Re: "Bootstrapping yacc in yacc" -> "Bootstrapping yacc in lex"!

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

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