Re: [Caml-list] subtyping, polymorphism, higher order types.....

[Daniel Weil <daniel.weil@athys.fr>]

I've spent these last week some time reading stuff on type classes and
Hashkell and I was going to raise the same question :
Is it possible to implement Type Classes (in the Haskell way) over 
Ocaml, at least over CamlLight ?

Type classes (which are not object classes) solve in a very elegant way 
the problem of overloading (especially for arythmetic operators). I'm 
not sure to have understood precisely what G'Caml does but I don't think 
G'Caml solve this problem.



Shaddin Doghmi wrote:

> In my experiences with ocaml, one of the major frustrations i constantly
> run into is the lack of subtyping. Polymorphic variants partially fix
> that problem, by allowing one to define types as immediate unions (i.e.
> without constructors) of other types, hence allowing a value to belong
> to both the subtype and the supertype, not to mention multiple unrelated
> types.
>
> However, there is still the issue of defining stuff such as "equality
> types", "comparables", and such, allowing overloading of functions on
> those. For example, when writing a large system with many datatypes, i
> tend to find myself commenting each type definition with stuff like
> "(*equality type*)",  to indicate to myself whether or not im allowed to
> use the default = operator on that type(for example, a set type would
> not get that annotation).  Haskell solves that problem through the use
> of type classes.
>
> Another problem i run into is the inability to define parametric higher
> order polymorphic types (is this the correct terminology?).. for
> example. lets say i want to define the following type:
>
> type 'a identifier = I of 'a *int | S of 'a*int
>
> but, what if i want to restrict 'a to a narrower class of types (as
> opposed to ad hoc types)... i would wish to say something like:
>
> type 'identifiable identifier = I of 'a*int | S of 'a*int
>
> where 'identifiable is a type class of some sort. I am not sure if
> Haskell solves this problem using type classes(seems like it could),
> Someone experienced in Haskell would probably know.
>
> I wonder if there are any plans to implement such a system in ocaml? if
> so, what approach would be taken?
>
> just a few thoughts.....
>
>
>
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-- 
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* Daniel Weil                                                       *
* ATHYS                                                             *
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* daniel.weil@athys.fr                                              *
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