Singleton type example
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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ExistentialQuantification #-}
type family Cond (b :: Bool) (t :: k) (e :: k) :: k where
Cond 'True t e = t
Cond 'False t e = e
data Booly :: Bool -> * where
Truey :: Booly 'True
Falsey :: Booly 'False
item :: forall b . Booly b -> Cond b Int [Int]
item Truey = 42
item Falsey = [1, 2, 3]
main :: IO ()
main = putStrLn $ show $ item Falsey
Haskell
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