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    <p>The key difference between what happens when you evaluate the definitions one by one at the REPL and when you place them inside <code>solve</code> is that in the first case, they are evaluated sequentially, thus the <code>y</code> the expression <code>(stream-map &lt;some-function&gt; y)</code> refers to is already in scope, whereas with internal definitions or <code>letrec</code>, it is not yet available.</p> <p>Funnily enough, MIT Scheme, which I used when going through SICP, had no such problem back then and still treats <code>letrec</code> and internal defines differently:</p> <pre><code>;; this is an error (letrec ((xs '(1 2 3)) (ys (map (lambda (x) (+ x 1)) xs))) ys) ;; this is still an error (and is treated as such by Guile), ;; yet evaluates to (2 3 4) in MIT Scheme (let () (define xs '(1 2 3)) (define ys (map (lambda (x) (+ x 1)) xs)) ys) </code></pre> <p>I'm not sure about the original "Revised Report On Algorithmic Language Scheme" or R2RS, but at least from R3RS on internal defines were supposed to be equivalent to <code>letrec</code>. Apparently this peculiarity of MIT's environment influenced the book... or perhaps it's the other way around.</p>
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    1. POProblem with circular definition in Scheme
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    1. USMichał Marczyk
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    1. POProblem with circular definition in Scheme
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    1. COThen why would I want to use internal defines at all? The restriction mentioned in R5RS (see my comment above) to allow defines only at the beginning of a <body> would be pointless if they were unavailable for the rest of the body. Even worse: When I type in the two blocks of expressions quoted in my post *in sequence* (i.e., with "y" and "dy" already defined in the global environment), I still get the same error message when evaluating (stream-ref (solve ...) 1000), even though Scheme is supposed to refer to the one in the global environment when the local one is unavailable.
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    2. COScheme is *not* supposed to refer to the global binding; `letrec` bindings (and internal defines, which are equivalent) are meant to be mutually recursive. *However*, quoting R5RS: "Just as for the equivalent letrec expression, it must be possible to evaluate each <expression> of every internal definition in a <body> without assigning or referring to the value of any <variable> being defined." There are all sorts of reasons why `letrec` expressions are useful and internal defines, being just an alternate syntax for `letrec`, are useful in the same situations.
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    3. COAnyway, as far as I can tell, this is what goes wrong with the code at hand. You could try to fix things with `delay` / `force`. The rationale behind R5RS' mandating such behaviour is a different matter, which I'm not sure I'm prepared to discuss extempore.
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