package reason-standard

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in-package search v0.2.0

reason-standard
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    Make
    
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    S
    
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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;
  }
end

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

This can be done by using the make function or the Make functor.

module Book = struct
  type t = {
    isbn: string;
    title: string;
  }
  
  module ByIsbn = (
    val Comparator.make ~compare:(fun bookA bookB ->
      String.compare bookA.isbn bookb.isbn
    )
  )
end

Then we can create a Set

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

module type T = sig ... end

T represents the input for the Make functor

type ('a, 'identity) t

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

This just is an alias for t

module type S = sig ... end

The output type of Make and make.

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.ofArray

val make : compare:('a -> 'a -> int) -> (module S with type t = 'a)

Create a new comparator by providing a compare function.

Examples

module Book = struct
  type t = {
    isbn: string;
    title: string;
  }
  
  module ByTitle = (
    val Comparator.make ~compare:(fun bookA bookB ->
      String.compare bookA.title bookb.title)
  )
end

let books = Set.empty (module Book.ByTitle)

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

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