Re: [Caml-list] What is an applicative functor?

[Alain Frisch <alain.frisch@lexifi.com>]

On 04/08/2011 10:20 AM, Jacques Garrigue wrote:
> Applicative functors have other advantages, like the fact you can refer to a
> type produced by a functor without having really applied it.
>
> For instance think of the following functor definition
>
>     module F(O : Set.OrderedType)(S : sig type t = Set.Make(O).t val union : t ->  t ->  t end) = ...
>
> If you want to do the same thing with generative functors, I believe you have to
> pass the result of Set.Make(O) around physically.
> I do think this is a significant weakness.

I can imagine uses for:

module F(O : Set.OrderedType)(S : Set.S with type elt = O.t) =

but I don't see a real-life case where one would want functor F to know 
that the type S.t was really produced by applying Set.Make.  Do you have 
a specific example in mind?


Alain