How can I declare a module (actually a Set.Make) in mli file?

Question

I have airport.mli and airport.ml.

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In airport.ml, I have

module AirportSet = Set.Make(struct type t = airport let compare = compare end);;

This is no problem.

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I then have a function

val get_all_airport : unit -> AirportSet.t;;

, which generates a AirportSet.

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so in airport.mli, I need to show the module AirportSet so AirportSet is recognized.

How can I do that?

Answer 1

module AirportSet : (Set.S with type elt = airport)

(The parens are actually unnecessary, putting them there so that you know this is a signature expected, in the general case of the form sig ... end).