#2855: Surprising type (family) type derived
-----------------------------+----------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Consider the following module
{{{
{-# LANGUAGE TypeFamilies #-}
module Bug5 where
class C a where
type T a
f :: T a -> T a -> T a
data D a = D { t :: T a }
g r = f (t r) (t r)
}}}
Now ask for the type of g
{{{
*Bug5> :t g
g :: (T a ~ T a1, C a1) => D a -> T a1
}}}
Why isn't the type
{{{
g :: (C a) => D a -> T a
}}}
?
The strange type is a minor nuisance, but it gets worse
{{{
{-# LANGUAGE TypeFamilies #-}
module Bug6 where
class C a where
type T a
type U a
f :: T a -> T a -> T a
x :: U a -> T a
data D a = D { t :: T a, u :: U a }
g r = f (t r) (x (u r))
}}}
This doesn't type check at all.
An even simpler example that fails:
{{{
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Bug7 where
class C a where
type T a
type U a
x :: U a -> T a
data D a = D (U a)
g :: (C a) => D a -> T a
g (D u) = x u
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2855>
GHC <http://www.haskell.org/ghc/>
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