Newcomer Examples

An example is worth a thousand words.

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

Use the option type

let possiblyNullValue1 = None
let possiblyNullValue2 = Some("Hello")

switch possiblyNullValue2 {
| None => Console.log("Nothing to see here.")
| Some(message) => Console.log(message)
}

Create a Parametrized Type

type universityStudent = {gpa: float}

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

Creating a JS Object

let student1 = {
  "name": "John",
  "age": 30,
}

Or using record:

type payload = {
  name: string,
  age: int,
}

let student1 = {
  name: "John",
  age: 30,
}

Modeling a JS Module with Default Export

See here.

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 an option. The conversion is done through the helper functions in ReScript's Js.Nullable module. In this case, toOption:

let greetByName = (possiblyNullName) => {
  let optionName = Js.Nullable.toOption(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.