Re: How to read an understand the bison report file ?

[Hans Aberg]

On 26 Jun 2012, at 13:23, Timothy Madden wrote:

> I wrote a grammar file for an approximation of the php language (with the 
> purpose of adding braces to single-statements under if, for, while, but that 
> is not included yet), that simply outputs the found tokens back to an output 
> stream.

If you want to write a grammar for a known language, there is a good chance 
somebody already has done so. You might inquiry in the Usenet newsgroup 
comp.compilers.

> My problem is I got like 233 shift/reduce conflicts (and 18 reduce/reduce 
> conflicts) and I do not really understand them. But searching the internet 
> shows most people have like 3 or 4 of them ... what did I do ?
> 
> Is there a way for me to understand that report file bison outputs ? I looked 
> a lot through it, but I can not get where the conflicts are comming from. It 
> does says something about the first rules, describing whitespace, which I 
> want to preserve as significant part of the grammar.
> 
> I have CenOS 6.2 64-bit (up-to-date), GNU bison 2.4.1 64-bit.
> The grammar and report files are attached.

Bison token preferences act on the tokens immediately before and after the 
parsing position '.' of the conflicting shift/reduce rules. So you might try to 
get rid some of them that way, by setting token precedences.

Reduce/reduce conflicts may call for the grammar to be rewritten - write a 
grammar for a more general language without them, and cut it down in the 
semantic actions. Or you may try to use the GLR parser - see the manual for an 
example, from parsing C++ I recall. It is possible to use the first method for 
that too, so it is part a matter of taste which method to choose.

Hans