my forall 1.hs

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GADTSyntax #-}

--https://stackoverflow.com/questions/3643172/using-maybe-type-in-haskell

-- https://en.wikibooks.org/wiki/Haskell/Classes_and_types

-- https://en.wikibooks.org/wiki/Haskell/Existentially_quantified_types

-- https://wiki.haskell.org/Data_declaration_with_constraint
  -- https://stackoverflow.com/questions/22622399/how-to-fix-illegal-datatype-context-use-xdatatypecontexts/22622591#22622591
    -- https://stackoverflow.com/questions/22622399/how-to-fix-illegal-datatype-context-use-xdatatypecontexts/22622591#22622591
    -- https://stackoverflow.com/questions/7438600/datatypecontexts-deprecated-in-latest-ghc-why/7438716#7438716

-- https://www.schoolofhaskell.com/school/to-infinity-and-beyond/pick-of-the-week/guide-to-ghc-extensions/data-type-extensions#gadtsyntax
-- https://en.wikibooks.org/wiki/Haskell/GADT

main = putStrLn $ show $ asdf rrr

rrr = Just 5
asdf rrr = case rrr of
  Just value -> value

--zxcv:: forall s. ShowBox -> s --??
zxcv yyy = case yyy of
  SB value -> value

{-
data Vl a = V a | G

s :: forall a. a->a
s x = x

-- 1
--aa :: [a -> Vl a]
--aa = [V, G] -- ??

-- 2
bbInt :: [Vl Integer]
bbInt = [V 5, V 6]
{-
bb :: forall a. [a -> Vl a]
bb = [V 5, V 6]
-}
--bbHetero = [V 5, V (), V 6.5]
-}
-- =========================

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

data ShowBox'' where
    SB'' :: forall a. Show a => a -> ShowBox''
heteroList'' :: [ ShowBox'' ]
heteroList'' = [ SB'' 4, SB'' (), SB'' 6 ]

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

data ShowBox' where
    SB' :: Show a => a -> ShowBox'
heteroList' :: [ShowBox']
heteroList' = [ SB' 4, SB' (), SB' 6 ]