Parsing makefiles (Bart Massey)
Fri, 1 Nov 1991 06:22:27 GMT

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Parsing makefiles (1991-11-01)
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Newsgroups: comp.compilers
From: (Bart Massey)
Keywords: yacc, lex, make
Organization: Department of Computer Science, University of Oregon
References: 91-10-122
Date: Fri, 1 Nov 1991 06:22:27 GMT

In article 91-10-122:
> Where can I find a "make" grammar?
> [ ... hard-to-yaccify
> distinction between lines that start with whitespace and lines that don't.]

Yeah, but you do this in the lexer.

BTW, the distinction is more brutal than that -- it's definitely "lines
that start with a tab and lines that don't." The following two scripts
(at least now, as I'm typing them) have very different effects when
executed by make

BAR=bletch; echo $$BAR

   BAR=bletch; echo $$BAR

(For those upon whom the distinction may be lost :-) (probably by the news
transport system), the first has a tab before the B, the second has a
space followed by a tab.) This is obviously a total design botch, but it
happened long ago.

If the lexer returns the appropriate token for lines that begin with a tab
(probably with its value just some sort of string containing the command)
then the parser hardly cares...

The moderator writes:
> For the make stuff, I suppose that you could have the lexer return the
> command lines as TAB-token REST-OF-LINE-token and just glom everything
> together, but by the time you've done that, there's hardly anything left
> for the parser to do.

Unless you write a modal lexer, though, that's kind of hard to
do, and not really worth it. You just want an
EXECUTABLE-LINE-token, whose value (LEX yylval) is some kind of
structure like

int ignore_errors;
int execute_silently;
char *to-execute;

There's still a *bit* for the parser to do, but you're right
about YACC probably being overkill.

Bart Massey

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