StrictVoidExample.hs

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

module Main where

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

-- Unlifted types
--
type Brancho :: Type -> Type
data Brancho e =
          MkBrancho !e
        | MkAnotherBrancho Int
        deriving Show

branchy1 :: Brancho ()
branchy1 = MkAnotherBrancho 0

branchy2 :: Brancho Void
branchy2 = MkAnotherBrancho 0

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