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From: Kaz Kylheku <563-365-8930@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: "Bootstrapping yacc in yacc" -> "Bootstrapping yacc in lex"!
Date: Mon, 15 Mar 2021 02:37:16 -0000 (UTC)
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On 2021-03-15, Rock Brentwood <rockbrentwood@gmail.com> wrote:
> It's a recurrent question that's come up in other forums "can yacc be
> bootstrapped in yacc?" Now, I'm adding a twist.
>
> I'll repeat one of my recent replies here. In the syntax for yacc files, laid
> out by the POSIX standard, there is no mandatory semi-colon at the ends of
> rules, so an extra look-ahead is required to determine whether an identifier
> is followed by a colon. If so, then this indicates the left-hand side of a new
> rule.
>
> A grammar rule has the form
>
> left-hand-side ":" stuff on the right optional ";"'s.
>
> If you see a ":" in the middle of the rules on the right, then you've actually
> sneaked on over into the *next* rule.
>
> Bison hacks the syntax, by making left-hand-side + ":" into a single token.

You could simply allow rules of this form

  ":" right side ...


With a semantic restrction that this must be preceded by a rule that
is not terminated with a semicolon, whose last element is a symbol:

  blah ":" previous rule material ending in symbol /* no semicolon */

  ":" next rule ";"

Then we make the AST transformation of moving "symbol" to be the head
of the following rule:

  -->


  blah ":" previous rule material ending in

  symbol ":" next rule ";"

Situations where it's not a symbol, or the prior rule has ended in
a semicolon (which is thus followed by a colon) are not syntax
errors, but are diagnosed semantically.
--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal