-- | Demonstration of an existentially-qualified type class
module Main (main) where

import Data.Constraint (Dict (Dict)) -- constraints
import Data.Kind (Type, Constraint)  -- base

-- A variant that contains a constraint and a term of unknown type.
data Z (c :: Type -> Constraint)
  = forall t. ZY (Dict (c t)) t
  | forall t. ZN t

-- Simpler variant that only carries a constraint
data W (c :: Constraint) = WY (Dict c) | WN

-- | Show if possible, determined dynamically.
sz :: Z Show -> String
sz (ZY Dict x) = show x
sz (ZN _) = "<<unshowable>>"

-- |
-- @
-- >>> main
-- ["2","()","<<unshowable>>"]
-- @
main :: IO ()
main = do
  let zs :: [Z Show] =
        [ ZY Dict (2 :: Int)
        , ZY Dict ()
        , ZN (const :: Int -> Int -> Int)
        ]
  print . map sz $ zs