You are doing something very delicate here, akin to overlapping instances. 

You have an instance

instance PatchInspect (PrimOf p)) => Conflict p

and a function

clever :: (Conflict (OnPrim p), ..) => ...

So if a constraint (Conflict blah) arises in the RHS of clever, the instance declaration will immediately apply; and then the type check fails.  But if it just so happens to precisely match the provided constraint (Conflict (OnPrim p)), you want to use the provided constraint.  In effect the type signature and the instance overlap.

Arguably, GHC should refrain from applying the instance if there is any possibility of a “given” constraint matching.  Currently it’s a bit random; but it’s a very weird situation.

But first, is this really what you intend?

Simon

From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Sittampalam, Ganesh
Sent: 24 February 2011 07:41
To: glasgow-haskell-users@haskell.org
Subject: weird behaviour of context resolution with FlexibleContexts and TypeFamilies

Hi,

If I build the code below with -DVER=2, I get a complaint about PatchInspect (PrimOf p) being missing from the context of cleverNamedResolve.

This doesn't happen with -DVER=1 or -DVER=3

I presume that type class resolution is operating slightly differently in the different cases, but it's quite confusing - in the original code joinPatches did something useful and I was trying to inline the known instance definition. I would have expected it to be consistent between all three cases, either requiring the context or not.

Is it a bug, or just one of the risks one takes by using FlexibleContexts?

I've tried this with GHC 6.12.3 and with 7.0.2RC2.

Cheers,

Ganesh

{-# LANGUAGE CPP, TypeFamilies, FlexibleContexts #-}
module Class ( cleverNamedResolve ) where

data FL p = FL p

class PatchInspect p where
instance PatchInspect p => PatchInspect (FL p) where

type family PrimOf p
type instance PrimOf (FL p) = PrimOf p

data WithName prim = WithName prim

instance PatchInspect prim => PatchInspect (WithName prim) where

class (PatchInspect (PrimOf p)) => Conflict p where
    resolveConflicts :: p -> PrimOf p

instance Conflict p => Conflict (FL p) where
    resolveConflicts = undefined

type family OnPrim p

#if VER==1
class FromPrims p where

instance FromPrims (FL p) where

joinPatches :: FromPrims p => p -> p
#else
#if VER==2
joinPatches :: FL p -> FL p
#else
joinPatches :: p -> p
#endif
#endif

joinPatches = id

cleverNamedResolve :: (Conflict (OnPrim p)
                      ,PrimOf (OnPrim p) ~ WithName (PrimOf p))
                   => FL (OnPrim p) -> WithName (PrimOf p)
cleverNamedResolve = resolveConflicts . joinPatches

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