Hi cafe. On a use case whim, I made the open-union package (https://github.com/RobotGymnast/open-union), copying the basic idea from extensible-effects:Data.OpenUnion1. I haven't uploaded to hackage yet, on the chance that there's already something like this around. Here's the basic functionality:

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}
import Data.OpenUnion

type MyUnion = Union (Char :| Int :| [()] :| Void)

showMyUnion :: MyUnion -> String
showMyUnion
    =  (\(c :: Char) -> "char: " ++ show c)
    @> (\(i :: Int) -> "int: " ++ show i)
    @> (\(l :: [()]) -> "list length: " ++ show (length l))
    @> typesExhausted

main :: IO ()
main = do
    putStrLn $ showMyUnion $ liftUnion (4 :: Int)
    putStrLn $ showMyUnion $ liftUnion 'a'
    putStrLn $ showMyUnion $ liftUnion [(), ()]

If any of the (@>) cases is omitted, a compile-time error occurs. If you try to lift a bad value to the union, a compile-time error occurs.

Any thoughts? Is there already something like this around?