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#lang racket
(require racket/stream)
(provide
stream-cons
stream-car
stream-cdr
the-empty-stream
stream-null?
display-line
show
display-stream
stream-ref
stream-for-each
stream-enumerate-interval
stream-map
stream-filter)
;; (define (delay b) (lambda () b))
;; (define (force f) (f))
;; (define (cons-stream a b) (cons a (delay b)))
;; (define (cons-stream a b) (stream-cons a b))
;; streams in racket by default are not eager for the head of the cons
(define (stream-car stream) (stream-first stream))
(define (stream-cdr stream) (stream-rest stream))
(define the-empty-stream (stream))
(define (stream-null? stream) (stream-empty? stream))
(define (display-line x) (newline) (display x))
(define (show x) (display-line x) x)
(define (display-stream s) (stream-for-each display-line s))
(define (stream-ref s n)
(if (= n 0)
(stream-car s)
(stream-ref (stream-cdr s) (- n 1))))
(define (stream-for-each proc s)
(if (stream-null? s)
'done
(begin (proc (stream-car s))
(stream-for-each proc (stream-cdr s)))))
(define (stream-enumerate-interval low high)
(if (> low high)
the-empty-stream
(stream-cons
low
(stream-enumerate-interval (+ low 1) high))))
(define (stream-map proc . argstreams)
(if (stream-null? (car argstreams))
the-empty-stream
(stream-cons
(apply proc (map stream-car argstreams))
(apply stream-map
(cons proc (map stream-cdr argstreams))))))
(define (stream-filter pred s)
(cond ((stream-null? s) the-empty-stream)
((pred (stream-car s)) (stream-cons (stream-car s) (stream-filter pred (stream-cdr s))))
(else (stream-filter pred (stream-cdr s)))))
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