At 11:25 10/10/03 +0200, Markus.Schnell@infineon.com wrote:
Hi Graham,
Instead, I replace the class instances by a single algebraic data type, whose members are functions corresponding to OO-style class methods.
could you give an example?
The code in a previous message of mine [1] was an example of sorts, though complicated by some other issues. Look for type 'DatatypeVal'. [1] http://haskell.org/pipermail/haskell-cafe/2003-October/005231.html A simpler example might be: Consider a class of values called shape, for which the following operations are defined: draw :: Shape -> Canvas -> Canvas flip :: Shape -> Shape move :: Shape -> Displacement -> Shape etc. One can imagine defining a Haskell type class with these methods, but then you get the type mixing problem noted previously. What I have found can work in situations like this is to define a type, thus: data Shape = Shape { draw :: Canvas -> Canvas , flip :: Shape , move :: Displacement -> Shape etc } then one would also need methods to create different kinds of shape, e.g.: makeSquare :: Point -> Displacement -> Shape makeCircle :: Point -> Length -> Shape etc. (Assuming appropriate type definitions for Point, Displacement, Length, etc.) #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact