impredicative.purs

module Main where

import Prelude hiding (show)
import Prelude as Prelude

import Data.Foldable (fold)
import Effect (Effect)
import TryPureScript (p, text, render)


newtype Identity a = Identity a

runIdentity :: forall a. Identity a -> a
runIdentity (Identity x) = x


-- DOES NOT WORK :-(

-- myWrappedShow1 :: Identity (forall a. Show a => a -> String)
-- myWrappedShow1 = Identity Prelude.show


-- WORKS :-)

newtype SomeShow = SomeShow (forall a. Show a => a -> String)

myWrappedShow2 :: Identity SomeShow
myWrappedShow2 = Identity (SomeShow Prelude.show)


main :: Effect Unit
main =
  render $ fold
    [ p $ text $ myShow 42
    , p $ text $ myShow true
    ]
  where
  myShow :: forall a. Show a => a -> String
  -- DOES NOT WORK :-(
  -- myShow = myWrappedShow1 # runIdentity
  -- WORKS :-)
  myShow = myWrappedShow2 # runIdentity >>> (\(SomeShow f) -> f)