onlyone.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators #-}

data Foo = Foo

data Bar = Bar

class OnlyOne ts

instance OnlyOne '[_1]

throwFoo :: OnlyOne (Foo ': ts) => Foo -> IO a
throwFoo = undefined

throwBar :: OnlyOne (Bar ': ts) => Bar -> IO a
throwBar = undefined

thing :: OnlyOne '[Foo, Bar] => IO Int
thing = do
  throwBar @'[] Bar
  throwFoo @'[Bar] Foo
  pure 5


-- > thing
-- 
-- <interactive>:5:1: error:
--     • No instance for (OnlyOne '[Foo, Bar])
--         arising from a use of ‘thing’
--     • In the expression: thing
--       In an equation for ‘it’: it = thing