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
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