I am working my way through the Wizard Book Structure and Interpretation of Programming in my free evenings. All was going well, until I tried to implement streams in Chapter Three. I am using (Dr)Racket, not Scheme, so this might be my problem.
The crux of the problem is that when I try to create streams I get a memory issue: "interactions disabled; out of memory". This suggests to me that there is something wrong with the way I am using delay.
Thanks in advance for any advice :)
I have tried the code suggested in the text:
#lang racket
(define (cons-stream a b)
(cons a (delay b)))
(define (car-stream s)
(car s))
(define (cdr-stream s)
(force (cdr s)))
where I am calling Racket's delay and force. I have also tried implementing delay and force myself following the text, but I come across the same memory error.
To test that this is all working I tried writing an infinite stream of 1s:
(define (ones) (cons-stream 1 (ones)))
and a very large interval:
(define (interval low high)
(if (> low high)
null
(cons-stream low
(interval (+ low 1) high))))
(define (first-integers n)
(interval 0 n))
(define test-interval
(first-integers 10000000000000000000000000))
To my understanding of the text, neither of these should cause problems because we only generate the streams when required. However, something I've done causes a memory issue and so I think the code tries to generate the entire stream!
We can not define
(define (cons-stream a b)
(cons a (delay b)))
because define creates a function cons-stream, so on every function call all arguments will be evaluated before the call is made. Which defeats the whole purpose -- b will be already evaluated.
See also How is delay implemented?.