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Re: How to record source properties for all symbols?

From: Mark H Weaver
Subject: Re: How to record source properties for all symbols?
Date: Tue, 05 Jun 2018 18:09:57 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)


Fis Trivial <address@hidden> writes:

> Mark H Weaver writes:
>> The problem is that there's no place to store the source information for
>> symbols in the standard S-expression representation.
>> The principal defining characteristic of symbols -- that "two symbols
>> are identical (in the sense of 'eqv?') if and only if their names are
>> spelled the same way" (R5RS ยง 6.3.3) -- combined with the fact that
>> 'eq?' is specified to be the same as 'eqv?' for symbols, leaves us no
>> way to distinguish two instances of the same symbol, and therefore no
>> way to store per-instance annotations such as source information.
>> Fixing this would require abandoning the plain S-expression
>> representation in favor of one in which symbols are represented by a
>> different data structure.  Our reader would need to be extended to
>> support the option of returning this new data representation instead of
>> plain S-expressions, and our macro expander would need to be modified to
>> accept this new representation as input.
> I still believe it's crucial to give user correct and detailed error
> message,

I agree.  I didn't intend to dispute the importance of good error
messages.  I merely intended to explain what the problem is, and why it
hasn't yet been done.

> After poking for a few days, I found that I have hard time understanding
> the code, can you give me some hints for reading the code so that I can
> understand how to encode the source information. Currently, I am still
> trying to make a baby step, encode source information into symbol.

It can't be done.  I tried to explain the reason in my earlier message,
but I guess you don't understand.  I don't have time right now to make
another attempt.

> I know that will break everything, but at least I will have basic
> understanding of underlying mechanic.

No, it simply won't work at all.

> Though I didn't find any keyword like `parser' or `lexer', but I tried
> to dig into `scm_read_expression', which returns a stringbuf.

What makes you think 'scm_read_expression' returns a stringbuf?
It returns an S-expression.

I appreciate your willingness to help with this, but I think that you've
chosen a task that's too difficult given your level of experience with
Guile.  If it was straightforward, it would have been done long ago.


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