Re: [Caml-list] One-value functors

[Goswin von Brederlow <goswin-v-b@web.de>]

Arnaud Spiwack <Arnaud.Spiwack@lix.polytechnique.fr> writes:

> Dear all,
>
> With the advent of first-class modules, I find myself more and more wanting to
> write functors with only one (value) component, just to benefit from type
> dependencies. Here is an example (randomly) extracted from Oleg Kiselyov and
> Jeremy Yallop:
>
>
>     type a and b
>     module type TC = sig type 'a t end
>     module type Subst = functor (A:TC) -> sig val x : a A.t -> b A.t end

What are you trying to do there? Why do you need a functor for that at
all? TC just having a type doesn't make sense to me.

> If this pattern had to become widespread, it would be worth agreeing for a name
> on the single component, though it's a bit unnecessary to need a name at all.
> It get very verbose, though, when trying to applying the functor. Supposing I
> have a module Subst:Subst :
>
>
>     let module S = Subst(sig type 'a t = 'a list) in
>     S.x ?

So S.x would be a function that turns for example an int list into a
float list, e.g. let x list = List.map float_of_int list?

How would you write Subst that it takes anything but 'a list (and
Obj.magic doesn't count)? How would the functor look like that would
accept 'a list or 'a array as argument? Seems to me TC needs to also
contain the map function for this to work. And then why not just use
(List.map float_of_int) as closure?

> I believe this would be something cool to have, so I came with two questions:
> 1/ Is there some demand for this kind of things (if not, I'm not sure it's
> worth pursuing, though of course demand could appear after supply)
> 2/ I don't see how to define the type syntax in camlp4, because of the inlining
> thing, could it be done?
>    (an alternative may be to have (X:A) => t be a module type, but then  (X:A)
> => (Y:B) => t is not a valid type, so we would need syntaxes for n-ary
> abstraction/applications)

Hard to say if I can't even understand what "this kind of thing" is.

MfG
        Goswin