How do I make an heterogeneous list in Haskell? (originally in Java)

Question

How to convert following Java implementation into Haskell?

The major purpose here is having a list that contains various elements which are sub-type of a particular in

Answer 1

This is possible but probably not desirable. There is such language extension as 'existential types' that allows dynamic polymorphism.

The idea of existential type is following:

data Foo = Foo a

Note that type variable "a" doesn't appear on the left side of ADT declaration. Simple example of possible implementation of dynamic polymorphic list and map function:

{-# LANGUAGE UnicodeSyntax, Rank2Types, GADTs, ConstraintKinds #-}

import Data.Constraint

-- List datatype:
data PList α where
   Nil  ∷ PList α
   (:*) ∷ α a ⇒ a → PList α → PList α

infixr 6 :*

-- Polymorphic map:
pmap ∷ (∀ a. α a ⇒ a → b) → PList α → [b]
pmap _ Nil      = []
pmap f (a :* t) = f a : pmap f t

main = let
        -- Declare list of arbitrary typed values with overloaded instance Show:
        l ∷ PList Show
        l = "Truly polymorphic list " :* 'a' :* 1 :* (2, 2) :* Nil
    in do
        -- Map show to list:
        print $ pmap show l

Output:

["\"Truly polymorphic list \"","'a'","1","(2,2)"]

Another example using this technique:

class Show a ⇒ Named a where
    name ∷ a → String

instance Named Int where
    name a = "I'm Int and my value is " ++ show a

instance Named Char where
    name a = "I'm Char and my value is " ++ show a

 main = let
        -- Declare list of arbitrary typed values with overloaded instance Named:
        l2 :: PList Named
        l2 = 'a' :* (1 ∷ Int) :* Nil
    in do 
        print $ pmap name l2
        print $ pmap show l2

Output:

["I'm Char and my value is 'a'","I'm Int and my value is 1"]
["'a'","1"]