bs.string.md

[@bs.string]

You can use bs.string together with polymorphic variant to constrain function's parameters.

For example:

[@bs.module "fs"]
external readFileSync: (
  ~name: string,
  [@bs.string] [`utf8 | `ascii]
) => string = "";

readFileSync(~name="data.txt", `utf8);

compiles to:

var Fs = require("fs");

Fs.readFileSync("data.txt", "utf8");

bs.string compiled `utf8 to the string "utf8".

Now, if we try pass an unknown encoding:

readFileSync(~name="data.txt", `binary);

we'll get a compile error.

When your string causes a syntax error in the polymorphic variant (this can happen, for example, when the string starts with a number or contains a special character), use bs.as:

[@bs.module "fs"]
external readFileSync: (
  ~name: string,
  [@bs.string] [`utf8 | `ascii | [@bs.as "ucs-2"] `ucs_2]
) => string = "";

readFileSync(~name="data.txt", `ucs_2);

This compiles to:

var Fs = require("fs");

Fs.readFileSync("data.txt", "ucs-2");