Re: [Caml-list] Applicative Functor Madness

[Arlen Cox <arlencox@gmail.com>]

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Based on some initial feedback, I'm providing some further information:

What I have is a whole bunch of functors that look like this:

module Make(S : OrderedType) = struct
  module SM = Map.Make(S)
  module SS = Set.Make(S)
  module SC = CustomFunctor.Make(S)
  (* where SC has exposed maps and sets in its interface *)
  ...
  (* many more similar modules/functors *)
  ...
  let foo () : SS.t =
    SC.run SS.empty
end

I don't really want to define the type of Make because I have to constrain
that SS, SM, and SC all preserve applicative identity in a type
constraint.  Then I have to do the same thing for every CustomFunctor as
well.  Worst, it's difficult to tell when things have been broken because
exactly which equality is missing is completely opaque in the type checking
-- especially when most of the uses of these modules is just plumbing (to
preserve applicative identity).

Thanks all,
Arlen

On Thu, Sep 20, 2018 at 10:29 AM Arlen Cox <arlencox@gmail.com> wrote:

> Hi everyone,
>
> I'm having some trouble getting some code that relies heavily on
> applicative functors to type check.  Does anyone know what I'm doing wrong
> with this?
>
> module type S = sig
>   module T : Set.OrderedType
>   module ST : module type of Set.Make(T)
> end
>
> module Make(T_in : Set.OrderedType) : S (* <- ERROR *)
>   with module T = T_in
>    and module ST = Set.Make(T_in)
> = struct
>   module T = T_in
>   module ST = Set.Make(T_in)
> end
>
> I get the following error message referencing the above point in the
> program.
>
> Error: In this `with' constraint, the new definition of ST
>        does not match its original definition in the constrained signature:
>        ...
>        Type declarations do not match:
>          type t = Set.Make(T_in).t
>        is not included in
>          type t = Set.Make(T).t
>        File "set.mli", line 68, characters 4-10: Expected declaration
>        File "set.mli", line 68, characters 4-10: Actual declaration
>
> It seems to me that since T = T_in, but applicative functors should make
> the type of Set.Make(T) = Set.Make(T_in).  Does this not work this way?
>
> Note that if I change the definition of S slightly, the same definition of
> Make now type checks:
>
> module type S = sig
>   module T : Set.OrderedType
>   module ST : Set.S with type elt = T.t
> end
>
> This solution is undesirable because I have a number of modules whose
> types would require an excessive number of "with module ... = ..."
> constraints to constrain in this way.  Is there a better way of getting
> this to type check?
>
> Thank you,
> Arlen
>
>
>

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