Pattern synonyms: help needed: capturing EvBinds in tcPat

[Richard Eisenberg]

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 <gergo at 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   \
>    `---= gergo at erdi.hu =-------'
> GDK is the GNU Not Unix Image Manipulation Program Tool Kit Drawing Kit
> 
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://www.haskell.org/mailman/listinfo/ghc-devs