ReasonML binding function with config having fixed string values

Question

Lets say, that I have this function in Javascript which can generate string based on proper configuration:

function func(config) {
  // ...
}

also, let's assume, that the config variable has structure as below (all of these can be not given to function call):

{
  "color": string,  // can be: "blue", "red", "green"
  "number": int,    // can be: any number
  "other": string,  // can be: "x", "y"
}

How to create proper binding for this? I'm stuck with:

[@bs.deriving abstract]
type options = {
  [@bs.optional]
  color: [@bs.string] [ | `blue | `red | `green ]
  [@bs.optional]
  number: int,
  [@bs.optional]
  other: [@bs.string] [ | `x | `y ]
}

[@bs.module]
external func: options => string = "func";

But it does not work when trying to use like this:

let config = MyModule.config(
  ~color=`blue,
  ~number=123,
  ~other=`x
);

let value = MyModule.func(config);

The color and other values are integers, not strings.

Answer 1

This is a case of a JavaScript idiom for named parameters (objects with optional fields), needing to be adapted to the OCaml/ReasonML idiom (functions with actual labelled parameters). You would do this in three steps. Step 1, as Glenn showed, define an external for the config:

type config;
[@bs.obj] external config: (
  ~color:[@bs.string] [`blue | `red | `green]=?,
  ~number:int=?,
  ~other:[@bs.string] [`x | `y]=?,
 unit,
) => config = "";

Step 2, bind to the JavaScript function using the JavaScript style of the config object:

[@bs.val] external func: config => string = "";

Step 3, wrap the JavaScript function binding in an OCaml-idiomatic function with labelled parameters:

let func(~color=?, ~number=?, ~other=?, ()) = ()
  |> config(~color?, ~number?, ~other?)
  |> func;

You can use it like this:

let result = func(~color=`blue, ());