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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)

Trac metadata

Trac field	Value
Version	7.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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