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...