Module TableclothComparator

Comparator provide a way for custom data structures to be used with Maps and Sets

Say we have a module Book which we want to be able to create a Set of

module Book = struct
  type t = {
    isbn: string;
    title: string;
  }

  let compare bookA bookB =
    String.compare bookA.isbn bookb.isbn
end

First we need to make our module conform to the S signature.

This can be done by using the Make functor.

module Book = struct
  type t = {
    isbn: string;
    title: string;
  }

  let compare bookA bookB =
    String.compare bookA.isbn bookb.isbn
  
  include Comparator.Make(struct 
    type nonrec t = t

    let compare = compare
  end)
end

Now we can create a Set of books

Set.fromList (module Book) [
  { isbn="9788460767923"; title="Moby Dick or The Whale" }
]

module type T

module type T = sig ... end

T represents the input for the Make functor

type t

type ('a, 'identity) t

type comparator

type ('a, 'identity) comparator = ( 'a, 'identity ) t

This just is an alias for t

module type S

module type S = sig ... end

The output type of Make.

type s

type ('a, 'identity) s =   (module S   with type identity = 'identity    and type t = 'a)

A type alias that is useful typing functions which accept first class modules like Map.empty or Set.fromArray

module Make

module Make (M : T) : S with type t := M.t

Create a new comparator by providing a module which satisifies T.