id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
2695,"bogus ""syntactically distinct contexts"" error",conal,,"{{{
{-# 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

}}}
",bug,closed,low,_|_,Compiler (Type checker),6.11,fixed,,conal,Unknown/Multiple,Unknown/Multiple,None/Unknown,Unknown,,,,