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This module provides some syntactic sugar to Marc Feeley's continuation interface. In this interface, continuations are a datatype separate from procedures. Hence it provides a continuation? predicate. I've stripped the prefix from the procedures continuation-capture and continuation-graft and renamed continuation-return throw. This latter procedure is accompanied in this module by a macro named catch, so that the usual catch-throw-semantics of other languages is available. For example, a typical use pattern of this module is as follows
(catch k ... (if ... (throw k val) ...))
But note, that in this pattern, k is a Scheme object of type continuation, and like every Scheme object it has indefinite extent, hence can be exported, saved in other data-structures etc. So this pattern is much more powerful than the corresponding pattern in other languages.
Some other procedures are provided as well, in particular continuation->procedure, which does what the name says, and continuations, which provides offline documentation to the module. Like in modules written in the design-by-contract style, the call (continuations) lists the exported symbols, and (continuations sym) provides documentation of the exported symbol sym.
Moreover, another interface to continuations is provided, recommended by Matt Might, albeit with different names: continuation and the infamous goto. The former is sort of a constructor, usually used as follows:
(let ((cc (continuation))) (if (continuation? cc) ... (throw cc val) ... ... do something with val ...))
Note, that the let is invoked twice, first after the call to continuation, then with the object val, which was thrown to cc.
The latter should be used with extreme care, off course, since it is almost as harmfull as the traditional goto, except, that it uses continuations instead of labels, hence can only jump to places visited before. But it is sometimes useful, e.g. in backtracking ....
continuations[procedure] (continuations [sym])
captures and returns the current continuation.
continuation?[procedure] (continuation? xpr)
continuation->procedure[procedure] (continuation->procedure cont)
transforms a continuation into a procedure
capture[procedure] (capture proc)
The same as call/cc but with a different datatype: Captures the current continuation as a continuation datatype (contrary to a procedure datatype in call/cc) and calls proc with that continuation as its only argument.
graft[procedure] (graft cont thunk)
tail-calls thunk with the implicit continuation cont.
throw[procedure] (throw cont . vals)
throws the values vals to the continuation cont.
catch[syntax] (catch cont xpr . xprs)
The same as let/cc of miscmacros but with a different datatype: Binds the cont variable to the current continuation as a continuation and executes the body xpr . xprs in this context.
goto[procedure] (goto cc)
The infamous goto, but with a continuation as argument instead of a label.
(define amb-stack '()) (define (amb-fail) (if (pair? amb-stack) (let ((back-track-point (car amb-stack))) (set! amb-stack (cdr amb-stack)) (goto back-track-point)) (error 'amb-fail "amb-stack exhausted"))) (define (amb-choose . choices) (let ((cc (continuation))) (cond ((null? choices) (amb-fail)) ((pair? choices) (let ((choice (car choices))) (set! choices (cdr choices)) (set! amb-stack (cons cc amb-stack)) choice))))) (define (amb-assert xpr) (if (not xpr) (amb-fail) #t)) (define (pythagoras . choices) (let ((a (apply amb-choose choices)) (b (apply amb-choose choices)) (c (apply amb-choose choices))) (amb-assert (= (* c c) (+ (* a a) (* b b)))) (amb-assert (< b a)) (list a b c))) (pythagoras 1 2 3 4 5 6 7) ;-> (4 3 5)
Jul 21, 2013
Copyright (c) 2013, Juergen Lorenz All rights reserved.
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- added continuation and goto, amb example
- initial import