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Date: Mon, 23 May 2011 21:34:13 +0200
From: Guillaume Yziquel <guillaume.yziquel@citycable.ch>
To: Hakan Suka <hakan40us@yahoo.com>
Cc: caml-list@inria.fr
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Subject: Re: [Caml-list] functor inside a class

Le Monday 23 May 2011 à 04:17:00 (-0700), Hakan Suka a écrit :
> Hi everyone,
> 
> But can I have a local module definition so that the I can have a tracker not 
> only for integers, but for any type being passed as input? The following does 
> not work:
> 
> class ['a] tracker size =
> let module LabelSet =
> Set.Make (struct type t = 'a let compare = compare end)
> in
> object
> val nums_to_track_tbl : (int32, LabelSet.t) Hashtbl.t =
> Hashtbl.create size
> 
> method get pos = Hashtbl.find_all nums_to_track_tbl pos
> 
> end

First of all, the scope in which you can refer to LabelSet is local to
the class declaration. However, the class method get has a type which
refers to LabelSet. That's out of scope.

More importantly, it seems that

	let f x = let module M = struct end in x

is syntactically valid, while it doesn't seem to work out the same way for
class declarations:

	class ['a] tracker = fun size -> let module M = struct end in object end

gives a syntax error.

You should create your local module somewhere else than before 'object'
'end'.

> If this is not possible, would it work in Ocaml 3.12 with first-class modules?
> 
> I have seen the following code in the manual which seems fairly close of what I 
> would need inside the parametrised class, but not sure if I can use the output 
> of make_set inside the module. I haven't tested this because I have Ocaml 3.11.2
> 
> let make_set (type s) cmp =
> let module S = Set.Make(struct
> type t = s
> let compare = cmp
> end) in
> (module S : Set.S with type elt = s) 
> 
> If that does not work, any suggestions on how to implement this?

The big point is that you need the 'get' method to return something which
is a 't' element of a module satisfying the 'Set.S with type t = 'a'
signature.

You need a type constructor for that. Set.Make(String).t is a valid
type, but I do not see how parametrise String for some other 'a type.

The following works:

	type 'a set_module = (module Set.S with type elt = 'a)

	let make_set (type s) ?(cmp = compare) () =
		let module S = Set.Make (struct type t = s let compare = cmp end) in
		(module S : Set.S with type elt = s)

	class ['a] tracker size = object
		val set = make_set ()   
		val nums_to_track_tbl : (int32, 'a set_module) Hashtbl.t = Hashtbl.create size
		method get pos = Hashtbl.find_all nums_to_track_tbl pos
	end

But it's a 'a set_module, not a 'a set, so first-class modules do not
seem so useful at first glance...

What you'd need is not a

	type 'a set_module = (module Set.S with type elt = 'a)

but something like

	type 'a set = (module Set.S with type elt = 'a).t

(which is not valid), and to my knowledge, this cannot be done.

However, I think there is a polymorphic-not-functorised code for maps
and sets lying out somewhere on the net. This is likely your best bet.

> Thanks,
> Hakan

-- 
     Guillaume Yziquel