UnliftedTypeExample.hs

{-# LANGUAGE UnliftedDatatypes #-}
{-# LANGUAGE GHC2024 #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}

module Main where

import Data.Kind
import GHC.Exts
import Data.Void

-- Unlifted types
--
type Branchy :: UnliftedType -> Type
data Branchy (e :: UnliftedType) =
          MkBranchy e
        | MkAnotherBranchy Int
        -- deriving not possible because of the UnliftedType field

type MyUnit :: UnliftedType
data MyUnit = MkMyUnit
        -- deriving not possible

type MyVoid :: UnliftedType
data MyVoid
        -- deriving not possible

branchy1 :: Branchy MyUnit
branchy1 = MkAnotherBranchy 0

branchy2 :: Branchy MyVoid
branchy2 = MkAnotherBranchy 0

main :: IO ()
main = do
    case branchy1 of
        MkBranchy MkMyUnit -> putStrLn "foo"
        MkAnotherBranchy _ -> putStrLn "foo"
    case branchy2 of
        -- compiler is smart enough to know one branch is impossible
        MkAnotherBranchy _ -> putStrLn "foo"