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haskellghcitype-families

Is it possible to display the results of applying a Haskell type family function?

For example, if I have these weird types:

{-# LANGUAGE TypeFamilies #-}
type family WeirdFamily a
type instance WeirdFamily () = Int
type instance WeirdFamily (a, b) = (a, WeirdFamily b)

Can I display (e.g. in GHCi) the result of WeirdFamily (Bool, (Char, ())) by typing something like:

:t WeirdFamily (Bool, (Char, ()))

into GHCi?

Solution

  • Use kind!.

    :kind! WeirdFamily (Bool, (Char, ()))
WeirdFamily (Bool, (Char, ())) :: *
= (Bool, (Char, Int))