Re: [Caml-list] Error: The type abbreviation t is cyclic while defining submodules

[Gabriel Scherer <gabriel.scherer@gmail.com>]

I think the problem is rather that the Set.S signature has abstract
types "t" (for sets) and "elt" (for elements), and that you mean "with
type elt = t" here.

Note that you don't need any of the signature constraints here:

module OrderedPoints = struct
  type nonrec t = t
  let compare = compare
end
module PointSet = Set.Make(OrderedPoints)

works. On the other hand, (module OrderedPoints : Set.OrderedType =
...) is not good as it makes OrderedPoints.t an abstract type, and
thus you cannot use it from client code afterwards.

Finally, if you want your code to type-check under an older OCaml
version without nonrec, you can work around the lack of it with
synonyms

type u = t
module OrderedPoints = struct
  type t = u
  let compare = compare
end

On Fri, Jun 10, 2016 at 4:18 PM, Sébastien Hinderer
<Sebastien.Hinderer@inria.fr> wrote:
> Gabriel Scherer (2016/06/10 16:05 -0400):
>> P.S.: Note that "nonrec" is only in definitions (struct .. end) and
>> signatures (sig .. end), not in equations such as "with type t = t"
>> which are a different construct, interpreted recursively -- not the
>> best choice for consistency, but well.
>
> So to make an equality between two types public one has to first
> introduce a signature, right?
>
> Now the code looks like this:
>
> type t = float * float
>
> let compare = Pervasives.compare
>
> (* Other functions on points *)
>
> module OrderedPoints : Set.OrderedType with type t = t = struct
>   type nonrec t = t
>   let compare = compare
> end
>
> module PointSet (* : Set.S with type t = t *) = Set.Make(OrderedPoints)
>
> Note the commented bit in the last line, when it is uncommented it does
> indeed not work.
>
> Thanks,
>
> Sébastien.