On Tue, Dec 1, 2009 at 4:21 PM, rodrigo.bonifacio
Thanks Luke.
In fact I, will have different implementations of the Transformation type. Something like:
data SelectScenarios = SelectScenarios {
scIds :: [Id]
}
What is this different type buying you? You can never "downcast" to it later.
And then I should be able to make SelectScenarios a kind of Transformation. So I think that I really need a class. What do you think about it?
instance Transformation SelectScenario where
(<+>) ....
So instead of making a type and an instance, just implement it directly as a Transformation: selectScenario :: [Id] -> Transformation selectScenario ids = Transformation { (<+>) = {- whatever implementation you gave for (<+>) above, using ids -} } If the only purpose of SelectScenario (your type) is to be used polymorphically as a Transformation, then this approach is isomorphic -- i.e. anything you can do with the existential type trick you can do with this approach. If SelectScecario is used for other purposes, then give an explicit cast function toTransformation :: SelectScenario -> Transformation toTransformation (SelectScenario ids) = Transformation { (<+>) = {- implementation of (<+>) just as if it were a class method -} } Existential types only buy you power when the quantified variable appears more than once on the right hand side, for example: forall a. Num a => (a,a). But even those can usually be factored out into more direct representations (I seem to recall Oleg has a proof that they always can, actually). Luke