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Types are the highlight of ReScript! They are:

  • Strong. A type can't change into another type. In JavaScript, your variable's type might change when the code runs (aka at runtime). E.g. a number variable might change into a string sometime. This is an anti-feature; it makes the code much harder to understand when reading or debugging.

  • Static. ReScript types are erased after compilation and don't exist at runtime. Never worry about your types dragging down performance. You don't need type info during runtime; we report all the information (especially all the type errors) during compile time. Catch the bugs earlier!

  • Sound. This is our biggest differentiator versus many other typed languages that compile to JavaScript. Our type system is guaranteed to never be wrong. Most type systems make a guess at the type of a value and show you a type in your editor that's sometime incorrect. We don't do that. We believe that a type system that is sometime incorrect can end up being dangerous due to expectation mismatches.

  • Fast. Many developers underestimate how much of their project's build time goes into type checking. Our type checker is one of the fastest around.

  • Inferred. You don't have to write down the types! ReScript can deduce them from their values. Yes, it might seem magical that we can deduce all of your program's types, without incorrectness, without your manual annotation, and do so quickly. Welcome to ReScript =).

The following sections explore more of our type system.


This let-binding doesn't contain any written type:

Reason (Old Syntax)ML (Older Syntax)JS Output
let score = 10;
let add = (a, b) => a + b;

ReScript knows that score is an int, judging by the value 10. This is called inference. Likewise, it also knows that the add function takes 2 ints and returns an int, judging from the + operator, which works on ints.

Type Annotation

But you can also optionally write down the type, aka annotate your value:

Reason (Old Syntax)ML (Older Syntax)JS Output
let score: int = 10;

If the type annotation for score doesn't correspond to our inferred type for it, we'll show you an error during compilation time. We won't silently assume your type annotation is correct, unlike many other languages.

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

Reason (Old Syntax)ML (Older Syntax)JS Output
let myInt = 5;
let myInt: int = 5;
let myInt = (5: int) + (4: int);
let add = (x: int, y: int) : int => x + y;
let drawCircle = (~radius as r: int): circleType => /* code here */;

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

Type Alias

You can refer to a type by a different name. They'll be equivalent:

Reason (Old Syntax)ML (Older Syntax)JS Output
type scoreType = int;
let x: scoreType = 10;

Type Parameter (Aka Generic)

Types can accept parameters, akin to generics in other languages. The parameters' name need to start with '.

The use-case of a parameterized type is to kill duplications. Before:

Reason (Old Syntax)ML (Older Syntax)JS Output
// this is a tuple of 3 items, explained next
type intCoordinates = (int, int, int);
type floatCoordinates = (float, float, float);

let a: intCoordinates = (10, 20, 20);
let b: floatCoordinates = (10.5, 20.5, 20.5);


Reason (Old Syntax)ML (Older Syntax)JS Output
type coordinates('a) = ('a, 'a, 'a);

let a: coordinates(int) = (10, 20, 20);
let b: coordinates(float) = (10.5, 20.5, 20.5);

Note that the above codes are just contrived examples for illustration purposes. Since the types are inferred, you could have just written:

Reason (Old Syntax)ML (Older Syntax)JS Output
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 in many places. Our array('a) type is such a type that requires a type parameter

Reason (Old Syntax)ML (Older Syntax)JS Output
// inferred as `array(string)`
let greetings = [|"hello", "world", "how are you"|];

If types didn't accept parameters, the standard library would need to define the types arrayOfString, arrayOfInt, arrayOfTuplesOfInt, etc. That'd be tedious.

Types can receive many arguments, and be composable.

Reason (Old Syntax)ML (Older Syntax)JS Output
type result('a, 'b) =
  | Ok('a)
  | Error('b);

type myPayload = {data: string};

type myPayloadResults('errorType) = array(result(myPayload, 'errorType));

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

Recursive Types

Just like a functions, a type can reference itself within itself:

Reason (Old Syntax)ML (Older Syntax)JS Output
type person = {
  name: string,
  friends: array(person)

To opt out of resursive type, use type nonrec person = ....

Mutually Recursive Types

Types can also be mutually recursive through and:

Reason (Old Syntax)ML (Older Syntax)JS Output
type student = {taughtBy: teacher}
and teacher = {students: array(student)};

Type Escape Hatch

ReScript's type system is robust and does not allow dangerous, unsafe stuff like implicit type casting, randomly guessing a value's type, etc. However, out of pragmatism, we expose a single escape hatch for you to "lie" to the type system:

Reason (Old Syntax)ML (Older Syntax)JS Output
external myShadyConversion: myType1 => myType2 = "%identity";

This declaration converts a myType1 of your choice to myType2 of your choice. You can use it like so:

Reason (Old Syntax)ML (Older Syntax)JS Output
external convertToFloat : int => float = "%identity";
let age = 10;
let gpa = 2.1 +. convertToFloat(age);

Obviously, do not abuse this feature. Use it tastefully when you're working with existing, overly dynamic JS code, for example.

More on externals here.

Note: this particular external is the only one that isn't preceded by a @bs annotation.