Modules are like mini files! They can contain type definitions, let bindings, nested modules, etc.


To create a module, use the module keyword. The module name must start with a capital letter. Whatever you could place in a .re file, you may place inside a module definition's {} block.

module School = { type profession = Teacher | Director; let person1 = Teacher; let getProfession = (person) => switch (person) { | Teacher => "A teacher" | Director => "A director" }; };

A module's contents (including types!) can be accessed much like a record's, using the . notation. This demonstrates modules' utility for namespacing.

let anotherPerson: School.profession = School.Teacher; print_endline(School.getProfession(anotherPerson)); /* "A teacher" */

Nested modules work too.

module MyModule = { module NestedModule = { let message = "hello"; }; }; let message = MyModule.NestedModule.message;

opening a module

Constantly referring to a value/type in a module can be tedious. Instead, we can "open" a module and refer to its contents without always prepending them with the module's name. Instead of writing:

let p: School.profession = School.getProfession(School.person1);

We can write:

open School; let p: profession = getProfession(person1);

The content of School module are made visible (not copied into the file, but simply made visible!) in scope. profession, getProfession and person1 will thus correctly be found.

Use open this sparingly, it's convenient, but makes it hard to know where some values come from. You should usually use open in a local scope:

let p = { open School; getProfession(person1); }; /* School's content isn't visible here anymore */

For an open followed by a single expression, we have a dedicated syntax sugar:

let p = School.(getProfession(person1));

Anything else needs to be written in the first way:

let p = { open School; print_endline("hello!"); getProfession(person1); };

Extending modules

Using include in a module statically "spreads" a module's content into a new one, thus often fulfill the role of "inheritance" or "mixin".

module BaseComponent = { let defaultGreeting = "Hello"; let getAudience = (~excited) => excited ? "world!" : "world"; }; module ActualComponent = { /* the content is copied over */ include BaseComponent; /* overrides BaseComponent.defaultGreeting */ let defaultGreeting = "Hey"; let render = () => defaultGreeting ++ " " ++ getAudience(~excited=true); };

Note: open and include are very different! The former brings a module's content into your current scope, so that you don't have to refer to a value by prefixing it with the module's name every time. The latter copies over the definition of a module statically, then also do an open.

Every .re file is a module

An OCaml/Reason file maps to a module; this unlocks some interesting expressivity that'd previously require code generation in other languages. The file implicitly forms a module React, which can be seen by other source files.

/* This typically compiles to module FileA below */ let a = 1; let b = 2; /* */ /* Maps FileA's implementation to a new API */ let alpha = FileA.a; let beta = FileA.b;

This contrived snippet expresses "copying" a file:

/* This typically compiles to module FileA below */ let a = 1; let b = 2; /* */ /* compiles to exactly's content with no runtime overhead! */ include FileA;

Note: Because files are modules, file names should, by convention, be capitalized so they match their module names. Uncapitalized file names are not invalid, but will be transformed into a capitalized module name. I.e. will be compiled into the module File. To simplify and minimize the disconnect here, the convention is therefore to capitalize file names too.


A module's type is called a "signature", and can be written explicitly. If a module is like a .re (implementation) file, then a module's signature is like a .rei (interface) file.


To create a signature, use the module type keyword. The signature name must start with a capital letter. Whatever you could place in a .rei file, you may place inside a signature definition's {} block.

/* Picking up previous section's example */ module type EstablishmentType = { type profession; let getProfession: profession => string; };

A signature defines the list of requirements that a module must satisfy in order for that module to match the signature. Those requirements are of the form:

  • let x: int; requires a let binding named x, of type int.

  • type t = someType; requires a type field t to be equal to someType.

  • type t; requires a type field t, but without imposing any requirements on the actual, concrete type of t. We'd use t in other entries in the signature to describe relationships, e.g. let makePair: t => (t, t) but we cannot, for example, assume that t is an int. This gives us great, enforced abstraction abilities.

To illustrate the various kinds of type entries, consider the above signature EstablishmentType which requires that a module:

  • Declare a type named profession.

  • Must include a function that takes in a value of the type profession and returns a string.


