id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
4981,inconsistent class requirements with TypeFamilies and FlexibleContexts,ganesh,simonpj,"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

}}}
",bug,closed,high,7.2.1,Compiler (Type checker),7.0.1,fixed,,ganesh@… dimitris@…,Unknown/Multiple,Unknown/Multiple,GHC rejects valid program,,"indexed-types/should_compile/T4981-V1.hs, T4981-V2.hs, T4981-V3.hs",,,