end}.
1582 The function @code{scm_dynwind_unwind_handler_with_scm} takes care that
1583 @var{data} is protected from garbage collection.
1586 @deftypefn {C Function} void scm_dynwind_rewind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
1587 @deftypefnx {C Function} void scm_dynwind_rewind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
1588 Arrange for @var{func} to be called with @var{data} as its argument when
1589 the current context is restarted by rewinding the stack. When @var{flags}
1590 contains @code{SCM_F_WIND_EXPLICITLY}, @var{func} is called immediately
1593 The function @code{scm_dynwind_rewind_handler_with_scm} takes care that
1594 @var{data} is protected from garbage collection.
1597 @deftypefn {C Function} void scm_dynwind_free (void *mem)
1598 Arrange for @var{mem} to be freed automatically whenever the current
1599 context is exited, whether normally or non-locally.
1600 @code{scm_dynwind_free (mem)} is an equivalent shorthand for
1601 @code{scm_dynwind_unwind_handler (free, mem, SCM_F_WIND_EXPLICITLY)}.
1605 @node Handling Errors
1606 @subsection How to Handle Errors
1608 Error handling is based on @code{catch} and @code{throw}. Errors are
1609 always thrown with a @var{key} and four arguments:
1613 @var{key}: a symbol which indicates the type of error. The symbols used
1614 by libguile are listed below.
1617 @var{subr}: the name of the procedure from which the error is thrown, or
1621 @var{message}: a string (possibly language and system dependent)
1622 describing the error. The tokens @code{~A} and @code{~S} can be
1623 embedded within the message: they will be replaced with members of the
1624 @var{args} list when the message is printed. @code{~A} indicates an
1625 argument printed using @code{display}, while @code{~S} indicates an
1626 argument printed using @code{write}. @var{message} can also be
1627 @code{#f}, to allow it to be derived from the @var{key} by the error
1628 handler (may be useful if the @var{key} is to be thrown from both C and
1632 @var{args}: a list of arguments to be used to expand @code{~A} and
1633 @code{~S} tokens in @var{message}. Can also be @code{#f} if no
1634 arguments are required.
1637 @var{rest}: a list of any additional objects required. e.g., when the
1638 key is @code{'system-error}, this contains the C errno value. Can also
1639 be @code{#f} if no additional objects are required.
1642 In addition to @code{catch} and @code{throw}, the following Scheme
1643 facilities are available:
1645 @deffn {Scheme Procedure} display-error frame port subr message args rest
1646 @deffnx {C Function} scm_display_error (frame, port, subr, message, args, rest)
1647 Display an error message to the output port @var{port}.
1648 @var{frame} is the frame in which the error occurred, @var{subr} is
1649 the name of the procedure in which the error occurred and
1650 @var{message} is the actual error message, which may contain
1651 formatting instructions. These will format the arguments in
1652 the list @var{args} accordingly. @var{rest} is currently
1656 The following are the error keys defined by libguile and the situations
1657 in which they are used:
1661 @cindex @code{error-signal}
1662 @code{error-signal}: thrown after receiving an unhandled fatal signal
1663 such as SIGSEGV, SIGBUS, SIGFPE etc. The @var{rest} argument in the throw
1664 contains the coded signal number (at present this is not the same as the
1665 usual Unix signal number).
1668 @cindex @code{system-error}
1669 @code{system-error}: thrown after the operating system indicates an
1670 error condition. The @var{rest} argument in the throw contains the
1674 @cindex @code{numerical-overflow}
1675 @code{numerical-overflow}: numerical overflow.
1678 @cindex @code{out-of-range}
1679 @code{out-of-range}: the arguments to a procedure do not fall within the
1683 @cindex @code{wrong-type-arg}
1684 @code{wrong-type-arg}: an argument to a procedure has the wrong type.
1687 @cindex @code{wrong-number-of-args}
1688 @code{wrong-number-of-args}: a procedure was called with the wrong number
1692 @cindex @code{memory-allocation-error}
1693 @code{memory-allocation-error}: memory allocation error.
1696 @cindex @code{stack-overflow}
1697 @code{stack-overflow}: stack overflow error.
1700 @cindex @code{regular-expression-syntax}
1701 @code{regular-expression-syntax}: errors generated by the regular
1705 @cindex @code{misc-error}
1706 @code{misc-error}: other errors.
1710 @subsubsection C Support
1712 In the following C functions, @var{SUBR} and @var{MESSAGE} parameters
1713 can be @code{NULL} to give the effect of @code{#f} described above.
1715 @deftypefn {C Function} SCM scm_error (SCM @var{key}, char *@var{subr}, char *@var{message}, SCM @var{args}, SCM @var{rest})
1716 Throw an error, as per @code{scm-error} (@pxref{Error Reporting}).
1719 @deftypefn {C Function} void scm_syserror (char *@var{subr})
1720 @deftypefnx {C Function} void scm_syserror_msg (char *@var{subr}, char *@var{message}, SCM @var{args})
1721 Throw an error with key @code{system-error} and supply @code{errno} in
1722 the @var{rest} argument. For @code{scm_syserror} the message is
1723 generated using @code{strerror}.
