On 11/09/2011 02:29 AM, Cedric Cellier wrote:
> For some reasons though, despite functors being one of the greatest 
> strength of the language, we do seam shy to use, recommand or brag 
> about them. I wonder if this is due to the lack of proper documentation ?
I don't think there is a lack of proper documentation: it is introduced 
right away in the "tutorial" part of the Ocaml manual (Part I), it is 
described in the language description (Part II) and there are a couple 
of examples in the std lib (Part IV). What do you think it lacks to be 
properly documented (at least w.r.t other features of Ocaml)?

I actually wonder if they just *look* too complicated? Maybe because 
they are verbose?
Each time you define a functor you also have to give the signature of 
its argument, which, compared to a simple "include" can look overkilling.
Concretely:

module A = struct
   ...
end

module B = struct
   include A
   ...
end

VS

module A = struct
   ...
end

module type B_INPUT = sig
   ... (can be big)
end

module Make_B (X:B_INPUT) = struct
   ...
end

For one module that's fine, but when you start having lots of modules 
and intricate interactions between them, that can start to be a pain in 
the ass.
In addition, this include->functor switch happens quite often when you 
have sources that were not written in the first place with functors in 
mind, so this is not an uncommon situation.

I think personally that this verbosity is actually a good thing because 
it forces to give some documentation which is particularly needed when 
many modules are interacting.
But then this documentation argument is a bit contradictory with what we 
sell to beginners when they learn Ocaml: "Ocaml is great because it has 
type inference, this removes verbosity!"...

-- 
Vincent Aravantinos
Postdoctoral Fellow, Concordia University, Hardware Verification Group
http://users.encs.concordia.ca/~vincent