Fork me on GitHub


Version 0.8.2


Monet is a library designed to bring great power to your JavaScript programming. It is a tool bag that assists Functional Programming by providing a rich set of Monads and other useful functions.

This library is inspired by those that have come before, especially the FunctionalJava and Scalaz projects.

While functional programming may be alien to you, this library is a simple way to introduce monads and pure functional programming into your daily practises.


Download the zip or tar ball.

Source code

The source is available at:


Simply download and add to your html pages or we also support bower. You can also include monet-pimp.js which contains extra functions on the Object.prototype for creating monads.

<script type="text/javascript" src="monet.js"></script>
<!-- Optionally -->
<script type="text/javascript" src="monet-pimp.js"></script>

Bower installation

Using bower:

bower install monet

or to install a specific version

bower install monet#0.8.2

A note on types

Well it’s JavaScript - there ain’t any

As you know JavaScript isn’t a strongly typed language. This kinda sucks. Types are a great help when it comes to functional programming as it makes the code more comprehensible and prevents a range of errors from being introduced.

Knowing the types of your functions and data is also important when writing documentation (such as this one), so we will invent some type annotations to make things more clear. We will only do this in the function definition and not in the concrete examples.

Generic Types

JavaScript doesn’t have generic types but it’s useful to know about them when dealing with Monads. For instance the List monad is a type that requires another type, such as a string or integer or some other type before it can be constructed. So you would have a List of Strings or a List of Integers or generically a List of As where A is a type you will supply. Now of course this is JavaScript and you can do as you please even though it doesn’t make sense. But to make things clearer (hopefully) we will attempt to do show generics or type parameters thusly:


Which means a List of As. Though of course you will have to keep track of the types yourself.


function x(a: A, b: B): C

And functions on a Monadic type that has been constructed with A

Maybe[A].fromNull(a: A): Maybe[A]
Anonymous functions

For functions that take other functions as parameters (which are called Higher order functions) we will use an abbreviated way to represent that function using a pseudo type lambda:

A -> B


function x(a: A -> B, c: B -> C): C

means that function x takes two parameters that are both functions themselves. a is a function that takes a type A and returns a type B and c is a function that takes a type B and returns a type C. The function x will return a type C.

The Unit type

Some functions (or lambdas) do not take a parameter, and some do not return anything. Will express this as:

() -> A


A -> ()

All monads

Everything that is a monad in will implement the following functions. The specific monads will be discussed in detail below.

bind alias: flatMap, chain

Monad[A].bind(f: A -> Monad[B]): Monad[B]

Performs a monadic bind.


Monad[A].map(f: A -> B): Monad[B]

unit alias: pure, of

Monad.unit(A): Monad[A]


Monad[A].ap(m: Monad[A -> B]): Monad[B]


Monad[Monad[A]].join(): Monad[A]

The inner and outer monads are the same type.


Monad[A].takeLeft(m: Monad[B]): Monad[A]

Performs a combination of both monads and takes the left one.

For example:

// result: Some(1)
// result: None
// result: None


Monad[A].takeRight(m: Monad[B]): Monad[B]

Performs a combination of both monads and takes the right one.

For example:

// result: Some(2)
// result: None
//result: None


The Maybe type is the most common way of representing nothingness (or the null type) with making the possibilities of NullPointer issues disappear.

Maybe is effectively abstract and has two concrete subtypes: Some (also Just) and None (also Nothing).

Creating an Maybe

var maybe = Maybe.Some(val);
var maybe = Maybe.None();
var maybe = Maybe.fromNull(val);  // none if val is null, some otherwise

or more simply with the pimped method on Object.

var maybe = "hello world".some()
var maybe = val.some()



Maybe[A].map(fn: A -> B) : Maybe[B]

map takes a function (A -> B) and applies that function to the value inside the Maybe and returns another Maybe.

For example:

Maybe.Some(123).map(function(val) {
	return val+1
=> 124

bind alias: flatMap, chain

Maybe[A].bind(fn: A -> Maybe[B]): Maybe[B]

bind takes a function that takes a value and returns an Maybe. The value to the function will be supplied from the Maybe you are binding on.

For example:

maybe.bind(function(val) {
	if (val == "hi") {
		return Maybe.Some("world")
	} else {
		return Maybe.None()

isSome alias: isJust

Maybe[A].isSome(): Boolean

isSome on a Some value will return true and will return false on a None.