Modules of the type EstablishmentType can contain more fields than the signature declares, just like the module School in the previous section (if we choose to assign it the type EstablishmentType. Otherwise, School exposes every field). This effectively makes the person1 field an enforced implementation detail! Outsiders can't access it, since it's not present in the signature; the signature constrained what others can access.

The type EstablishmentType.profession is abstract: it doesn't have a concrete type; it's saying "I don't care what the actual type is, but it's used as input to getProfession". This is useful to fit many modules under the same interface:

module Company: EstablishmentType = { type profession = CEO | Designer | Engineer | ...; let getProfession = (person) => ...; let person1 = ...; let person2 = ...; };

It's also useful to hide the underlying type as an implementation detail others can't rely on. If you ask what the type of Company.profession is, instead of exposing the variant, it'll only tell you "it's Company.profession".

Extending module signatures

Like modules themselves, module signatures can also be extended by other module signatures using include:

module type BaseComponent = { let defaultGreeting: string; let getAudience: (~excited: bool) => string; }; module type ActualComponent = { /* the BaseComponent signature is copied over */ include BaseComponent; let render: unit => string; };

Note: BaseComponent is a module type, not an actual module itself!

If you do not have a defined module type, you can extract it from an actual module using include (module type of ActualModuleName). For example, we can extend the List module from the OCaml standard library, which does not define a module type.

module type MyList = { include (module type of List); let myListFun: list('a) => list('a); };

Every .rei file is a signature

Similar to how a file implicitly defines a module React, a file React.rei implicitly defines a signature for React. If React.rei isn't provided, the signature of defaults to exposing all the fields of the module. Because they don't contain implementation files, .rei files are used in the ecosystem to also document the public API of their corresponding modules.

/* file (implementation. Compiles to module React) */ type state = int; let render = (str) => str;
/* file React.rei (interface. Compiles to signature of module React) */ type state = int; let render: str => str;

Module Functions (functors)

Modules can be passed to functions! It would be the equivalent of passing a file as a first-class item. However, modules are at a different "layer" of the language than other common concepts, so we can't pass them to regular functions. Instead, we pass them to special functions called "functors".

The syntax for defining and using functors is very much like the syntax for defining and using regular functions. The primary differences are:

  • Functors use the module keyword instead of let.

  • Functors take modules as arguments and return a module.

  • Functors require annotating arguments.

  • Functors must start with a capital letter (just like modules/signatures).

Here's an example MakeSet functor, that takes in a module of the type Comparable and returns a new set that can contain such comparable items.

module type Comparable = { type t; let equal: (t, t) => bool; }; module MakeSet = (Item: Comparable) => { /* let's use a list as our naive backing data structure */ type backingType = list(Item.t); let empty = []; let add = (currentSet: backingType, newItem: Item.t) : backingType => /* if item exists */ if (List.exists((x) => Item.equal(x, newItem), currentSet)) { currentSet /* return the same (immutable) set (a list really) */ } else { [ newItem, ...currentSet /* prepend to the set and return it */ ] }; };

Functors can be applied using function application syntax. In this case, we're creating a set, whose items are pairs of integers.

module IntPair = { type t = (int, int); let equal = ((x1, y1), (x2, y2)) => x1 == x2 && y1 == y2; let create = (x, y) => (x, y); }; /* IntPair abides by the Comparable signature required by MakeSet */ module SetOfIntPairs = MakeSet(IntPair);

Module functions types

Like with module types, functor types also act to constrain and hide what we may assume about functors. The syntax for functor types are consistent with those for function types, but with types capitalized to represent the signatures of modules the functor accepts as arguments and return values. In the previous example, we're exposing the backing type of a set; by giving MakeSet a functor signature, we can hide the underlying data structure!

module type Comparable = ... module type MakeSetType = (Item: Comparable) => { type backingType; let empty: backingType; let add: (backingType, Item.t) => backingType; }; module MakeSet: MakeSetType = (Item: Comparable) => { ... };

Tips & Tricks

Modules and functors are at a different "layer" of language than the rest (functions, let bindings, data structures, etc.). For example, you can't easily pass them into a tuple or record. Use them judiciously, if ever! Lots of times, just a record or a function is enough.