Re: [Caml-list] recursive module type

["Michaël Lienhardt" <michael.lienhardt@inria.fr>]

Thanks

I also thought about the SetConverter solution.
This is what I will use.

I was asking if such a recursive definition was possible because I have a tendency to compare the ocaml module system with Java-like objects.
 From that point of view, signatures are like tree-structured Java interfaces that can declare types, values and functions in addition to methods (functors).
While typing in object-oriented languages is recursive by default, I understand that it is not the case in ocaml because of type inference:
  annotating modules with their signatures then enable recursive modules.
Then, because signatures are types, and also have some similarities with Java interfaces, I imagined that some kind of recursion is possible.

Michael Lienhardt


Le 04/05/2013 14:47, Jacques Garrigue a écrit :
>
> On 2013/05/04, at 21:23, Michaël Lienhardt <michael.lienhardt@inria.fr> wrote:
>
>> Hello,
>>
>> I would like to define some recursive module type in caml, like
>> module type S = sig
>> ...
>> module Toto(M : S) : sig
>>   ...
>> end
>> end
>>
>> More precisely, I'd like to extend the Set module available in the ocaml distribution with a function that convert a set into another set.
>> Intuitively, I came up with the following signature S, which is not accepted by ocaml (Error: Unbound module type S):
>>
>>   module type S = sig
>>
>>     include Set.S
>>
>>     module Convert : functor(SetTarget : S) -> sig
>>       val convert : (elt -> SetTarget.elt) -> t -> SetTarget.t
>>     end
>>   end
>
> You first problem is syntactic.
> You cannot define recursive signatures, just recursive modules.
> But the signature may refer recursively to the module.
> So you can attempt to write the following:
>
> module rec M : sig
>    module type S = sig
>       include Set.S
>       module Convert : functor(SetTarget :M.S) -> sig
>         val convert : (elt -> SetTarget.elt) -> t -> SetTarget.t
>       end
>     end
> end = M;;
>
> unfortunately, this does not work:
> Fatal error: exception Env.Recmodule
>
> And this is not surprising, because this would let you indeed define
> a recursive signature (i.e. a signature that unfolds infinitely).
>
> Jacques Garrigue
>