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I've been slowly working on porting the llvm package to use data kinds, type families and type-nats and ran into a minor issue when trying to remove the two newtypes used for classifying values ( ConstValue and Value ) by introducing a new Value type parameterized by its constness. CallArgs only accepts Value 'Variable a arguments and provides a function for casting a Value 'Const a to a Value 'Variable a . I'd like to generalize CallArgs to allow each argument to be either 'Const or 'Variable . Is this possible to encode this somehow using type families? I think it's probably doable with fundeps. {-# LANGUAGE DataKinds #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} data Const = Const | Variable data Value (c :: Const) (a :: *) type family CallArgs a :: * type instance CallArgs (a -> b) = forall (c :: Const) . Value c a -> CallArgs b type instance CallArgs (IO a) = IO (Value 'Variable a) ... which fails to compile: /tmp/blah.hs:10:1: Illegal polymorphic or qualified type: forall (c :: Const). Value c a In the type instance declaration for `CallArgs' Where the following solution works (equivalent to the legacy code), but requires the user to cast the each constant Value : type family CallArgs' a :: * type instance CallArgs' (a -> b) = Value 'Variable a -> CallArgs' b type instance CallArgs' (IO a) = IO (Value 'Variable a)

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