voodoo.hs

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE TypeApplications #-}

class Voodoo tag where
    -- Let's define an operation that requires TypeApplications in order to work
    voodoo :: Int -> String

-- Let's also have an instance (we could have more, of course)
data Tag
instance Voodoo Tag where
    voodoo = show

-- Now, this works:
--
test1 = voodoo @Tag 3

-- But this doesn't:
--
-- test2 = voodoo 3 @Tag
--
--     • Cannot apply expression of type ‘String’
--        to a visible type argument ‘Tag’

-- Can we define voodoo' that will work? (Using obnoxious amounts of black
-- magic, if necessary)
voodoo' :: Int -> (forall tag. Voodoo tag => String)
voodoo' = undefined

test3 = voodoo' 3 @Tag