typeapp.hs

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

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

-- When do I need TypeApplication?
-- When I need some intermediate types that change how
-- the internal function behaves.
-- Could maybe have sth like "runAPI @MyAPI input"

import GHC.TypeLits (Symbol, KnownSymbol, symbolVal)
import Data.Proxy (Proxy(..))

-- The canonical example: `a` isn't exported so f doesn't
-- know which read instance to pick. f "True" @Bool works,
-- f "1" @Bool fails.
f :: forall a. (Read a, Show a) => String -> String
f = (show :: a -> String) . read

data a :<|> b = a :<|> b
infixr 8 :<|>

data (a :: k) :> (b :: *)
infixr 9 :>

data Const (s :: Symbol)

class PrintThing layout where
  pt :: String

instance forall s a. (KnownSymbol s) => PrintThing (Const s) where
  pt = symbolVal (Proxy :: Proxy s)

instance forall a b. (PrintThing a, PrintThing b) => PrintThing (a :<|> b) where
  pt = (pt @a) ++ (pt @b)

-- Tests

type Test = (Const "1")  :<|> (Const "2")
type Test2 = (Const "hello")  :<|> (Const " yous")

hello = do
  print (pt @Test)
  print (pt @Test2)