Re: bison and/or antlr ?

On Fri, 13 Jul 2007 11:20:29 -0400, Chris F Clark
<cfc@shell01.TheWorld.com> wrote:


>George Neuner <gneuner2@comcast.net> writes:
>
>> Just to be sure, you are referring to _semantic_ predicates? ...
>> I do find _syntactic_ predicates to be indispensable, ...
>
>No, I was actually referring to syntactic predicates. I agree that
>they are sometimes indispensable, but they also put little loop-holes
>in your grammar, where something in one context parses one way (means
>one thing) and in a different context parses (means) another.


I actually make use of syntactic predicates to deliberately restrict
reuse of subgrammars where there is a possibility of semantic
confusion.




>problem is (particularly with PEG's, which are syntactic predicates on
>steroids) is that there is no check to verify that you haven't
>introduced some corner case you didn't intend to. I don't think that
>there are any PEG generators that even warn you if some rule is
>unreachable. That scares me.


It would scare me also. But I don't use PEGs and that may be why I
like syntax predicates more than you do - I find them indispensable
for CFG. I have rarely written grammars so complex that I couldn't
analyze them with moderate effort - though a DICOM parser I once wrote
pushed the limits.




>By the way, I do feel the same about the precedence and associativity
>declarations in LR parsing. They are great tools, but not for use
>with abandon. They also drop you off the cliff where your tool can't
>tell you that you have mis-specified what you want. Used with care
>that's okay. However, it is too easy to program carelessly.


Which is why I don't use declarations. Although it's more cumbersome
to code precedence hierarchies into the grammar, it's portable and it
makes the grammar easier (IMO) to analyze.


George

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