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

[adam vogt]

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