Why is my Map type not exported by Reason?

Question

Following this question, I have created a file (and hence module) that defines a concrete Map type:

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

type scores = StringMap.t(int);

Now, I want to use the type in another file:

/* Demo.re */
let currentScores = Scores.scores.empty;

Js.log(currentScores);

However, this gives me the error:

The value scores can't be found in Scores

If I add a constant (e.g. let n = 123; and Js.log(Scores.n);) then it works.

What am I missing here?

Turns out `empty` lives on `StringMap`, not `scores`. I find this surprising though. Surely to make a `Map` instance the key-type must be known? — sdgfsdh, Feb 16 '18 at 22:15

score 2 · Accepted Answer · answered Feb 26 '18 at 16:57

scores is a type, and types, even record types, do not have fields on the type itself. Furthermore, types and values live in different namespaces, so while the scores type exists, the scores value does not, hence the error "The value scores can't be found in Scores".

Modules, on the other hand, can have "fields", so that's why it lives there. And you could of course also alias empty the same way you've aliased the Scores.t type:

type scores = StringMap.t(int);
let empty = StringMap.empty;

Finally, you ask "Surely to make a Map instance the key-type must be known?". Indeed it does, and you've made it known. You specified the key type when you made the StringMap module (Map.Make({ type t = string; ...0);). You do not need to specify the value type (int), however. That will be inferred.

Why is my Map type not exported by Reason?

1 Answers1