Help needed: parsing pattern synonym contexts

Dr. ERDI Gergo gergo at
Wed Jul 2 11:45:29 UTC 2014


On Tue, 1 Jul 2014, Simon Marlow wrote:

> The s/r conflicts can also be a problem, depending on what you're trying to 
> parse.  It's generally a good idea to get rid of them if you can, but at the 
> least you should understand why they exist (use happy --info) and document 
> them in Parser.y.pp.

OK I've uploaded the code to wip/T9023 and the happy --info report to 

Loads of shift/reduce conflicts come from state 214. Looking at them, I 
would have thought that the solution would be to parse pattern synonym 
definitions like we parse data constructors as types: by parsing it as a 
single pattern and then splitting it. A quick shortcut to this should be 
to just disable the parsing rule for infix pattern synonyms; so I tried 

As expected, this cuts down on the number of shift/reduce conflicts in 
that state considerably. The rest then seems to be coming from state 570:

 	patsyn_context -> forall .                          (rule 157)
 	patsyn_context -> forall . context '=>'             (rule 158)

Unfortunately, I don't have a quick workaround for that one yet.


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