A simple object system in Haskell

oo.hs

-- > :load oo.hs 
-- [1 of 1] Compiling Main             ( oo.hs, interpreted )
-- Ok, modules loaded: Main.
-- > :main
-- Hello, World
-- > :t (newGreeter "Hi, " |> sayHello)
-- (newGreeter "Hi, " |> sayHello) :: String -> IO ()


-- This would be an `interface` in Java.
data Greeter = Greeter {
    terminalSymbol :: String,
    sayHello :: String -> IO ()
  }


-- This would be the class definition
newGreeter :: String -> Greeter
newGreeter greetingStart = this
  where this = Greeter {
                      terminalSymbol = "!",
                      sayHello = privateMember
                    }
        privateMember personName = putStrLn (greetingStart ++ personName ++ (this /> terminalSymbol))


-- This would be the dot (.) in languages like Java or Python.
(/>) = flip ($)


main :: IO ()
main = do greeter /> sayHello $ "World"
          modifiedGreeter /> sayHello $ "World"
  where greeter = newGreeter "Hello, "
        -- Simple to replace existing implementation (prototypal inheritance?).
        modifiedGreeter = greeter {
          sayHello = \_ -> putStrLn "Modified!"
        }


-- I'm not sure I can represent internal state using monads, but that would be
-- the next logical step.

-- I also, don't see how would be possible to implement inheritance as in Java,
-- but it's probably doable to model prototypal inheritance.