For example:

//result: true

isNone alias: isNothing

Maybe[A].isNone(): Boolean

isNone on a None value will return true and will return false on a Some.

For example:

//result: true

some alias: just

Maybe[A].some(): A

some will ‘reduce’ the Maybe to its value. But warning! It will throw an error if you attempt to do this on a none. Use orSome instead.

For example:

//result: "hi"

orSome alias: orJust

Maybe[A].orSome(a:A) : A

Will return the containing value inside the Maybe or return the supplied value.

=> "hi"
=> "bye"


Maybe[A].orElse(Maybe[A]): Maybe[A]

Returns the Maybe if it is a Some otherwise returns the supplied Maybe.


Maybe[A].ap(Maybe[A->B]): Maybe[B]

The ap function implements the Applicative Functor pattern. It takes as a parameter another Maybe type which contains a function, and then applies that function to the value contained in the calling Maybe.

It may seem odd to want to apply a function to a monad that exists inside another monad, but this is particular useful for when you have a curried function being applied across many monads.

Here is an example for creating a string out of the result of a couple of Maybes. We use curry() which is a pimped method on Function so we can partially apply.

var person = function (forename, surname, address) {
    return forename + " " + surname + " lives in " + address

var maybeAddress = Maybe.just('Dulwich, London')
var maybeSurname = Maybe.just('Baker')
var maybeForename = Maybe.just('Tom')

var personString = maybeAddress

// result: "Tom Baker lives in Dulwich, London"

For further reading see this excellent article.


Maybe[A].toEither(fail: E): Either[E,A]

Converts a Maybe to an Either


Maybe[A].toValidation(fail: E): Validation[E,A]

Converts a Maybe to a Validation.


Maybe[A].toList: List[A]

Converts to a list, returns an Empty list on None.


Either (or the disjunct union) is a type that can either hold a value of type A or a value of type B but never at the same time. Typically it is used to represent computations that can fail with an error. Think of it as a better way to handle exceptions. We think of an Either as having two sides, the success is held on the right and the failure on the left. This is a right biased either which means that map and flatMap (bind) will operate on the right side of the either.

Creating an Either

var success = Either.Right(val);
var failure = Either.Left(val);

or with the pimped methods on object:

var success = val.right()
var failure = "some error".left()



Either[E,A].map(fn: A -> B): Either[E,B]

This will apply the supplied function over the right side of the either, if one exists, otherwise it returns the Either untouched.

For example:

Right(123).map(function (e) {return e+1})
// result: Right(124)
Left("grr").map(function (e) {return e+1})
// result: Left("grr")

flatMap alias: bind, chain

Either[E,A].flatMap(fn: A -> Either[E,B]): Either[E,B]

This will perform a monadic bind over the right side of the either, otherwise it will do nothing.


Either[E,A].ap(v: Either[E, A -> B]): Either[E,B]

This takes an either that has a function on the right side of the either and then applies it to the right side of itself. This implements the applicative functor pattern.


Either[E,A].cata(leftFn: E -> X, rightFn: A ->X): X

The catamorphism for either. If the either is right the right function will be executed with the right value and the value of the function returned. Otherwise the left function will be called with the left value.

For example:

	var result = either.cata(function(failure) {
		return "oh dear it failed because " + failure
	}, function(success) {
		return "yay! " + success


Either[A,B].bimap(leftFn: A->C, rightFn: B->D): Either[C,D]


Either[E,A].isRight(): Boolean

Returns true if this Either is right, false otherwise.


Either[E,A].isLeft(): Boolean

Returns true if this Either is left, false otherwise.

Either[E,A].right(): A

Returns the value in the right side, otherwise throws an exception.


Either[E,A].left(): E

Returns the value in the left side, otherwise throws an exception.


Either[E,A].toValidation(): Validation[E,A]

Converts the Either to a Validation.


Either[E,A].toMaybe(): Maybe[A]

Converts to a Maybe dropping the left side.


Validation is not quite a monad as it doesn’t quite follow the monad rules, even though it has the monad methods. It that can hold either a success value or a failure value (i.e. an error message or some other failure object) and has methods for accumulating errors. We will represent a Validation like this: Validation[E,A] where E represents the error type and A represents the success type.

