Please find the code for streams and the integration below. The macro
definition of cons-stream is not from SICP since it is implementation
dependent (for this reason I have also not tried the code on other Scheme
implementations except Guile). So this could be buggy.
I don't think it is only an issue of delay/force mismatch - the strange
thing is that the code *works* at the REPL, but does *not work* inside a
procedure definition.
<code>
;; Basic stream operations
(define-macro (cons-stream a b)
`(cons ,a (delay ,b)))
(define (stream-car stream) (car stream))
(define (stream-cdr stream) (force (cdr stream)))
(define the-empty-stream '())
(define stream-null? null?)
(define (stream-ref s n)
(if (= n 0)
(stream-car s)
(stream-ref (stream-cdr s) (- n 1))))
;; Other generic stream operations
(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 (add-streams s1 s2)
(stream-map + s1 s2))
(define (scale-stream stream factor)
(stream-map (lambda (x) (* x factor)) stream))
;; Integral with delayed integrand
(define (integral delayed-integrand initial-value dt)
(define int
(cons-stream initial-value
(let ((integrand (force delayed-integrand)))
(add-streams (scale-stream integrand dt)
int))))
int)
;; The troublesome procedure
(define (solve f y0 dt)
(define y (integral (delay dy) y0 dt))
(define dy (stream-map f y))
y)
;; This works
(define y (integral (delay dy) 1 0.001))
(define dy (stream-map (lambda (x) x) y))
(stream-ref y 1000)
;; This doesn't work
;(stream-ref (solve (lambda (x) x) 1 0.001) 1000)
</code>
Linas Vepstas-3 wrote:
>
> On 4 May 2010 11:59, user8472 <head_over_he...@freenet.de> wrote:
> [...]
>> streams.scm:601:14: While evaluating arguments to stream-map in
>> expression
>> (stream-map f y):
>> streams.scm:601:14: Variable used before given a value: y
>> ABORT: (unbound-variable)
>>
>> So something is still not quite right.
>
> My bad, I completely failed to notice that delayed evaluation
> was a part of the problem. Can you simplify the example to
> its barest elements that still provoke the error? Hopefully,
> this will provide a clue to what the true as to what the
> problem is. (Not having seen the definition of "integral",
> I'm wondering whether some failure to initialize y to y0
> might have something to do with it ... some missing force
> somewhere ... but that's a wild and probably wrong guess).
>
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