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Re: [Scheme-reports] Formal Comment: what is the required behavior of 'lazy'?
Thanks! This had already been fixed in the repository and
The expression (delay-force expression) is conceptually similar to
(delay (force expression)), with the difference that forcing the
result of delay-force will in effect result in a tail call to (force
expression), while forcing the result of (delay (force expression))
may not. Thus iterative lazy algorithms that may result in a long
series of chains of delay and force can be rewritten using
delay-force to prevent consuming unbounded space during evaluation.
(where lazy has been renamed delay-force).
This seems overly coy. What matters is heap space, not whether the
call to 'force' is a tail call. How about something like:
(delay-force expression) is identical to (delay (force expression)),
with the additional requirement that implementations must support
tail-recurive nesting of delay-force (where expression returns the
result of a second use of delay-force) to arbitrary depths. This
allows delay-force to be used to write lazy, iterative loops as in
the stream-filter example below. See section 7.3 for an example of
how delay-force can be implemented.
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