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From: Kaz Kylheku <157-073-9834@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: Why do some versions of bison require {} here?
Date: Wed, 2 Jan 2019 19:29:46 +0000 (UTC)
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On 2019-01-01, Philipp Klaus Krause <pkk@spth.de> wrote:
>    | addressmod
>         {
>         }
>    ;
>
> for their yacc / bison to accept the rule.
> What is happening here? Why is the {} required? Why only on the last
> rule, while the first one is okay?
>
> Philipp
> [Adding the empty action forces bison to reduce the rule rather than just
> shifting and saving state for later.  I couldn't guess why that would matter
> in this case.  Are there precedence rules?  With your change does the grammar compile
> cleanly or does it have conflicts? -John]

What, John? Shouldn't a missing action be equivalent to { $$ = $1 }?

Here, found it in Bison's manual, "3.4.6 Actions":

  If you don’t specify an action for a rule, Bison supplies
  a default: $$ = $1.

That should imply that there in fact is an action that requires a
reduction. Unless Bison handles this case specially without a reduce
somehow?

--
TXR Programming Lanuage: http://nongnu.org/txr
Music DIY Mailing List:  http://www.kylheku.com/diy
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[Oops, you're quite right.  It's been a decade since I revised
flex+bison and the brain cells are tired.  Now I'm just baffled
unless there is some syntax error that the extra rule is somehow
fixing. -John]