A Typeclass Box in Haskell. Allows for a single type to represent any type that responds to a typeclass.

TypeclassBox.hs

{-# LANGUAGE GADTs, ConstraintKinds, FlexibleInstances, RankNTypes #-}

-- Our typeclass with a simple foo :: a -> String func
class Foo a where
    foo :: a -> String

-- Our test data types and their instances of Foo
data MyFoo = MyFoo String
data MyBar = MyBar Int

instance Foo MyFoo where
    foo (MyFoo s) = s

instance Foo MyBar where
    foo (MyBar i) = show i


-- Our Typeclass Box (TBox). Needs GADTs. `a` is existential. `c` is a contraint
data TBox c where
     TBox :: c a => a -> TBox c

-- A TBox can be represented as a Foo (it is whatever its inner representation of Foo is).
-- Notice that `a` is existential. We don't know what it is, we just know that we can call `foo` on it.
-- Needs FlexibleInstances.
instance Foo (TBox Foo) where
    foo (TBox a) = foo a


main = do
    -- We can now make a list of TBox where each inner type is different
    let listOfFoo = (TBox (MyFoo "hello") : TBox (MyBar 123) : []) :: [TBox Foo]
    -- AND we can call `foo` on all of them via map
    mapM_ putStrLn $ fmap foo listOfFoo

When compared to this gist, it is interesting to see how little you need to do in Haskell to get the same done. This is mostly due to the fact that type-classes are part of Haskell, as opposed to Scala where they are just a trick that you can do with implicits. Additionally, Scala's syntax is very verbose when compared to Haskell's.

Generalized TBox added. Pretty cool stuff.

Thank you for this