axionbuster · July 6, 2025 23:26
diff --git a/Main.hs b/Main.hs
 -- | 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
	-- \| 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