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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. But note, that in this pattern, the continuation is a Scheme object, 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, the latter, although dangerous, is sometimes useful, e.g. in backtracking ....
continuations[procedure] (continuations [sym])
captures and returns the current continuation. Typically 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.
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. Typically used as follows
(catch k ... (if ... (throw k val) ...))
goto[procedure] (goto cc)
The infamous goto, but with a continuation as argument instead of a label.
;; multiple operators sharing common state (define-values (amb-fail amb-choose amb-assert) (let ((amb-stack '())) (values (lambda () (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"))) (lambda 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))))) (lambda (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)
(define-syntax iterate (syntax-rules () ((_ var iterator xpr . xprs) (let ((it iterator) (it-cont #f)) (let loop () (let ((cc (continuation))) (if (continuation? cc) ;; first let-pass (if (continuation? it-cont) (throw it-cont (void)) (it (lambda (val) (catch next-cc (throw cc (cons next-cc val)))))) ;; second let-pass (cc is now pair) (let ((next-cont (car cc)) (next-val (cdr cc))) (set! it-cont next-cont) (let ((var next-val)) xpr . xprs) (loop))))))))) ;; define an iterator for tree, i.e. a function of yield, which returns ;; one tree-item at each pass (define (tree-iterator tree) (lambda (yield) (let walk ((tree tree)) (if (pair? tree) (begin (walk (car tree)) (walk (cdr tree))) (yield tree))))) (iterate var (tree-iterator '(3 . ((4 . 5) . 6))) (print var)) ; prints 3 4 5 6 in sequence
Jul 21, 2013
Copyright (c) 2013, Juergen Lorenz All rights reserved.
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- added continuation and goto with the amb example
- initial import