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-boun...@haskell.org
[mailto:glasgow-haskell-users-boun...@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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