#6088: GeneralizedNewtypeDeriving + TypeFamilies + Equality constraints
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Reporter: Lemming | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler (Type
checker)
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: 3046 |
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I have the following module that uses generalized newtype deriving:
{{{
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
class C a
newtype A n = A Int
class Pos n
instance (Pos n) => C (A n)
newtype B n = B (A n)
deriving (C)
}}}
This module can be compiled, and GHCi shows me what generalized newtype
deriving generated for B:
{{{
*Main> :info B
newtype B n = B (A n) -- Defined at NewtypeSuperclass.hs:13:9
instance Pos n => C (B n) -- Defined at NewtypeSuperclass.hs:14:14
}}}
This is what I expected.
Now I want to translate the Pos type class to an equality constraint on a
type function value:
{{{
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE EmptyDataDecls #-}
class C a
newtype A n = A Int
type family Pos n
data True
instance (Pos n ~ True) => C (A n)
newtype B n = B (A n)
deriving (C)
}}}
Now I get the compiler error:
{{{
Couldn't match type `Pos n' with `True'
arising from the 'deriving' clause of a data type declaration
When deriving the instance for (C (B n))
}}}
It seems that the equality constraint disallows generalized newtype
deriving.
Is this an implementation issue or is there a deep theoretic problem?
I would certainly prefer to obtain something like:
{{{
*Main> :info B
newtype B n = B (A n)
instance (Pos n ~ True) => C (B n)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6088>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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