Creating a Validation

var success = Validation.success(val);
var failure ="some error");

or with pimped methods on an object

var success = val.success();
var failure = "some error".fail();



Validation[E,A].map(fn:A -> B): Validation[E,A]

map takes a function (A -> B) and applies that function to the value inside the success side of the Validation and returns another Validation.

For example:

Validation.success(123).map(function(val) { return val + 1})
//result: Success(124)

bind alias: flatMap, chain

Validation[E,A].bind(fn:A -> Validation[E,B]) : Validation[E,B]

bind takes a function that takes a value and returns an Validation. The value to the function will be supplied from the Validation you are binding on.

For example:

validation.bind(function(val) {
	if (val == "hi") {
		return Validation.success("world")
	} else {
		return"wow, you really failed.")


Validation[E,A].isSuccess() : Boolean

Will return true if this is a successful validation, false otherwise.


Validation[E,A].isFail() : Boolean

Will return false if this is a failed validation, true otherwise.


Validation[E,A].success() : A

Will return the successful value.


Validation[E,A].fail() : E

Will return the failed value, usually an error message.


Validation[E,A].ap(v: Validation[E, A->B]) : Validation[E,B]

Implements the applicative functor pattern. ap will apply a function over the validation from within the supplied validation. If any of the validations are fails then the function will collect the errors.

var person = function (forename, surname, address) {
    return forename + " " + surname + " lives at " + address

var validateAddress = Validation.success('Dulwich, London')
var validateSurname = Validation.success('Baker')
var validateForename = Validation.success('Tom')

var personString = validateAddress.ap(validateSurname

// result: "Tom Baker lives at Dulwich, London"

var result =["no address"])
	.ap(["no surname"])
// result: Validation(["no address", "no surname"])


Validation[E,A].cata(failureFn: E->X, successFn: A->X): X

The catamorphism for validation. If the validation is success the success function will be executed with the success value and the value of the function returned. Otherwise the failure function will be called with the failure value.

For example:

var result = v.cata(function(failure) {
	return "oh dear it failed because " + failure
}, function(success) {
	return "yay! " + success


Validation[E,A].toEither(): Either[E,A]

Converts an Either to a Validation


Validation[E,A].toMaybe(): Maybe[A]

Converts to a Maybe dropping the failure side.

Immutable lists

An immutable list is a list that has a head element and a tail. A tail is another list. The empty list is represented by the Nil constructor. An immutable list is also known as a “cons” list. Whenever an element is added to the list a new list is created which is essentially a new head with a pointer to the existing list.

Creating a list

The easiest way to create a list is with the pimped method on Array, available in monet-pimp.js.

var myList = [1,2,3].list()

which is equivalent to:

var myList = List(1, List(2, List(3, Nil)))

As you can see from the second example each List object contains a head element and the tail is just another list element.



List[A].cons(a: A) : List[A]

cons will prepend the element to the front of the list and return a new list. The existing list remains unchanged.

For example:

var newList = myList.cons(4)
// newList.toArray() == [4,1,2,3]
// myList.toArray() == [1,2,3]

cons is also available as a pimped method on Object.prototype:

var myList = ["a","b","c"].list()
var newList = "z".cons(myList)
newList.toArray() == ["z","a","b","c"]


List[A].map(fn: A->B): List[B]

Maps the supplied function over the list.

var list = [1,2,3].list().map(function(a) {
	return a+1
// list == [2,3,4]

flatMap alias: bind

List[A].flatMap(fn: A -> List[B]): List[B]

Maps the supplied function over the list and then flattens the returned list. The supplied function must return a new list.

List[A].head(): A

Returns the head of the list.

For example:

//result: 1


List[A].headMaybe(): Maybe[A]

Returns the optional head of the list.

For example:

// result: Some(1)

// result: None()


List[A].foldLeft(initialValue: B)(fn: (acc:B, element:A) -> B): B

foldLeft takes an initial value and a function and will ‘reduce’ the list to a single value. The supplied function takes an accumulator as its first value and the current element in the list as its second argument. The returned value from the function will be pass into the accumulator on the subsequent pass.

