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How do I declare a map type in Reason ML?

One advantage of Reason ML over JavaScript is that it provides a Map type that uses structural equality rather than reference equality.

However, I cannot find usage examples of this.

For example, how would I declare a type scores that is a map of strings to integers? 

/* Something like this */
type scores = Map<string, int>;

And how would I construct an instance?

/* Something like this */
let myMap = scores();

let myMap2 = myMap.set('x', 100);
Solution

  • The standard library Map is actually quite unique in the programming language world in that it is a module functor which you must use to construct a map module for your specific key type (and the API reference documentation is therefore found under Map.Make):

    module StringMap = Map.Make({
  type t = string;
  let compare = compare
});

type scores = StringMap.t(int);

let myMap = StringMap.empty;
let myMap2 = StringMap.add("x", 100, myMap);

    There are other data structures you can use to construct map-like functionality, particularly if you need a string key specifically. There's a comparison of different methods in the BuckleScript Cookbook. All except Js.Dict are available outside BuckleScript. BuckleScript also ships with a new Map data structure in its beta standard library which I haven't tried yet.