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copied!<p>If you don't want to go the macro route, you could always just abandon <code>cons-stream</code> and rewrite <code>lazy-list</code> as follows:</p> <pre><code>(define (lazy-list from) (cons from (λ() (lazy-list (+ from 1))))) </code></pre> <p>This is probably the easiest, most pragmatic solution, but it's only good for making lazy lists of incrementing numbers. You could generalize this by passing in a function that will generate successive elements of the list when called:</p> <pre><code>(define (lazy-list-gen generator) (cons (generator) (λ() (lazy-list-gen generator)))) (define (lazy-list from) (lazy-list-gen (λ() (let ((ret from)) (set! from (+ from 1)) ret)))) </code></pre> <p>This works pretty well:</p> <pre><code>> (define x (lazy-list 1)) > (car-stream x) 1 > (car-stream (cdr-stream x)) 2 </code></pre> <p>But there's a bug in the code:</p> <pre><code>... continuing from above ... > (car-stream (cdr-stream x)) 3 </code></pre> <p>This error happens because the call to <code>cdr-stream</code> calls <code>generator</code> again. We can solve this by caching the return value of the lambda:</p> <pre><code>(define (lazy-list-gen generator) (cons (generator) (let ((gen-cache #f)) (λ() (cond ((not gen-cache) (set! gen-cache (lazy-list-gen generator)))) gen-cache)))) </code></pre> <p>Now it works as it should:</p> <pre><code>> (define x (lazy-list 1)) > (car-stream x) 1 > (car-stream (cdr-stream x)) 2 > (car-stream (cdr-stream x)) 2 > (car-stream (cdr-stream (cdr-stream x))) 3 > (car-stream (cdr-stream x)) 2 </code></pre>

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