Re: [Caml-list] Are record types generative?

[Jeremy Yallop <yallop@gmail.com>]

On 23 January 2018 at 14:54, Oleg <oleg@okmij.org> wrote:
> Although both M2 and S are two instances of m2, and hence both
> S.v and M2.v  should have the record type t = {l:int}, they are not
> regarded equal.

Yes: record definitions are indeed generative in OCaml, unlike in Standard ML.

> The problem is easy to fix:
>
> module F3(S:m2 with type t = M2.t) = struct
>   let res = S.v = M2.v
> end
> ;;
>
>   module F3 :
>   functor (S : sig type t = M2.t = { l : int; } val v : t end) ->
>     sig val res : bool end
>
> let _ = let module M = F3(M2) in M.res;;
>
> but even in this simple case the fix is ugly (and becomes uglier in
> the real code). Maybe there is a way to avoid adding too many sharing
> constraints?

One approach is to ensure that record and variant declarations do not
appear in signatures, so that type members in signatures are just
equations that refer to top-level types.  Then your example could be
written like this:

    type s = {l:int}

    module type m2 = sig
      type t = s
      val v : t
    end

    module M2 : m2 = struct
      type t = s
      let v = {l=1}
    end

    module F2(S:m2) = struct
      let res = S.v = M2.v
    end

Things become a little trickier if the type definition refers to other
elements in the signature

   module type m3 = sig
      type a
      type t = {l:a}
       ...

Losing equalities between different instances of 't' may in fact be
the desired behaviour here, since 'a' can be instantiated differently
in each implementation of the module.  But if it's important to keep
equalities then parameterising the record can help:

   type 'b s = {l:'b}

   module type m3 = sig
      type a
      type t = a s
       ...

> BTW, the new manual has a section about common polymorphism
> pitfalls. Modules also have a fair share of dark corners (along with
> the objects). Perhaps there could be a section in the manual about
> not so obvious aspects of modules (in the first approximation,
> just collecting questions and answers like the present one).

I think that'd be very useful.