Heterogeneous List in Haskell (simple version)

HeterogeneousList.hs

{-# LANGUAGE ExistentialQuantification #-}

-- This is adapted from http://en.wikibooks.org/wiki/Haskell/Existentially_quantified_types
-- See also https://www.haskell.org/haskellwiki/Heterogenous_collections#Existential_types

module HeterogeneousList where

data ShowBox = forall s. Show s => SB s
 
heteroList :: [ShowBox]
heteroList = [SB (), SB (5::Integer), SB True]

instance Show ShowBox where
  show (SB s) = show s        -- (*) see the comment in the text below
 
f :: [ShowBox] -> IO ()
f = mapM_ print

main :: IO ()
main = f heteroList