[Haskell-cafe] Re: Class Interfaces in OOHaskell?

6 Jul 2007

      I conquered the below problem, but now I have another question:

How can one have two interface-classes that reference each other? For example,

type Inter1 = Record (
  MkFoo :=: Inter2 -> IO ()
  :*: HNil )

type Inter2 = Record (
  MkBar :: Inter1 -> IO ()
  :*: HNil )

Obviously this is cyclical, but is there a nice way to get around it?
I think I could wrap them up in a datatype (ie, data InterOne =
InterOne Inter1, and modify definitions accordingly) but are there any
alternative methods?

Scott

On 7/6/07, Scott West  wrote:
...
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