Description
i can run in racket , but not jupyter .
; 如下的这个在racket里会正常，但在jupyter里会造成死机。
(require racket)
;(define (delay exp)
; (lambda () exp))
; ;(memo-proc (lambda ()
; ; exp)))
;
;(define (force delayed-object)
; (delayed-object))
;
;(define (memo-proc proc)
; (let ((already-run? false) (result false))
; (lambda ()
; (if (not already-run?)
; (begin (set! result (proc))
; (set! already-run? true)
; result)
; result))))
(define (delay ex)
(lambda () ex))
(define (force delayed-object)
(delayed-object))
(define (stream-car stream) (car stream))
(define (stream-cdr stream) (force (cdr stream)))
; this won't work as a simple function
;(define (cons-stream a b)
; (cons a (delay b)))
; This is scheme syntax for macro
; http://stackoverflow.com/questions/5610480/scheme-sicp-r5rs-why-is-delay-not-a-special-form
(define-syntax cons-stream
(syntax-rules ()
[(cons-stream x y) (cons x (delay y))]))
(define the-empty-stream '())
(define (stream-null? stream)
(null? stream))
(define (stream-filter pred stream)
(cond ((stream-null? stream) the-empty-stream)
((pred (stream-car stream))
(cons-stream (stream-car stream)
(stream-filter pred (stream-cdr stream))))
(else (stream-filter pred (stream-cdr stream)))))
(define (stream-ref s n)
(if (stream-null? s) the-empty-stream
(if (= n 0)
(stream-car s)
(stream-ref (stream-cdr s) (- n 1)))))
(define (stream-map proc . argstreams)
(if (stream-null? (car argstreams))
the-empty-stream
(cons-stream (apply proc (map stream-car argstreams))
(apply stream-map
(cons proc (map stream-cdr argstreams))))))
(define (stream-for-each proc s)
(if (stream-null? s)
'done
(begin (proc (stream-car s))
(stream-for-each proc (stream-cdr s)))))
; Neil, 2012-05-10
(define (stream-subseq stream a b)
(cond ((stream-null? stream) the-empty-stream)
((= a b) the-empty-stream)
((> a b) the-empty-stream)
(else (cons-stream (stream-ref stream a)
(stream-subseq stream (+ a 1) b)))))
(define (display-line x)
(newline)
(display x))
(define (display-stream s)
(stream-for-each display-line s))
; examples
;(let ((x (delay (+ 1 2))))
; (for ([i (in-range 1 10)])
; (display (force x))))
;
(define (integers-starting-from n)
(cons-stream n (integers-starting-from (+ n 1))))
(define integers
(integers-starting-from 1))
;(display-line (stream-ref integers 0))
(let ((x (stream-subseq integers 10000 10010)))
(display-stream x))
(define odd-numbers
(stream-filter odd? integers))
(display-stream (stream-subseq odd-numbers 50 60))
;(let ((x (cons-stream 1 (cons-stream 2 '(3)))))
; (display-stream x))
(define (stream-add s n)
(stream-map (lambda (x)
(+ x n)) s))
(define (add-streams s1 s2)
(stream-map + s1 s2))
(define fib
(cons-stream 1
(cons-stream 1
(add-streams fib
(stream-cdr fib)))))
(display-stream (stream-subseq fib 150 160))
(define (divisible? x y)
(= (remainder x y) 0))
(divisible? 10 2)
(define (sieve stream)
(cons-stream
(stream-car stream)
(sieve (stream-filter
(lambda (x)
(not (divisible? x (stream-car stream))))
(stream-cdr stream)))))
(define primes
(sieve (integers-starting-from 2)))
(display-stream (stream-subseq primes 1000 1010))
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