Hrvoje> Can you give two or three simple examples of call/cc usage? I assume you mean code---I enumerated some applications in previous posts. Note that any code using catch/throw is also an example for call/cc, but the reverse does not hold. Here's an implementation of coroutines, courtesy of Matthias Blume, Princeton. (define (new-coroutine) (let ((x '())) (lambda (cmd) (if cmd (set! x cmd) (x #f))))) (define (init-coroutine! c proc . args) (c (lambda (ign) (apply proc args)))) (define (transfer! from to) (call-with-current-continuation (lambda (c) (from c) (to #f)))) Here's an implementation of generators from the Scheme 48 distribution. ; Copyright (c) 1993, 1994 by Richard Kelsey and Jonathan Rees. ; Copyright (c) 1996 by NEC Research Institute, Inc. See file COPYING. ; Internal variable representing the failure stack. (define (fail) (*fail*)) (define *fail* (lambda () (error "You didn't do (init)."))) ; For the alternation operator, Icon's a | b or McCarthy's (amb a b), ; we write (either a b). (define-syntax either (syntax-rules () ((either) (fail)) ((either x) x) ((either x y ...) (%either (lambda () x) (lambda () (either y ...)))))) (define (%either thunk1 thunk2) ;Macro auxiliary (saving-failure-state (lambda (restore) ((call-with-current-continuation (lambda (k) (set! *fail* (lambda () (restore) (k thunk2))) thunk1)))))) (define (saving-failure-state proc) (let ((save *fail*)) (proc (lambda () (set! *fail* save))))) ; (one-value x) is Prolog's CUT operator (define-syntax one-value (syntax-rules () ((one-value x) (%one-value (lambda () x))))) (define (%one-value thunk) (saving-failure-state (lambda (restore) (let ((value (thunk))) (restore) value)))) ; (all-values a) returns a list of all the possible values of the ; expression a. Prolog calls this "bagof"; I forget what Icon calls it. (define-syntax all-values (syntax-rules () ((all-values x) (%all-values (lambda () x))))) (define (%all-values thunk) (let ((results '())) (either (let ((new-result (thunk))) (set! results (cons new-result results)) (fail)) (reverse results)))) ; Generate all the members of list l. E.g. ; (all-values (+ (member-of '(10 20 30)) (member-of '(1 2 3)))) ; => '(11 12 13 21 22 23 31 32 33) (define (member-of l) (if (null? l) (fail) (either (car l) (member-of (cdr l))))) ; Crufty initialization hack that allows you to type failing ; expressions at the R-E-P loop (if there is an R-E-P loop). E.g. try ; evaluating the sequence ; (either 1 2) ; (fail) ; (+ (fail) 10) (define (init) (set! *fail* #f) ;for GC purposes (either 'initialized (let loop () (either 'failed (loop))))) (display "Type (init) at the read-eval-print loop.") (newline) An implementation of an exception system, documentation is in ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/prop/exceptions.ss Documentation is in: ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/prop/exceptions Here's a (not-so-simple) implementation of control representation, also from the Scheme 48 implementation: ; no copyright notice ; ,open signals escapes ; Changes by jar: ; Added Uses of Scheme 48's WITH-CONTINUATION primitive, so that unreachable ; continuations can be reclaimed by the GC. ; ; Renamed reset-thunk -> *reset ; call/ct -> *shift ; ; Note: the meta-continuation ought to be thread-specific. ; Alternatively, the threads package could be defined in terms of ; shift and reset. This would have the advantage of making the threads ; package itself re-entrant. It would be nice to rehabilitate the ; runnable-threads queue, currently a piece of global state, as local ; to a particular invocation of WITH-MULTITASKING. ;Date: Wed, 29 Dec 1993 13:54:52 +0100 ;From: Olivier Danvy <danvy(a)daimi.aau.dk&gt; ;To: jar(a)martigny.ai.mit.edu ;Subject: little Christmas gift ;Reply-To: danvy(a)daimi.aau.dk ; ;Hi again: ; ;Here is a contribution for the Scheme48 library: the shift and reset ;operators from "Abstracting Control" (LFP90) and "Representing Control" ;(MSCS92). In his POPL94 paper, Andrzej Filinski observed that since the ;meta-continuation is single-threaded, it can be globalized in a ;register. Andrzej has programmed this both in SML and in Scheme. I ;only have prettified the Scheme definition a wee bit. (define-syntax reset (syntax-rules () ((_ ?e) (*reset (lambda () ?e))))) (define-syntax shift (syntax-rules () ((_ ?k ?e) (*shift (lambda (?k) ?e))))) (define *meta-continuation* (lambda (v) (error "You forgot the top-level reset..."))) (define *abort (lambda (thunk) (with-continuation null-continuation ;JAR hack (lambda () (let ((val (thunk))) (*meta-continuation* val)))))) (define null-continuation #f) (define *reset (lambda (thunk) (let ((mc *meta-continuation*)) (call-with-current-continuation (lambda (k) (begin (set! *meta-continuation* (lambda (v) (set! *meta-continuation* mc) (k v))) (*abort thunk))))))) (define *shift (lambda (f) (call-with-current-continuation (lambda (k) (*abort (lambda () (f (lambda (v) (reset (k v)))))))))) ;---------- ; ;Reminder: reset specifies a control delimiter. shift grabs the current ;continuation up to the current control delimiter, and reifies it as a ;composable procedure. If the procedure is not used, shift has the ;effect of aborting up to the current control delimiter. ; ;Examples: ; ;(+ 10 (reset (+ 2 3))) ;--> ;15 ; ;(+ 10 (reset (+ 2 (shift k 3)))) ;--> ;13 ; ;(+ 10 (reset (+ 2 (shift k (k 3))))) ;--> ;15 ; ;(+ 10 (reset (+ 2 (shift k (+ 100 (k 3)))))) ;--> ;115 ; ;(+ 10 (reset (+ 2 (shift k (+ 100 (k (k 3))))))) ;--> ;117 ; ; ;Other reminder: shift and reset are weaker than Matthias's control and ;prompt, in that they can be CPS-transformed. ; ;Have a happy holiday, ; ;-- Olivier ; ;PS: This definition is not unlike David Espinoza's implementation of monadic ;effects, ie, it has no interpretive or translation overhead. ; JAR's notes: ; ; ; CWCC defined in terms of SHIFT ; ; (define cwcc ; (lambda (p) ; (shift k (k (p (lambda (x) ; (shift k1 (k x)))))))) ; ; ; Monads from shift and reset (from Filinski, POPL '94) ; ; (define (reflect meaning) ; (shift k (extend k meaning))) ; ; (define (reify thunk) ; (reset (eta (thunk)))) ; ; Example: nondeterminism monad. ; ; > (define (eta x) (list x)) ; > (define (extend f l) (apply append (map f l))) ; > ; > (define-syntax amb ; (syntax-rules () ((amb ?x ?y) (*amb (lambda () ?x) (lambda () ?y))))) ; ; > (define (*amb t1 t2) ; (reflect (append (reify t1) (reify t2)))) ; > ; > (reify (lambda () (amb 1 2))) ; '(1 2) ; > (reify (lambda () (+ (amb 1 2) 3))) ; '(4 5) ; > ; > (define cwcc call-with-current-continuation) ; > (reify (lambda () ; (+ 1 (cwcc (lambda (k) ; (* 10 (amb 3 (k 4)))))))) ; '(31 51) ; > -- Cheers =8-} Chipsy Friede, Völkerverständigung und überhaupt blabla