dyn.hs

-- See https://travis.athougies.net/posts/2017-04-26-close-the-world.html

{-# OPTIONS_GHC -fprint-explicit-foralls #-}

{-# LANGUAGE TypeApplications #-}

import Data.Maybe
import Data.Typeable

data Dyn where
  Dyn :: (Show a, Typeable a) => a -> Dyn

x1, x2, x3 :: Dyn
x1 = Dyn (3 :: Int)
x2 = Dyn "Fred"
x3 = Dyn (Just (0 :: Integer))

xs :: [Dyn]
xs = [x1, x2, x3]

is :: forall a. Typeable a => Dyn -> Maybe a
is (Dyn a) = cast a

only :: forall a. Typeable a => [Dyn] -> [a]
only = mapMaybe (\(Dyn x) -> cast x)

main :: IO ()
main = do
  print $ map (is @Int) xs
  print $ mapMaybe (is @Int) xs
  print $ only @Int xs
  print $ only @String xs
  print $ only @(Maybe Integer) xs