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authorLudovic Courtès <ludo@gnu.org>2013-04-04 14:14:25 +0800
committerLudovic Courtès <ludo@gnu.org>2013-04-05 22:54:14 +0200
commit55e26a49dbc5fa7ccbf218305d88b0b37db4db3f (patch)
tree39c618a29dc54a15d5c5c757430d89c01071886b /doc/ref/api-control.texi
parentd888b531681c8528a2daafe0bea70c0a42313da6 (diff)
downloadguile-55e26a49dbc5fa7ccbf218305d88b0b37db4db3f.tar.gz
Add `call/ec' and `let/ec'.
Based on a patch by Nala Ginrut <nalaginrut@gmail.com>, with suggestions from Mark H. Weaver. * module/ice-9/control.scm (call-with-escape-continuation, call/ec): New procedures. (let-escape-continuation, let/ec): New macros. * module/ice-9/futures.scm (let/ec): Remove. * test-suite/tests/control.test ("escape-only continuations")["call/ec", "let/ec"]: New tests. * doc/ref/api-control.texi (Prompt Primitives): Document `call/ec', `let/ec', and their long names.
Diffstat (limited to 'doc/ref/api-control.texi')
-rw-r--r--doc/ref/api-control.texi49
1 files changed, 48 insertions, 1 deletions
diff --git a/doc/ref/api-control.texi b/doc/ref/api-control.texi
index 320812dfb..278a03dcb 100644
--- a/doc/ref/api-control.texi
+++ b/doc/ref/api-control.texi
@@ -574,9 +574,56 @@ both.
Before moving on, we should mention that if the handler of a prompt is a
@code{lambda} expression, and the first argument isn't referenced, an abort to
-that prompt will not cause a continuation to be reified. This can be an
+that prompt will not cause a continuation to be reified. This can be an
important efficiency consideration to keep in mind.
+@cindex continuation, escape
+One example where this optimization matters is @dfn{escape
+continuations}. Escape continuations are delimited continuations whose
+only use is to make a non-local exit---i.e., to escape from the current
+continuation. Such continuations are invoked only once, and for this
+reason they are sometimes called @dfn{one-shot continuations}.
+
+The constructs below are syntactic sugar atop prompts to simplify the
+use of escape continuations.
+
+@deffn {Scheme Procedure} call-with-escape-continuation proc
+@deffnx {Scheme Procedure} call/ec proc
+Call @var{proc} with an escape continuation.
+
+In the example below, the @var{return} continuation is used to escape
+the continuation of the call to @code{fold}.
+
+@lisp
+(use-modules (ice-9 control)
+ (srfi srfi-1))
+
+(define (prefix x lst)
+ ;; Return all the elements before the first occurrence
+ ;; of X in LST.
+ (call/ec
+ (lambda (return)
+ (fold (lambda (element prefix)
+ (if (equal? element x)
+ (return (reverse prefix)) ; escape `fold'
+ (cons element prefix)))
+ '()
+ lst))))
+
+(prefix 'a '(0 1 2 a 3 4 5))
+@result{} (0 1 2)
+@end lisp
+@end deffn
+
+@deffn {Scheme Syntax} let-escape-continuation k body @dots{}
+@deffnx {Scheme Syntax} let/ec k body @dots{}
+Bind @var{k} within @var{body} to an escape continuation.
+
+This is equivalent to
+@code{(call/ec (lambda (@var{k}) @var{body} @dots{}))}.
+@end deffn
+
+
@node Shift and Reset
@subsubsection Shift, Reset, and All That