https://qiita.com/acple@github/items/216542d1c4844c2ec879

Existential.purs

module Main where

import Prelude

import Data.Exists (Exists, mkExists, runExists)
import Data.Foldable (traverse_)
import Data.Maybe (Maybe(..))
import Effect (Effect)
import Effect.Class.Console (log)

----------------------------------------------------------------

-- 任意のaに対して値とshowをセットで扱う
newtype ShowBox' a = SB { value :: a, show :: a -> String }

-- aがどこかにきえた！
newtype ShowBox = ShowBox (Exists ShowBox')

instance showSB :: Show ShowBox where
  show (ShowBox sb) = sb # runExists \(SB { show: sh, value }) -> sh value -- なかのshowにvalueをぶつけるだけ

-- コンストラクタのような
mkShowBox :: forall a. Show a => a -> ShowBox
mkShowBox = ShowBox <<< mkExists <<< SB <<< { value: _, show }

main :: Effect Unit
main = do
  let hetero = [ mkShowBox 1, mkShowBox "abc", mkShowBox [ unit, unit ], mkShowBox { x: Just 123, y: 1000.0 } ]
  traverse_ (log <<< show) hetero