{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
{-# OPTIONS -fallow-overlapping-instances #-}

{-

-- (C) 2004-2005, Oleg Kiselyov & Ralf Laemmel
-- Haskell's overlooked object system

A variation on the shapes example.
We make shapes opaque in an existential envelope.
Thereby, all shapes can be inserted into a normal homogeneous list.

-}

module ShapesExists where

import OOHaskell
import Shapes


-- The polymorphic scribble loop.

main =
  do
       -- set up list of shapes.
       s1 <- mfix (rectangle (10::Int) (20::Int) (5::Int) (6::Int))
       s2 <- mfix (circle (15::Int) (25::Int) (8::Int))
       let scribble = [ HideShape s1
                      , HideShape s2 ]
       
       -- iterate through the list
       -- and handle shapes polymorphically
       mapM_ ( \(HideShape shape) -> do
                  shape # draw
                  (shape # rMoveTo) 100 100
                  shape # draw )
             scribble

       -- call a rectangle specific function
       arec <- mfix (rectangle (0::Int) (0::Int) (15::Int) (15::Int))
       arec # setWidth $ 30
       arec # draw


-- The well-quantified existential wrapper

data OpaqueShape =
 forall x. ( HasField (Proxy Draw) x (IO ())
           , HasField (Proxy RMoveTo) x (Int -> Int -> IO ())
           ) => HideShape x