On Tue, 15 Jun 2004 12:05:46 +0100, Graham Klyne <[EMAIL PROTECTED]> wrote:
At 11:30 15/06/04 +0200, Wolfgang Jeltsch wrote:
I'd say, classes in Haskell are similar to interfaces in Java.
I started my Haskell programming with that viewpoint, but got tripped up by it.
A Java interface can be used as a type in its own right, but a Haskell
class cannot. For example, you can't have a list of Eq's, only a list of
some type that happens to be an Eq. The different list members can't be
differently-typed instances of Eq.
Well, just with existential types.
To emulate a Java interface, I have ended up creating algebraic data types
whose components are functions. (I don't claim that's a good approach to
Haskell programming, just what I ended up doing.)
My approach was to create an existential types for each class and instanciate it
from the class like:
class FooClass f where
bar :: f -> Integer
setBar :: Integer -> f -> f
data Foo = forall f . FooClass f => Foo f
instance FooClass Foo where
bar (Foo f) = bar f
setBar b (Foo f) = Foo $ setBar b f
I am not sure if this is the way to go, but at least it works (except for read issues). The
advantage I see in this approach is that I can stick all types that are instanciated from
FooClass in a list and can deal with them as there would be no exitential type constructor
"in the way".
Cheers,
Georg
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