[Haskell-cafe] Class Interfaces in OOHaskell?

[Scott West]

Hello all,

Looking at the OOHaskell black (grey?) magic, and wondering if there
would be an interesting way to construct class interfaces using the
OOHaskell paradigm?

I'm trying to do it as so (assume relevant types/proxies declared):

type FigureInter = Record ( Draw :=: IO ()
                        :*: HNil
                            )

figure self = do
  return emptyRecord
  where
    _ = narrow self :: FigureInter

abstrFigure self = do
  super <- figure self
  visible <- newIORef True
  returnIO
    $   setVisible .=. (\b -> writeIORef visible b)
    .*. isVisible  .=. readIORef visible
    .*. draw       .=. return ()
    .<. super

but ghci complains (you know how it likes to complain), with

    Couldn't match expected type `Record t2'
           against inferred type `F (Proxy Draw) (m ())'
    In the second argument of `(.*.)', namely `draw .=. (return ())'
    In the second argument of `(.*.)', namely
        `(isVisible .=. (readIORef visible)) .*. (draw .=. (return ()))'
    In the first argument of `(.<.)', namely
        `(setVisible .=. (\ b -> writeIORef visible b))
       .*.
         ((isVisible .=. (readIORef visible)) .*. (draw .=. (return ())))'

Anyone have any tips they care to share? :)

Scott