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Re: SOME (and other mapping functions)
- To: email@example.com, firstname.lastname@example.org
- Subject: Re: SOME (and other mapping functions)
- From: email@example.com (Bill St. Clair)
- Date: Wed, 5 Jan 1994 12:02:52 -0600
At 12:37 AM 1/5/94 +0000, Philip L. Stubblefield wrote:
>In article <firstname.lastname@example.org>
>pazzani@pan.ICS.UCI.EDU (Michael Pazzani) writes:
>> I prefer to use mapping functions when possible instead of do or loop.
>> However, it appears that I pay a penalty for my stylistic preference
>> because when using some lambda expressions as the function argument
>> (e.g., one that references a lexical variable other than the parameter)
>> space gets allocated.
>> (defun greater-some (x list)
>> (some #'(lambda(e)(> e x)) list))
>> (time (greater-some 7 '(1 2 3 4 5 6 7 8)))
>> (GREATER-SOME 7 '(1 2 3 4 5 6 7 8)) took 0 milliseconds (0.000 seconds) to run.
>> 40 bytes of memory allocated.
>> Is there any declaration, etc that can avoid this spoce allocation.
>You can use the DYNAMIC-EXTENT declaration to inform MCL that a named
>function has dynamic extent, i.e., that it will never be referenced
>once the establishing construct has been exited (CLtL/2 pp. 42-46).
>Given this declaration, MCL can allocate the function on the stack
>(CLtL/2 pp. 232-236, especially p. 232). The only disadvantage is that
>you must use FLET or LABELS to define the function so that it has a
>name. For example:
>? (defun greater-some-1 (x list)
> (flet ((greater-than-x (e)
> (> e x)))
> (declare (dynamic-extent #'greater-than-x))
> (some #'greater-than-x list)))
Genera (Symbolics Lisp) has a way for SOME to declare that its first argument
is a function that can safely have dynamic extent. All its built-in
mapping functions contain the appropriate declarations, and
the compiler uses them to automagically stack-cons the closure
in (some #'(lambda (3) (> e x)) list). MCL does not have this feature,
so Phillip's dynamic-extent declaration is the best you can do.
Bill St. Clair