#2203: TFs in class instances heads
-------------------------+--------------------------------------------------
Reporter: chak | Owner: chak
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.9 | Severity: normal
Keywords: | Testcase:
Architecture: Multiple | Os: Multiple
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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`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2203>
GHC <http://www.haskell.org/ghc/>
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