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This library contains two interfaces to continuations, Marc Feeley's and Matt Might's, both with some syntactic sugar.
In Mark Feeley's 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 my other modules, the call (continuations) lists the exported symbols, and (continuations sym) provides documentation of the exported symbol sym.
And then there is the infamous goto, which albeit dangerous is sometimes useful, e.g. in backtracking ....
Matt Might's interface is proveded in two flavors, with continuations, checked by continuation?, and escape procedures, checked by escape-procedure?. This makes possible code written in an idiom similar to the setjmp-longjmp-pair in C.
captures and returns the current continuation as an escape procedure. Typically used as follows
(let ((cc (escape-procedure))) (cond ((escape-procedure? cc) ;; normal body ;; possibly calling (cc val) ... ((ok? cc) ;; exceptional case ;; do something with cc ...))
Note, that the let is invoked twice, first after the call to escape-procedure, then with the object val, which was bound to cc by calling (cc val).
- escape-procedure? xprprocedure
type predicate, defined simultaneously with escape-procedure
- call/ep receiverprocedure
like call/cc, but reifies the captured continuation as a typed escape-procedure
- (let/ep k xpr . xprs)syntax
like let/cc from miscmacros, but reifies the captured continuation k as a typed escape-procedure
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? xprprocedure
- continuation->procedure contprocedure
transforms a continuation into a procedure
- capture procprocedure
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 cont thunkprocedure
tail-calls thunk with the implicit continuation cont.
- throw cont val ....procedure
throws the values val .... to the continuation cont.
- (catch cont xpr ....)syntax
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 .... in this context. Typically used as follows
(catch k ... (if ... (throw k val) ...))
- goto ccprocedure
The infamous goto, but with a continuation as argument instead of a label.
- call receiverprocedure
The same as call/cc, but implemented via capture.
- continuations sym ..procedure
(import continuations) (define 1+ #f) ;; ... with catch (define (init) (+ 1 (catch cont (set! 1+ (continuation->procedure cont)) (throw cont 0)))) ;; ... with continuation (define (init-again) (+ 1 (let ((cc (continuation))) (cond ((continuation? cc) (set! 1+ (continuation->procedure cc)) (throw cc 0)) (else cc))))) (define (search ok? lst) (catch return (for-each (lambda (item) (if (ok? item) (throw return item))) lst) #f)) (define (search-with-goto ok? lst) (let ((start (continuation))) (cond ((null? lst) #f) ((ok? (car lst)) (car lst)) (else (set! lst (cdr lst)) (goto start))))) ;; nonlocal return: throw and catch in different procedures (define (treat ok?) (lambda (item cont) (if (ok? item) (throw cont item)))) (define (handled-search handle lst) (catch return (for-each (lambda (item) (handle item return)) lst) #f)) (define (product . nums) (let ((cc (escape-procedure))) (cond ((escape-procedure? cc) ; continuation cc just created (print "NORMAL BODY") (cond ((null? nums) 1) ((zero? (car nums)) (cc 0)) (else (* (car nums) (apply product (cdr nums)))))) ((number? cc) ; cc has been thrown a number (print "EXCEPTIONAL CASE") cc) )))
Jun 23, 2019
Copyright (c) 2013-2019, Juergen Lorenz All rights reserved.
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- call/ep and let/ep added
- typo in documentation procedure corrected
- ported and simplyfied from chicken-4 version 1.4.1