bogus "syntactically distinct contexts" error

{-# LANGUAGE TypeOperators, FlexibleContexts
  , MultiParamTypeClasses, FunctionalDependencies
  , TypeFamilies
  -- , ScopedTypeVariables
  #-}

-- The ScopedTypeVariables is there just as a bug work-around.  Without:
-- 
--     Mutually dependent functions have syntactically distinct contexts
--     When matching the contexts of the signatures for
--       dZero :: forall b a s.
--                (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) =>
--                a :> b
--       pureD :: forall b a s.
--                (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) =>
--                b -> a :> b
--     The signature contexts in a mutually recursive group should all be identical
--     When generalising the type(s) for dZero, pureD
-- 
-- This bug was introduced between ghc 6.9.20080622 and 6.10.0.20081007.

{-# OPTIONS_GHC -fno-warn-missing-methods #-}


import Control.Applicative

class AdditiveGroup v where
  zeroV   :: v
  (^+^)   :: v -> v -> v
  negateV :: v -> v

class AdditiveGroup v => VectorSpace v s | v -> s where
  (*^)  :: s -> v -> v


-- | Mapping from all elements of @a@ to the results of some function
class HasTrie a where
    data (:->:) a :: * -> *

instance HasTrie a => Functor ((:->:) a)

instance HasTrie a => Applicative ((:->:) a)

class VectorSpace v s => HasBasis v s where
  type Basis v :: *

-- | Linear map, represented a as a memo function from basis to values.
type u :-* v = Basis u :->: v

data a :> b = D { powVal :: b, derivative :: a :-* (a :> b) }

dZero :: (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) => a:>b
dZero = pureD zeroV

pureD :: (AdditiveGroup b, HasBasis a s, HasTrie (Basis a)) => b -> a:>b
pureD b = b `D` pure dZero

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	conal
Operating system
Architecture