more continuations

pull/2/head
Matthew Butterick 9 years ago
parent 93db2015af
commit 83a1090754

@ -18,7 +18,7 @@
(gather-unique-ids #'(PROGRAM-LINE ...)))])
#'(#%module-begin
(define UNIQUE-ID 0) ...
(run PROGRAM-LINE ...))))
(run PROGRAM-LINE ... (line #f (statement "end"))))))
; #%app and #%datum have to be present to make #%top work
(define-macro (basic-top . ID)
@ -53,13 +53,11 @@
(with-handlers ([end-program-signal? void])
(for/fold ([program-counter 0])
([i (in-naturals)])
(if (= program-counter (vector-length lines))
(basic:end)
(let* ([line-thunk ($line-thunk (vector-ref lines program-counter))]
[maybe-line-number (line-thunk)])
(if (number? maybe-line-number)
(find-index maybe-line-number)
(add1 program-counter))))))))
(let* ([line-thunk ($line-thunk (vector-ref lines program-counter))]
[maybe-line-number (line-thunk)])
(if (number? maybe-line-number)
(find-index maybe-line-number)
(add1 program-counter)))))))
(define return-stack empty)
@ -70,8 +68,8 @@
(struct $line (number thunk) #:transparent)
(define-macro (line NUMBER . STATEMENTS)
#'($line NUMBER (λ () (with-handlers ([end-line-signal? (λ _ #f)])
. STATEMENTS))))
#'($line NUMBER (λ () (with-handlers ([end-line-signal? (λ _ #f)])
. STATEMENTS))))
(define-macro statement
[(statement ID "=" EXPR) #'(set! ID EXPR)]
@ -170,31 +168,27 @@
#'(basic:for VAR START-VALUE END-VALUE 1)]
[(_ VAR START-VALUE END-VALUE STEP-VALUE)
#'(begin
(cond
[(and (pair? for-stack)
(eq? 'VAR (car (car for-stack))))
;; we're already in the midst of a loop, so abort the rest of the line
(raise-end-line-signal)]
[else
;; initialize the loop counter
(statement VAR "=" START-VALUE)
;; create a point for the `next` statement to return to, using a continuation
(let/cc return-point-for-next
(push-for-stack (cons 'VAR
(λ () ; thunk that increments counter & teleports back to beginning of loop
(define next-val (+ VAR STEP-VALUE))
(when (<= next-val END-VALUE)
(set! VAR next-val)
(return-point-for-next))))))
(raise-end-line-signal)]))])
(define (handle-next [stack-selector-proc car])
(unless (pair? for-stack)
(error 'next "for-stack is empty"))
(let ([for-thunk (cdr (stack-selector-proc for-stack))])
(unless (for-thunk)
(pop-for-stack))))
(define-macro basic:next
[(_ VAR) #'(handle-next (λ(stack) (assq 'VAR stack)))] ; named `next` means find var in stack
[(_) #'(handle-next)]) ; plain `next` implies var on top of stack
;; initialize the loop counter
(statement VAR "=" START-VALUE)
;; create a point for the `next` statement to return to, using a continuation
(let/cc return-point-for-next
(push-for-stack (cons 'VAR
(procedure-rename
(λ () ; thunk that increments counter & teleports back to beginning of loop
(define next-val (+ VAR STEP-VALUE))
(if (<= next-val END-VALUE)
(begin
(set! VAR next-val)
(return-point-for-next #f)) ; return value for subsequent visits to line
(pop-for-stack))) (format-datum "~a-incrementer" 'VAR))))
#f))]) ; return value for first visit to line
(define (handle-next [which #f])
(unless (pair? for-stack) (error 'next "for-stack is empty"))
(define for-thunk (cdr (if which
(assq which for-stack)
(car for-stack))))
(for-thunk))
(define-macro (basic:next VAR ...)
#'(handle-next 'VAR ...))

@ -1,5 +1,8 @@
#lang br/demo/basic
10 for A=1 to 5 step 3
10 for A=1 to 3
20 print A
30 next
21 for B=5 to 8
22 print B
23 next B
30 next A
40 print "yay"
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