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Re: #line 0.000000oo

From: Paul Eggert
Subject: Re: #line 0.000000oo
Date: Wed, 13 Nov 2002 12:46:52 -0800

> From: Akim Demaille <address@hidden>
> Date: 13 Nov 2002 10:57:25 +0100
> exp: exp '+' exp
> {
>    /* Something. */
> #line 123 "foo.y"
>    $$ = $1 + $4;
> }
> should complain in foo.y:123.  But it is clear that the #line must be
> forwarded inside the action.

If that's the case, Bison is currently mishandling #line.  E.g., for this:

  exp: '+'
  #line 1000 "foo.y"
     $$ = $1 + $4;
  #line 2000 "foo.y"
     $$ = $1 + $4;

the Bison output is currently this:

  foo.y:1000.1-1001.15: integer out of range: `$4'
  foo.y:1000.1-1003.15: integer out of range: `$4'

so the second "#line" directive did not affect the Bison diagnostics.

> Now, you seem to suggest that once the '}' closed, we should restore
> the previous location context,

No, I wasn't proposing that.  All I'm saying is that either #line
should consistently affect Bison diagnostics, or it should
consistently be ignored by Bison (other than passing it through to the
C source code).

> We can do this with an intermediary parser between our scanner, and
> the actual parser.  The reason why we might want a secondary parser is
> that anyway, some day we want %if, %else, %endif.

OK, it sounds like you are proposing two preprocessors for Yacc C
code: a new one that uses %if/%endif and is preprocessed by Yacc, to
go along with the existing one that uses #if/#endif and is
preprocessed by the C compiler.  I can certainly see the need for
having a separate preprocessor for Yacc, and I also see why you
can't use a single C preprocessor for both purposes.

But in that case, we should use "%line" uniformly for Yacc line
numbers, to go along with the future "%if" and "%endif".  This makes
it clearer to users that there are two preprocessors here.  We should
just pass #line through in C code, as we do now, and make it an error
to use #line outside of C code (just as it's an error to use #if
outside of C code).  The "%line" notation makes a lot more sense to me
than the current approach, which uses the same #line notation to mean
two quite different things.

> I'm really against supporting tortured syntax which only concrete
> use will be the test suite.

It's OK to implement a subset of the C rules, so long as we document
whatever we implement.  But the current subset is pretty restrictive,
and it will make it a bit of a pain for humans to use %line.  For
example, currently trailing white space is not allowed in #line; nor
is the short form supported.

If we implement %line ourselves, how about if we use the following
subset of the C rules instead:

  * Comments are not allowed.
  * Newline (including backslash-newline) is not allowed.
  * There is no preprocessor-like expansion (e.g., you can't say
    `%line __STDC__ "foo"' as you can in C).

Another way of putting it is that we alter Bison as follows:

  * Arbitrary white space is allowed between tokens.
  * The file name must have properly escaped quotes.
  * The file name is optional, and defaults to the previous file name.

In other words, the regular expression changes from this:

  ^"#line "{int}" \"".*"\"\n"

to this:


where {w} is any horizontal white space character ([ \f\t\v]).

If we play our cards right, we won't need to parse the escape
sequences in that string: just print them as-is in error messages
(without quoting them), and escape M4 characters like [ and ] when
sending the string via M4 to C code.

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