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 \ `---= gergo@erdi.hu =-------' GDK is the GNU Not Unix Image Manipulation Program Tool Kit Drawing Kit