Re: Why are ambiguous grammars usually a bad idea? Why are languages usually defined and implemented with ambiguous grammars?

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From	gah4 <gah4@u.washington.edu>
Newsgroups	comp.compilers
Subject	Re: Why are ambiguous grammars usually a bad idea? Why are languages usually defined and implemented with ambiguous grammars?
Date	Mon, 27 Dec 2021 19:45:06 -0800 (PST)
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(snip about conflicts in grammars)
(then I wrote)
> As far as I know, the main cause of ambiguous grammar in programming languages
> is the nested if-then-optional-else structure. If you require else, then it isn't ambiguous,
> but people like the optional else. That usually comes out as a shift-reduce conflict,
> and parser generators know how to handle that.

(snip)

> But you already have the reply from John Levine ...
> [See previous message, where we fixed that with "fi" in 1968. -John]

I first learned if-then-else from PL/I, where it was designed in
about 1963.    I now suspect that people get used to the one they
learned first, and find it more obvious.

There is also some language, and I forget now which, with an ELIF
construct, such that you can make a nested if-then-else sequence
without the increase of nesting level, and so need only one ENDIF.

IF e THEN s; ELIF e THEN s; ELIF e THEN s; ENDIF;

(No comment on my preference for ENDIF vs. FI.)

[That would also be Algol 68, using "fi"
It didn't need the semicolons because they are separators,
not terminators as in PL/I and C.
If you've written scripts in the Bourne shell or its descendants
such as bash and zsh, it deliberately looks a lot like Algol 68.
A request from your moderator: if anyone is planning another round
in the semicolon terminator vs. separator war, please don't.
-John]

Thread

Re: Why are ambiguous grammars usually a bad idea? Why are languages usually defined and implemented with ambiguous grammars? gah4 <gah4@u.washington.edu> - 2021-12-27 04:18 -0800
  Re: Why are ambiguous grammars usually a bad idea? Why are languages usually defined and implemented with ambiguous grammars? gah4 <gah4@u.washington.edu> - 2021-12-27 19:45 -0800
  Re: Why are ambiguous grammars usually a bad idea? Why are languages usually defined and implemented with ambiguous grammars? Christopher F Clark <christopher.f.clark@compiler-resources.com> - 2021-12-28 16:18 +0200

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