Help needed: parsing pattern synonym contexts

[Simon Marlow]

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.

Cheers,
Simon

On 30/06/2014 14:04, Simon Peyton Jones wrote:
> Gergo
>
> It's just a question of chasing down the reduce/reduce errors.  You can give a flag to Happy that makes it dump a file with all the info about it parsing states, and where the reduce/reduce errors come from, and you can go from there.
>
> If you don't know how to interpret that file, just commit your best try to your wip/ branch and maybe some other ghc devs will help you.  Maybe me.  But without being able to reproduce it, it's hard to help.
>
> Simon
>
> | -----Original Message-----
> | From: Dr. ERDI Gergo [mailto:gergo at erdi.hu]
> | Sent: 25 June 2014 14:20
> | To: Simon Peyton Jones
> | Cc: GHC Devs
> | Subject: RE: Help needed: parsing pattern synonym contexts
> |
> | On Tue, 24 Jun 2014, Simon Peyton Jones wrote:
> |
> | > In the latter case, what happened to the shift/reduce and
> | > reduce/reduce errors reported by Happy?  Esp the latter.  If you are
> | > getting more you need to track them down.
> |
> | I think I've figured out what might be causing the problem.
> |
> | First, a couple figures. With these rules:
> |
> | pattern_synonym_decl
> |          : 'pattern' con vars0 patsyn_token pat
> |          | 'pattern' varid conop varid patsyn_token pat
> |
> | pattern_synonym_sig
> |          : 'pattern' patsyn_stuff '::' ctype
> |
> | patsyn_stuff
> |          : constr_stuff
> |
> | I'm getting 112 new reduce/reduce conflicts.
> |
> | If I add the context like you recommended:
> |
> | pattern_synonym_sig
> |          : 'pattern' patsyn_context patsyn_stuff '::' ctype
> |
> | patsyn_context :: { LHsContext RdrName }
> |          : forall
> |          | forall context '=>'
> |
> |
> | then I get 54 new shift/reduce conflicts and no (new) reduce/reduce
> | conflicts.
> |
> | My feeling is the problem is that patterns don't need any special
> | parentheses around type annotations, which means the following is a
> | legal pattern synonym definition:
> |
> | pattern Single x = [x] :: [Int]
> |
> | and I think that the difference (the '=' or '<-') is too 'deep' between
> | this and something like
> |
> | pattern Single a :: [a]
> |
> | Unfortunately, I still have no idea how to solve this problem...
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