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beautiful-racket/beautiful-racket-demo/basic-demo-2/for.rkt

30 lines
1017 B
Racket

#lang br
(require "let.rkt" "line.rkt")
(provide b-for b-next)
(define thunk-table (make-hasheq))
(define-macro-cases b-for
[(_ LOOP-ID START END) #'(b-for LOOP-ID START END 1)]
[(_ LOOP-ID START END STEP)
#'(b-let LOOP-ID (let/cc loop-cc
(hash-set! thunk-table
'LOOP-ID
(λ ()
(define next-val (+ LOOP-ID STEP))
(if (next-val . in-closed-interval? . START END)
(loop-cc next-val)
(hash-remove! thunk-table 'LOOP-ID))))
START))])
(define (in-closed-interval? x start end)
(if (< start end)
(<= start x end)
(<= end x start)))
(define-macro (b-next LOOP-ID)
#'(begin
(unless (hash-has-key? thunk-table 'LOOP-ID)
(raise-line-error "next without for"))
(define thunk (hash-ref thunk-table 'LOOP-ID))
(thunk)))