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Date: Fri, 08 Apr 2011 19:35:15 +0200
From: Alain Frisch <alain@frisch.fr>
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To: Till Varoquaux <till@pps.jussieu.fr>
CC: Andreas Rossberg <rossberg@mpi-sws.org>,
        Gerd Stolpmann <info@gerd-stolpmann.de>, Dawid Toton <d0@wp.pl>,
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Subject: Re: [Caml-list] What is an applicative functor?

On 04/08/2011 06:43 PM, Till Varoquaux wrote:
> I tend to consider using Impure functors as very poor coding hygiene..
> I am sure there are compelling use cases but I've yet to come across
> one.

A functor that implements hash-consing over some type could be one such 
example:

module HashCons(X : Hashtbl.HashedType) : sig
   type s
   val mk: X.t -> s
   val get: s -> X.t
end = struct
   type s = X.t
   module H = Hashtbl.Make(X)
   let tbl = H.create 16
   let mk x = try H.find tbl x with Not_found -> H.add tbl x x; x
   let get x = x
end

The intention is that values of type s are physical equal whenever they 
wrap equal values of type t. Unfortunately, this invariant can be broken 
because the functor is applicative. It would be good to be able to mark 
it as being generative instead.


Alain