An excerpt from SICP book, For the last line given there is problem with scheme code compilation, As I see it, the functions sqr and shower are both functions of single argument that return a single value. And hence should work. For the below given code in scheme:
(define (stream-car stream)
(car stream))
(define (stream-cdr stream)
(force (cdr stream)))
;; cons-stream needs to be a macro
(define-syntax cons-stream
(syntax-rules ()
[(_ a b) (cons a (delay b))]
))
(define the-empty-stream '())
(define stream-null? null?)
;; define stream-for-each
;; run a procedure on each element of stream
(define (stream-for-each proc stream)
(if (stream-null? stream)
'done
(begin (proc (stream-car stream))
(stream-for-each proc (stream-cdr stream)))))
(define test-stream (cons-stream 2 (cons-stream 3 (cons-stream 4 '()))))
;; displays passed argument but with a newline
(define (display-line x)
(newline)
(display x))
(define (display-stream stream)
(stream-for-each display-line stream))
;; stream map
(define (stream-map proc stream)
(if (stream-null? stream)
the-empty-stream
(cons-stream (proc (stream-car stream))
(stream-map proc (stream-cdr stream)))))
(define (sqr x)
(* x x))
(define (stream-enumerate-interval low hi)
(if (> low hi)
'()
(cons-stream low (stream-enumerate-interval (+ 1 low) hi))))
(define (shower x)
(display-line x)
x)
;; this works
(define yyy (stream-map sqr (stream-enumerate-interval 1 10)))
;; reason why below wont work?
;; (define xxx (stream-map shower (stream-enumerate-interval 1 10)))
Also as per SICP book, it defines a different version of stream-map that allows procedures that accepts procedures that can take multiple arguments.
I fail to understand the need for a different stream-map and why above stream-map code on the last line won't work
Both of them work, but since stream-map is lazy you won't get them computed in full unless you do something to force it. Eg. lets make stream->list:
(define (stream->list stream)
(if (stream-null? stream)
'()
(cons (stream-car stream)
(stream->list (stream-cdr stream)))))
(stream->list yyy)
; ==> (1 4 9 16 25 36 49 64 81 100)
(stream->list zzz)
; ==> (1 2 3 4 5 6 7 8 9 10) (and as a side effect prints then one line at a time)
The fact is the actual computation on the rest of the list is never ever done unless you try to access the value. stream->list forces the computation of the whole stream so then it gets computed.
Notice that the sicp streams always force the first value regardless. This is a simplification that other stream libraries have fixed so in SRFI-41 you don't have any side effect unless you access the result.
#!r6rs
(import (rnrs)
(srfi :41))
;; nothing is displayed after this expression is evaluated
(define squares (stream-map (lambda (x) (shower (* x x)))
(stream-from 0)))
(stream-car squares)
; ==> 0 (displays "\n0")
(stream->list 4 squares)
; ==> (0 1 4 9)
; (displayes "\n1\n4\n9" since "0" is already calculated.)