Guide

An example is worth a thousand words.

This section is dedicated to newcomers trying to figure out general idioms & conventions in Reason and BuckleScript. If you're a beginner who's got a good idea for an example, please suggest an edit!

Using the option type

option is a variant that comes with the standard library. It obviates the need for null values in other languages.

let possiblyNullValue1 = None;
let possiblyNullValue2 : option string = Some "Hello@";

switch possiblyNullValue2 {
| None => print_endline "Nothing to see here."
| Some message => print_endline message
};

Creating a parametrized type

type universityStudent = {gpa: float};

type response 'studentType = {status: int, student: 'studentType};

let result: response universityStudent = fetchDataFromServer ();

Creating a JS Object

Assuming you're compiling to JS, of course.

let obj1 = {
  "name": "John",
  "age": 30
};
/* Compiles to a JS object that looks exactly like what you're seeing */

Note that the above isn't a record; the keys are quoted in string. That's Reason syntax sugar for bs.obj. The type is inferred. Next example explicitly types it.

Typing a JS Object

type payload = Js.t {.
  name: string,
  age: int
};
let obj1: payload = {"name": "John", "age": 30};

Note that {. name: string, age: int} is the syntax for a Reason/OCaml object type declaration (not a record!). It's lifted into Js.t so that BuckleScript sees the whole type and compiles it correctly to a regular JavaScript object. Ordinary, non-lifted OCaml objects are compiled into something else (rarely needed currently).

Binding to a JS Module with Default Export

Assuming the module's called store.js, and has a default export, plus a method called getDate.

type store = Js.t {. getDate : (unit => float) [@bs.meth]};
external store : store = "./store" [@@bs.module];
Js.log store;
Js.log (store##getDate ());

Checking for JS nullable types using the option type

For a function whose argument is passed a JavaScript value that's potentially null or undefined, it's idiomatic to convert it to a Reason option. The conversion is done through the helper functions in Bucklescript's Js.Nullable module. In this case, to_opt:

let greetByName possiblyNullName => {
  let optionName = Js.Nullable.to_opt possiblyNullName;
  switch optionName {
  | None => "Hi"
  | Some name => "Hello " ^ name
  }
};

This check compiles to possiblyNullName == null in JS, so checks for the presence of null or undefined.