For example, say you wanted to add up a list of integers, your initial value would be 0 and your function would return the sum of the accumulator and the passed in element.

var myList = [1,2,3,4].list()
var sum = myList.foldLeft(0)(function(acc, e) {
	return e+acc
// sum == 10

foldRight(initialValue)(function(e, acc))

List[A].foldRight(initialValue: B)(fn: (element: A, acc: B) -> B): B

Performs a fold right across the list. Similar to foldLeft except the supplied function is first applied to the right most side of the list.

append alias: concat()

List[A].append(list: List[A]) : List[A]

Will append the second list to the current list. Both list must be of the same type.

For example:

var list1 = [1,2,3].list()
var list2 = [4,5,6].list()
var list3 = list1.append(list2)
// list3.toArray() == [1,2,3,4,5,6]


List[Monad[A].sequence(Monad): Monad[List[A]]

Will sequence a list of monads. The signature above is slightly hard to represent, but this function will sequence a list of any type of monad, but you will need to supply the name of the monad you are sequencing.

Note: This version of sequence will only work with Monads that can cope with eager evaluations. For lazy monads such as IO and Reader please use lazySequence or the explicit versions, such as sequenceIO.

For example:

[1.right(), 2.left()].list().sequence(Either) // For Eithers
[1.some(), 2.none()].list().sequence(Maybe)

Or you can use the convenience methods like sequenceMaybe or sequenceEither below. Note that since Validation is not a true monad it will not work as expected for this method; use sequenceValidation instead.


List[Monad[A].lazySequence(Monad): Monad[List[A]]

This is the same as sequence except it caters for Monads that require laziness, such as IO and Reader.


List[Maybe[A]].sequenceMaybe(): Maybe[List[A]]

Takes a list of Maybes and turns it into a Maybe List. If the list contains at least one None value then a None will be returned, otherwise a Some will be returned with a list of all the values.

For example:

var sequenced = [Some(1), Some(2), Some(3)].list().sequenceMaybe()
// sequenced == Some([1,2,3]) <- That's an immutable list not an array

var sequenced = [Some(1), Some(2), None, Some(3), None].list().sequenceMaybe()
// sequenced == None

This is the same as calling:

[Some(1), Some(2)].sequence(Maybe)


List[Either[E,A]].sequenceEither(): Either[E, List[A]]

This will sequence a List of Eithers stopping on the first Left that it finds. It will return either a List of the Right values or the first Left value it encounters.

For example:

[1.right(), 2.right(), 3.right()].list().sequenceEither() == Right(List(1,2,3))
[1.right(), 2.left(), 3.left()].list() == Left(2)

Note: Unlike sequenceValidation it does not accumulate the Left (or “failing”) values, but rather stops execution and returns the first Left.


List[Validation[E,A]].sequenceValidation(): Validation[List[E], List[A]]

Takes a list of Validations and turns it into a Validation List. It will collect all the success values into a list on the Success side of the validation or it accumulates the errors on the Failure side, if there are any failures.

var sequenced = ["a".success(), "b".success(), "c".success()]
// sequenced == Success(["a", "b", "c"])

var sequenced = ["a".success(),
// sequenced == Fail(["c","d"])


List[IO[A]].sequenceIO(): IO[List[A]]

Will sequence a list of IO actions.


List[Reader[A]].sequenceReader(): Reader[List[A]]

Will sequence a list of Readers.


List[A].reverse(): List[A]

Returns a new list reversed.

var list = [1,2,3].list().reverse()
// list.toArray() == [3,2,1]

Non Empty Lists

Much like the immutable list, a Non Empty List can never be empty. It implements the comonad pattern. It has a guaranteed head (total) and a guaranteed (total) tail.

Creating a NonEmptyList

var nonEmptyList = NonEmptyList(1, [2,3,4].list())
// alias
var nonEmptyList = NEL(1, [2,3,4].list())
// or
var nonEmptyList = NonEmptyList(1, Nil)
// or fromList which returns a Maybe[NonEmptyList].
var maybeNonEmptyList = NonEmptyList.fromList([1,2,3,4].list())

Trying to create an empty NonEmptyList will throw an exception.



NEL[A].map(fn: A -> B): NEL[B]

Maps a function over a NonEmptyList.

bind alias: flatMap, chain

NEL[A].bind(fn: A -> NEL[B]): NEL[B]

Performs a monadic bind over the NonEmptyList.

head alias: copure, extract

NEL[A].head(): A

Returns the head of the NonEmptyList. Also known as copure or extract this is part of the comonad pattern.


