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Re: optimizing lazy sequences: srfi-41 vs delayed continuation with valu

From: Mark H Weaver
Subject: Re: optimizing lazy sequences: srfi-41 vs delayed continuation with values + promise vs delayed continuation with cons + lambda
Date: Fri, 02 Mar 2018 18:22:00 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)


Amirouche Boubekki <address@hidden> writes:
> Now, I got another idea what if I replace the 'cons' returned
> by the traversi procedures by 'values' and replace the
> lambda with 'delay' with some use of 'force' at the correct
> places. It will result in the following procedures:
> (define-public (list->stream lst)
>   (let loop ((lst lst))
>     (if (null? lst)
>         (delay (values #f #f))
>         (delay (values (car lst) (loop (cdr lst)))))))
> (define-public (stream->list stream)
>   (let loop ((stream stream)
>              (out '()))
>     (call-with-values (lambda () (force stream))
>       (lambda (value next)
>         (if next
>             (loop next (cons value out))
>             (reverse! out))))))
> (define-public (stream-map proc stream)
>   (let loop ((stream stream))
>     (call-with-values (lambda () (force stream))
>       (lambda (value next)
>         (if next
>             (delay (values (proc value) (loop next)))
>             (delay (values #f #f)))))))

This code assumes that promises can store multiple values.  Although
Guile's legacy core promises *accidentally* support multiple values
today, there's no guarantee that they will continue to do so in the
future.  None of the standards allow this, and Guile's manual states in
the documentation for 'delay' that "The effect of <expression> returning
multiple values is unspecified."

Supporting multiple values in promises makes them more complex, and
inevitably less efficient.

SRFI-45 promises are simpler than Guile's legacy core promises in two
ways: (1) they do not include built-in thread synchronization, and (2)
they do not support multiple values.

I would recommend using SRFI-45 promises.  Although they are implemented
in Scheme, last I checked I found that they were about as fast as
Guile's legacy core promises implemented in C, presumably because of the
built-in thread synchronization in our core promises.

In any case, I would strongly recommend against writing code that
assumes that Guile's promises can hold multiple values.


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