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Re: read-all ?

From: Ludovic Courtès
Subject: Re: read-all ?
Date: Tue, 22 Jan 2013 11:01:35 +0100
User-agent: Gnus/5.130005 (Ma Gnus v0.5) Emacs/24.2 (gnu/linux)


Andy Wingo <address@hidden> skribis:

> On Tue 22 Jan 2013 10:15, Andy Wingo <address@hidden> writes:


>> Patch attached.  I didn't update the docs because it wasn't clear to me
>> that (ice-9 rdelim) is actually the right place to put it.
>> What do you think?  Should we perhaps put it in a new (ice-9 ports)?

I would avoid adding a new module, unless there are other things to put
in there.

>> Are the names right?

‘read-all’ doesn’t convey the idea that it’s textual (unlike the R6RS

Perhaps ‘port-contents-as-string’, or ‘read-all-string’, or...?

> +(define* (read-all! buf #:optional
> +                    (port (current-input-port))
> +                    (start 0) (end (string-length buf)))
> +  "Read all of the characters out of PORT and write them to BUF.
> +Returns the number of characters read.
> +
> +This function only reads out characters from PORT if it will be able to
> +write them to BUF.  That is to say, if BUF is smaller than the number of
> +available characters, then BUF will be filled, and characters will be
> +left in the port."
> +  (check-arg buf (string? buf) "not a string")
> +  (check-arg start (index? start) "bad index")
> +  (check-arg end (index? end) "bad index")
> +  (check-arg start (<= start end) "start beyond end")
> +  (check-arg end (<= end (string-length buf)) "end beyond string length")
> +  (let lp ((n start))
> +    (if (< n end)
> +        (let ((c (read-char port)))
> +          (if (eof-object? c)
> +              (- n start)
> +              (begin
> +                (string-set! buf n c)
> +                (lp (1+ n)))))
> +        (- n start))))

As you note, this is fairly inefficient, like ‘get-string-n!’.

Given that ‘string-set!’ is (unduly) costly, I wonder if consing all the
chars and then calling ‘list->string’ wouldn’t be more efficient in time
(it’d be less efficient in space.)



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