suzanne.soy/racket
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avoid 'begin' for the stepper's sake

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svn: r1574
This commit is contained in:
Matthew Flatt 2005-12-09 22:00:13 +00:00
parent edd5e54a4f
commit 1117ce0d06
1 changed files with 6 additions and 1 deletions
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7
collects/htdp/world.ss
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@ -230,7 +230,7 @@
(define-syntax (update s) (define-syntax (update s)
(syntax-case s (produce) (syntax-case s (produce)
[(_ pict produce exp) (syntax (begin (update-frame pict) exp))] [(_ pict produce exp) (syntax (update-frame/proc pict (lambda () exp)))]
[(_ pict pict2 ... produce exp) [(_ pict pict2 ... produce exp)
(raise-syntax-error 'update "you can place only one picture in the canvas" s)] (raise-syntax-error 'update "you can place only one picture in the canvas" s)]
[(_ stmt produce) [(_ stmt produce)
@ -240,6 +240,11 @@
[_ [_
(raise-syntax-error 'update "use as (update <image> produce <expression>)")])) (raise-syntax-error 'update "use as (update <image> produce <expression>)")]))
(define (update-frame/proc pict thunk)
(begin
(update-frame pict)
(thunk)))
(define (update-frame pict) (define (update-frame pict)
(unless the-frame (error 'update SEQUENCE-ERROR)) (unless the-frame (error 'update SEQUENCE-ERROR))
(check-result 'update (lambda (x) (beg:image? x)) "image" pict) (check-result 'update (lambda (x) (beg:image? x)) "image" pict)