1725 Care should be taken that any code in between the failing operation
1726 and the call to these routines doesn't change @code{errno}.
1729 @deftypefn {C Function} void scm_num_overflow (char *@var{subr})
1730 @deftypefnx {C Function} void scm_out_of_range (char *@var{subr}, SCM @var{bad_value})
1731 @deftypefnx {C Function} void scm_wrong_num_args (SCM @var{proc})
1732 @deftypefnx {C Function} void scm_wrong_type_arg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value})
1733 @deftypefnx {C Function} void scm_wrong_type_arg_msg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value}, const char *@var{expected})
1734 @deftypefnx {C Function} void scm_memory_error (char *@var{subr})
1735 @deftypefnx {C Function} void scm_misc_error (const char *@var{subr}, const char *@var{message}, SCM @var{args})
1736 Throw an error with the various keys described above.
1738 In @code{scm_wrong_num_args}, @var{proc} should be a Scheme symbol
1739 which is the name of the procedure incorrectly invoked. The other
1740 routines take the name of the invoked procedure as a C string.
1742 In @code{scm_wrong_type_arg_msg}, @var{expected} is a C string
1743 describing the type of argument that was expected.
1745 In @code{scm_misc_error}, @var{message} is the error message string,
1746 possibly containing @code{simple-format} escapes (@pxref{Writing}), and
1747 the corresponding arguments in the @var{args} list.
1751 @subsubsection Signalling Type Errors
1753 Every function visible at the Scheme level should aggressively check the
1754 types of its arguments, to avoid misinterpreting a value, and perhaps
1755 causing a segmentation fault. Guile provides some macros to make this
1758 @deftypefn Macro void SCM_ASSERT (int @var{test}, SCM @var{obj}, unsigned int @var{position}, const char *@var{subr})
1759 @deftypefnx Macro void SCM_ASSERT_TYPE (int @var{test}, SCM @var{obj}, unsigned int @var{position}, const char *@var{subr}, const char *@var{expected})
1760 If @var{test} is zero, signal a ``wrong type argument'' error,
1761 attributed to the subroutine named @var{subr}, operating on the value
1762 @var{obj}, which is the @var{position}'th argument of @var{subr}.
1764 In @code{SCM_ASSERT_TYPE}, @var{expected} is a C string describing the
1765 type of argument that was expected.
1768 @deftypefn Macro int SCM_ARG1
1769 @deftypefnx Macro int SCM_ARG2
1770 @deftypefnx Macro int SCM_ARG3
1771 @deftypefnx Macro int SCM_ARG4
1772 @deftypefnx Macro int SCM_ARG5
1773 @deftypefnx Macro int SCM_ARG6
1774 @deftypefnx Macro int SCM_ARG7
1775 One of the above values can be used for @var{position} to indicate the
1776 number of the argument of @var{subr} which is being checked.
1777 Alternatively, a positive integer number can be used, which allows to
1778 check arguments after the seventh. However, for parameter numbers up to
1779 seven it is preferable to use @code{SCM_ARGN} instead of the
1780 corresponding raw number, since it will make the code easier to
1784 @deftypefn Macro int SCM_ARGn
1785 Passing a value of zero or @code{SCM_ARGn} for @var{position} allows to
1786 leave it unspecified which argument's type is incorrect. Again,
1787 @code{SCM_ARGn} should be preferred over a raw zero constant.
1790 @node Continuation Barriers
1791 @subsection Continuation Barriers
1793 The non-local flow of control caused by continuations might sometimes
1794 not be wanted. You can use @code{with-continuation-barrier} to erect
1795 fences that continuations can not pass.
1797 @deffn {Scheme Procedure} with-continuation-barrier proc
1798 @deffnx {C Function} scm_with_continuation_barrier (proc)
1799 Call @var{proc} and return its result. Do not allow the invocation of
1800 continuations that would leave or enter the dynamic extent of the call
1801 to @code{with-continuation-barrier}. Such an attempt causes an error
1804 Throws (such as errors) that are not caught from within @var{proc} are
1805 caught by @code{with-continuation-barrier}. In that case, a short
1806 message is printed to the current error port and @code{#f} is returned.
1808 Thus, @code{with-continuation-barrier} returns exactly once.
1811 @deftypefn {C Function} {void *} scm_c_with_continuation_barrier (void *(*func) (void *), void *data)
1812 Like @code{scm_with_continuation_barrier} but call @var{func} on
1813 @var{data}. When an error is caught, @code{NULL} is returned.
1818 @c TeX-master: "guile.texi"