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POProblem with circular definition in Scheme
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copied!<p>I am currently working through SICP using Guile as my primary language for the exercises. I have found a strange behavior while implementing the exercises in chapter 3.5. I have reproduced this behavior using Guile 1.4, Guile 1.8.6 and Guile 1.8.7 on a variety of platforms and am certain it is not specific to my setup.</p> <p>This code works fine (and computes e):</p> <pre><code> (define y (integral (delay dy) 1 0.001)) (define dy (stream-map (lambda (x) x) y)) (stream-ref y 1000) </code></pre> <p>The following code <em>should</em> give an identical result:</p> <pre><code> (define (solve f y0 dt) (define y (integral (delay dy) y0 dt)) (define dy (stream-map f y)) y) (stream-ref (solve (lambda (x) x) 1 0.001) 1000) </code></pre> <p>But it yields the error message:</p> <pre><code>standard input:7:14: While evaluating arguments to stream-map in expression (stream-map f y): standard input:7:14: Unbound variable: y ABORT: (unbound-variable) </code></pre> <p>So when embedded in a procedure definition, the (define y ...) does not work, whereas outside the procedure in the global environment at the REPL it works fine.</p> <p>What am I doing wrong here? I can post the auxiliary code (i.e., the definitions of integral, stream-map etc.) if necessary, too. With the exception of the system-dependent code for cons-stream, they are all in the book. My own implementation of cons-stream for Guile is as follows:</p> <pre><code>(define-macro (cons-stream a b) `(cons ,a (delay ,b))) </code></pre>

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