NEL[A].tail(): List[A]

Returns the tail of the NonEmptyList.

tails alias: cojoin

NEL[A].tails(): NEL[NEL[A]]

Returns all the tails of the NonEmptyList. Also known as cojoin this is part of the comonad pattern. A list is considered a tail of itself.

For example:

NEL(1, [2,3,4].list()).tails()
//result: [
//          [ 1, 2, 3, 4 ],
//          [ 2, 3, 4 ],
//          [ 3, 4 ],
//          [ 4 ]
//        ]

mapTails alias: cobind, coflatMap

NEL[A].mapTails(fn: NEL[A] -> B): NEL[B]

Maps a function over the tails of the NonEmptyList. Also known as cobind this is part of the comonad pattern.

For example:

nonEmptyList.cobind(function (nel) {
            return nel.foldLeft(0)(function(a,b){
                return a+b
//result: [10,9,7,4]

append alias: concat

NEL[A].append(n: NEL[A]): NEL[A]

Appends two NonEmptyLists together.


NEL[A].reverse(): NEL[A]

Reverses the NonEmptyList.


NEL.fromList(List[A]): Maybe[NEL[A]]

Returns an optional NonEmptyList. If the supplied List is empty the result will be a None, otherwise a NonEmptyList wrapped in a Some (or Just).


The IO monad is for isolating effects to maintain referential transparency in your software. Essentially you create a description of your effects of which is performed as the last action in your programme. The IO is lazy and will not be evaluated until the perform (alias run) method is called.

Creating an IO

var ioAction = IO(function () { return $("#id").val() })


IO alias: io

IO[A](fn: () -> A): IO[A]

The constructor for the IO monad. It is a purely functional wrapper around the supplied effect and enables referential transparency in your software.

bind alias: flatMap

IO[A](fn: A -> IO[B]): IO[B]

Perform a monadic bind (flatMap) over the effect. It takes a function that returns an IO. This will happen lazily and will not evaluate the effect.

Examples: see below


IO[A](fn: A -> B): IO[B]

Performs a map over the result of the effect. This will happen lazily and will not evaluate the effect.

run alias: perform

Evaluates the effect inside the IO monad. This can only be run once in your programme and at the very end.

“Pimped” functions

Wraps a supplied function in an IO. Assumes no arguments will be supplied to the function.

function() { return $("#id") }.io()


Returns a function that will return an IO when one parameter is supplied.

function(id) { return $(id) }.io1()

or more simply



Say we have a function to read from the DOM and a function to write to the DOM. This example uses jQuery.

var read = function(id) {
	return $(id).text()

var write = function(id, value) {

On their own both functions would have a side effect because they violate referential transparency. The read function is dependent on an ever changing DOM and thus subsequent calls to it would not produce the same result. The write function obviously mutates the DOM and so it too is not referentially transparent, as each time it is called, an effect occurs.

We can modify this functions so that instead of performing these side-effects they will just return an IO with the yet-to-be-executed function inside it.

var read = IO(function (id) { return $(id).text() })

var write = function(id) {
	return IO(function(value) {

You can call write(id) until you are blue in the face but all it will do is return an IO with a function inside.

We can now call map and flatMap to chain this two effects together. Say we wanted to read from a div covert all the text to uppercase and then write back to that div.

var toUpper = function (text) { return text.toUpperCase() }
var changeToUpperIO = read("#myId").map(toUpper).flatMap(write("#myId"))

So what is the type of changeToUpperIO? Well it is the IO type. And that means at this stage, nothing has been executed yet. The DOM has not been read from, the text has not been mapped and the DOM has not been updated. What we have is a referentially transparent description of our programme.

In other pure functional languages such as Haskell we would simply return this type back to the runtime, but in JavaScript we have to manage this ourselves. So now let’s run our effect.

Now our DOM should be updated with the text converted to upper case.

It becomes much clearer which functions deal with IO and which functions simply deal with data. read and write return an IO effect but toUpper simply converts a supplied string to upper case. This pattern is what you will often find in your software, having an effect when you start (i.e. reading from a data source, network etc), performing transformations on the results of that effect and finally having an effect at the end (such as writing result to a database, disk, or DOM).


