I took a look here and would like to help, but I'm confused: What makes you think the dictionary for (Eq [b]) would be in scope? I can see where the (Eq b) comes from (the theta that pops out of patSynSig), but where does (Eq [b]) come from?
Richard On Sep 8, 2013, at 4:31 AM, Dr. ERDI Gergo <ge...@erdi.hu> wrote: > Hi, > > Based on feedback from Simon PJ a couple weeks ago, I started a substantial > rewrite of the typecheker of pattern synonyms, to support existential types. > My latest code (full of 'stash' commits and a huge amount of 'traceTc' calls) > is available in the pattern-synonyms-wip branch of > http://github.com/gergoerdi/ghc.git. > > What I'm struggling with, is capturing the evidence bindings during the > typechecking of a pattern which is a pattern synonym occurance. For example, > suppose I have the following module: > > {-# LANGUAGE GADTs, PatternSynonyms #-} > module ConstrEx where > > data T where > MkT :: (Eq b) => b -> b -> T > > pattern P b b' = (MkT b b', ()) > > f (P x y) = [x] == [y] > > > The names in the following will refer to the output of pattern-synonyms-wip > with --ddump-tc-trace. > > The right-hand side of 'f' requires an instance Eq [b], and is given an > instance $dEq_an5 :: Eq b, so it binds a dictionary '$dEq_an7 = > GHC.Classes.$fEq[] @[b] [$dEq_an5]'. This makes sense. However, inside > 'tcPatSynPat', which is the version of 'tcPat' for pattern synonym > occurances, the 'checkConstraints'-wrapped call to (eventually) > 'thing_inside' doesn't capture this binding (I get back an empty 'TcEvBinds'). > > This causes problems further on because even though $dEq_an5 is going to be > in scope while generating code for the right-hand side (since 'tcInstPatSyn', > the function that instantiates 'P x y' into '(MkT x y, ())', substitutes the > name of the dictionary bound by MkT, $dEq_amS, to $dEq_an5), but $dEq_an7 is > not, since its binding is not emitted by 'tcPatSynPat' (since it doesn't get > it from 'checkConstraints'). Looking at the tc-trace log, I see '$dEq_an7 = > GHC.Classes.$fEq[] @[b] [$dEq_an5]' on line 2070, way after the call to > 'checkConstraints' returns on line 1811. > > I realize the above description is basically incomprehensible unless you go > through the trouble of reading through 'tcPatSynDecl', 'tcPatSynPat' and > 'tcInstPatSyn', but I don't yet know how to even formulate my question more > succinctly. > > Any help is appreciated. > > Thanks, > Gergo > > -- > > .--= ULLA! =-----------------. > \ http://gergo.erdi.hu \ > `---= ge...@erdi.hu =-------' > GDK is the GNU Not Unix Image Manipulation Program Tool Kit Drawing Kit > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://www.haskell.org/mailman/listinfo/ghc-devs _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
