Re: [Haskell-cafe] "Generating" type synonyms without Template Haskell

Hi Tom, You could push the use of that type family into the definition of MyProduct so that the type family can be fully applied:

{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies, TypeOperators #-}

data MyProduct f a b c = MyProduct { foo :: MkF f a , bar :: MkF f b , baz :: MkF f c }

type family MkF (f :: k) (x :: *) :: *

type instance MkF () x = x type instance MkF (f ': fs) x = f (MkF fs x) type instance MkF '[] x = x type instance MkF f x = f x

Then your example signatures can be written as

type MP f = MyProduct f Int Bool String

type A = MP () type B = MP Maybe type C = MP [[], IO] type D = MP P

I'm not sure this approach gains you much over just writing out the type signatures has you had them. Adam