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Drumroll! You probably came to Reason for the types! (Or the friendly community, or the interesting tweets, or the hype.)

Type Annotations

Types can be inferred or explicitly written down by choice.

let score: int = 10;

You can also wrap an expression in parentheses and annotate it:

let myInt = 5;
let myInt = (5 : int);
let myInt = (5 : int) + (4 : int);
let add (x: int) (y: int) :int => x + y;
let drawCircle radius::(r: int) :unit => ...;

Note: in the last line, radius::(r: int) is a labeled argument. More on this here.

Type Aliases

You can refer to a type by a different name:

type scoreType = int;
let x: scoreType = 10;

Mutually Recursive Types

Just like functions, types can be mutually recursive through and:

type student = {taughtBy: teacher}
and teacher = {students: list student};

Note that there's no semicolon ending the first line and no type on the second line.

Type Arguments

Types can be "parameterized" (akin to generics in other languages). It's as if a type is a function that takes in arguments and returns a new type. The parameters need to start with '.

Types with parameters allow us to kill duplications. Before:

/* this is a tuple of 3 items, explained next */
type intCoordinates = (int, int, int);
type floatCoordinates = (float, float, float);

let buddy: intCoordinates = (10, 20, 20);


type coordinates 'a = ('a, 'a, 'a);

/* apply the coordinates "type function" and return the type (int, int, int) */
type intCoordinatesAlias = coordinates int;
let buddy: intCoordinatesAlias = (10, 20, 20);

/* or, more commonly, write it inline */
let buddy: coordinates float = (10.5, 20.5, 20.5);

In practice, types are inferred for you. So the more concise version of the above example would be nothing but:

let buddy = (10, 20, 20);

The type system infers that it's a (int, int, int). Nothing else needed to be written down.

Type arguments appear everywhere.

/* inferred as `list string` */
let greetings = ["hello", "world", "how are you"];

If types didn't accept parameters (aka, if we didn't have "type functions"), the standard library will need to define the types listOfString, listOfInt, listOfTuplesOfInt, etc.

Types can receive more arguments, and be composable.

type result 'a 'b =
| Ok 'a
| Error 'b;

type myPayload = {data: string};

type myPayloadResults 'errorType = list (result myPayload 'errorType);

let payloadResults: myPayloadResults string = [
  Ok {data: "hi"},
  Ok {data: "bye"},
  Error "Something wrong happened!"


Exceptions are just a special kind of variant, "thrown" in exceptional cases (don't abuse them!). When you have ordinary variants, you often don't need exceptions, since you can just use variants types such as type result above.

try (somethingThatThrows ()) {
| Not_found => print_endline "Item not found!"
| Invalid_argument message => print_endline message

You can make your own exceptions like you'd make a variant (exceptions need to be capitalized too).

exception InputClosed string;
raise (InputClosed "the stream has closed!");


Although functions are the preferred way of working within Reason, it's also possible to use objects.

An object encapsulates data that it stores within fields, and has methods that can be invoked against the data it has.

Declaring an object type

An object can have an object type to define its structure.

type tesla = {
  color: string

The extra dot at the beginning is to indicate that this is a closed object type, which means that an object based on this type must have exactly this public structure.

type car 'a = {
  color: string
} as 'a;

Two dots, also called an elision, indicate that this is an open object type, and therefore can also contain other values and methods. An open object is also polymorphic and therefore requires a parameter.

An object type is not required to create an object.

Creating an object
type tesla = {
  drive: int => int

let obj:tesla = {
  val hasEnvy = {contents: false};
  pub drive speed => {
    this#enableEnvy true;
  pri enableEnvy envy => {
    hasEnvy.contents = envy

This object is of object type tesla and has a public method drive. It also contains a private method enableEnvy that is only accesible from within the object.

The following example shows an open object type which uses a type as parameter. The object type parameter is required to implement all the methods of the open object type.

type tesla 'a = {
  drive: int => int
} as 'a;

let obj:
  tesla {. drive: int => int, doYouWant: unit => bool}
  = {
  val hasEnvy = {contents: false};
  pub drive speed => {
    this#enableEnvy true;
  pub doYouWant () => hasEnvy.contents;
  pri enableEnvy envy => {
    hasEnvy.contents = envy