id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
2203,TFs in class instances heads,chak,chak,"Ganesh posted the following example on haskell-cafe:
{{{
{-# LANGUAGE ScopedTypeVariables, TypeFamilies, FlexibleInstances #-}

module Test1a where

class Foo a where
   type TheFoo a
   foo :: TheFoo a -> a
   foo' :: a -> Int

class Bar b where
   bar :: b -> Int

instance Foo a => Bar (Either a (TheFoo a)) where
   bar (Left a) = foo' a
   bar (Right b) = foo' (foo b :: a)

instance Foo Int where
   type TheFoo Int = Int
   foo = id
   foo' = id

val :: Either Int Int
val = Left 5

res :: Int
res = bar val
}}}
It fails to type check as the type of `bar` cannot be inferred.  However, GHC should reject the instance due to the TF in the head despite `FlexibleInstances`.

Moreover, the corrected code
{{{
{-# LANGUAGE ScopedTypeVariables, TypeFamilies, UndecidableInstances #-}

module Test1a where

class Foo a where
   type TheFoo a
   foo :: TheFoo a -> a
   foo' :: a -> Int

class Bar b where
   bar :: b -> Int

instance (b ~ TheFoo a, Foo a) => Bar (Either a b) where
   bar (Left a) = foo' a
   bar (Right b) = foo' (foo b :: a)

instance Foo Int where
   type TheFoo Int = Int
   foo = id
   foo' = id

val :: Either Int Int
val = Left 5

res :: Int
res = bar val
}}}
requires `UndecidableInstances`, although it shouldn't.

We should be able to allow equalities of the form `tv ~ F tv1 .. tvn` with tv and tvi being distinct type variables without requiring `UndecidableInstances`.",bug,closed,normal,6.12 branch,Compiler (Type checker),6.9,fixed,,ganesh@… claus,Unknown/Multiple,Unknown/Multiple,,Unknown,T2203a & T2203b,,,