The Reader monad is a wonderful solution to inject dependencies into your functions. There are plenty of great resources to get your teeth into the Reader monad such as these great talks.

The Reader monad provides a way to “weave” your configuration throughout your programme.

Creating a Reader

Say you had this function which requires configuration:

function createPrettyName(name, printer) {
	return printer.write("hello " + name)

Calling this function from other functions that don’t need the dependency printer is kind of awkward.

function render(printer) {
	return createPrettyName("Tom", printer)

One quick win would be to curry the createPrettyName function, and make render partially apply the function and let the caller of render supply the printer.

function createPrettyName(name, printer) {
	return printer.write("hello " + name)

function render() {
	return createPrettyName("Tom")

This is better, but what if render wants to perform some sort of operation on the result of createPrettyName? It would have to apply the final parameter (i.e. the printer) before createPrettyName would execute.

This where the Reader monad comes in. We could rewrite createPrettyName thusly:

function createPrettyName(name) {
	return Reader(function(printer) {
		return printer.write("hello " + name)

To sweeten up the syntax a little we can also write:

function createPrettyName(name, printer) {
	return printer.write("hello " + name)

So now, when a name is supplied to createPrettyName the Reader monad is returned and being a monad it supports all the monadic goodness.

We can now get access to the result of createPrettyName through a map.

function reader() {
	return createPrettyName("Tom").map(function (s) { return "---"+s+"---"})

The top level of our programme would co-ordinate the injecting of the dependency by calling run on the resulting Reader.

reader().run(new BoldPrinter())



Reader[A].map(f: A -> B): Reader[B]

Maps the supplied function over the Reader.

bind alias: flatMap, chain

Reader[A].bind(f: A -> Reader[B]): Reader[B]

Performs a monadic bind over the Reader.


Reader[A].ap(a: Reader[A->B]): Reader[B]

Applies the function inside the supplied Reader to the value A in the outer Reader. Applicative Functor pattern.



Executes the function wrapped in the Reader with the supplied config.


The Free monad is a monad that is able to separate instructions from their interpreter. There are many applications for this monad, and one of them is for implementing Trampolines, (which is a way to make recursion constant stack for languages that don’t support tail call elimination, like JavaScript!).

Please see Ken Scambler’s excellent talk and example project to get an in-depth understanding of this very useful monad.

Creating a Free monad

The Free monad has two constructors, Suspend and Return, which represents the continuation of a calculation and the completion of one, respectively.

Return(a: A): Free[F[_], A]
Suspend(f: F[Free[F,A]]): Free[F[_], A]

var a = Return(1)
var sum = Suspend(Identity(Return(1))
// or
var sum = Free.liftF(Identity(1))

As you may see, Return wraps a value A, where as Suspend, wraps a Functor containing another Free.



Free[F[_], A].map(f: A -> B): Free[F[_],B]

Performs a map across the value inside the functor.

bind alias: flatMap, chain

Free[F[_],A].bind(f: A -> Free[F[_], B]): Free[F[_],B]

Performs a monadic bind over the Free.


Free.liftF(F[A]): Free[F,A]

Lifts a Funtor F into a Free.


Free[F[_],A].resume(): Either[F[Free[F,A]] , A]

Evalutates a single layer in the computation, returning either a suspension or a result.


Free[F[_],A].go(f: F[Free[F, A]] -> Free[F, A]) : A

Runs the computation to the end, returning the final result, using the supplied functor f to extract the next Free from the suspension.


Free[Function, A].run(): A

This function only makes sense for Tampolined computations where the supplied functor is a Function. This will run the computation to the end returning the result A.

Other useful functions


fn.compose(f1) alias fn.o(fn1)

Function composition. f.compose(g) is equivalent to:

function compose(x) {
	return f(g(x))


Function composition flipped. f.andThen(g) is equivalent to:

function compose(x) {
	return g(f(x))


This method on function will curry that function so that it can be partially applied. This implementation is quite flexible and allows a method to be applied in the following ways:

var sum = function(a,b,c) {
    return a+b+c

sum(1) // will return a function that takes b and c

sum(1)(2) // will return a function that takes c

// or nearly any other combination...
// will return 6


Written and maintained by Chris Myers @cwmyers. Follow Monet.js at @monetjs.