Subject: bug#18520: string ports should not have an encoding

From: [email-masked] (Ludovic Courtès)
Date: Mon, 22 Sep 2014 22:39:29 +0200

David Kastrup <[email-masked]> skribis:

> [email-masked] (Ludovic Courtès) writes:
>
>> David Kastrup <[email-masked]> skribis:
>>
>>> I'm currently migrating LilyPond over to GUILE 2.0.  LilyPond has its
>>> own UTF-8 verification, error flagging, processing and indexing.
>>
>> Do I understand correctly that LilyPond expects Guile strings to be byte
>> vectors, which it can feed with UTF-8 byte sequences that it built by
>> itself?
>
> Not really.  LilyPond reads and parses its own files but it does divert
> parts through GUILE occasionally in the process.  Some stuff is passed
> through GUILE with time delays and parts wrapped into closures and
> flagged with machine-identifiable source locations.

OK.

>>> If you take a look at
>>> <URL:http://git.savannah.gnu.org/cgit/lilypond.git/tree/scm/parser-ly-from-scheme.scm>,
>>> ftell on a string port is here used for correlating the positions of
>>> parsed subexpressions with the original data.  Reencoding strings in
>>> utf-8 is not going to make this work with string indexing since ftell
>>> does not bear a useful relation to string positions.
>>
>> AIUI the result of ‘ftell’ is used in only one place, while
>> ‘port-line’ and ‘port-column’ are used in other places.
>
> The ftell information is wrapped into an alist together with a closure
> corresponding to the source location.  At a later point of time, the
> surrounding string may be interpreted, and the source location is
> correlated with the closure and the closure used instead of a call to
> local-eval (which does not have the same power of evaluating materials
> in a preserved lexical environment as a closure has).
>
>> The latter seems more appropriate to me when it comes to tracking
>> source location.
>
> For error messages, yes.  For associating a position in a string with a
> previously parsed closure, no.

But wouldn’t a line/column pair be as suitable as a unique identifier as
the position in the file?

Also, if the result of ‘ftell’ is used as a unique identifier, does it
really matter whether it’s an offset measured in bytes or in character?

(Trying to make sure I understand the problem.)

Thanks,
Ludo’.