[Haskell-cafe] Evaluating type expressions in GHCi

[Brandon Moore]

>From: Sean Leather <leather at cs.uu.nl>
>
>I would like to ask GHCi for the type that a type expression will evaluate to, once all definitions of type synonyms and (when possible) type families have been inlined.
>
>
>It appears that I can do some part of this for type T by using ":t undefined :: T":
...
>undefined :: F (Int, Bool) :: (Bool, Int)
>
>I also get what I expect here:
>
>ghci> :t undefined :: F (a, Bool)
>undefined :: F (a, Bool) :: (F a, Int)
>
>Of course, this doesn't work on types of kinds other than *.


In the absence of interpreter support, you can work around that by making
appropriate type constructors.
{-# LANGUAGE TypeFamilies, KindSignatures, EmptyDataDecls #-}

type family G a :: * -> *
type instance G Int = Maybe
type instance G Bool = []

data Wrap1 (t :: * -> *)

ghci> :t undefined :: Wrap1 (G Int)
undefined :: Wrap1 (G Int) :: Wrap1 Maybe

The development version of ghci seems to support type declarations,
which would make this easier.

https://github.com/ghc/ghc/commit/3db757241ce7fb99c096c30481aefa86bb9855a1

Brandon