Dear Luke, thanks for your answers....
If SelectScecario is used for other purposes, then give an explicit cast function
Sure, as I mentioned, we have different transformations and it would be worth to filter a list of transformations by a particular type or even apply the list of transformations in a particular order considering their type.
toTransformation :: SelectScenario -> Transformation toTransformation (SelectScenario ids) = Transformation { (<+>) = {- implementation of (<+>) just as if it were a class method -} }
I understand your idea, but I will have to implement several variations of "toTransformation", one for each kind of transformation. Moreover, I couldn't realize how is possible to define a function that could be applied to different transformations without using type classes--- I have to restrict the types of argument of such a function. Moreover, I couldn't figure out what are the benefits of your solution. Please, if possible, could you elaborate that a bit more, in order that I could understand why your design is better (I mean, more legible, reusable or concise)? Thanks in advance, Rodrigo. Em 01/12/2009 22:44, Luke Palmer < lrpalmer@gmail.com > escreveu: On Tue, Dec 1, 2009 at 4:21 PM, rodrigo.bonifacio wrote:
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