inconsistent class requirements with TypeFamilies and FlexibleContexts
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.
{-# 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
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | ganesh@earth.li |
| Operating system | |
| Architecture |