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

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2011/04/08, at 17:04, Alain Frisch wrote:

> On 04/08/2011 08:50 AM, Andreas Rossberg wrote:
>> Nitpick: the Make functor is still applicative. All functors in OCaml
>> are applicative.
> 
> Nitpick²: except if you compile with -no-app-funct.
> 
>> Aside: In general, you really want to have both: functors with impure
>> bodies better be generative, while functors with pure bodies should be
>> applicative.
> 
> I'm not sure the benefits of applicative functors are worth the extra complexity they induce, especially if you want to support applicative functors in the same system as well. As far as I understand, applicative functors are useful primarily to give precise types to higher-order functors (which are rare enough).
> 
> It would be interested to see how much effort would be needed to adapt large code bases to compile with -no-app-funct (it probably amounts to giving a name to the result of a few functor applications).

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.

Jacques Garrigue