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Implementation of Maybe and Result monads compatible with async/await. Learn more about monads here.

Maybe is a container dedicated for the handling of a data which might be missing. Typically, it is used for representation of optional values. It allows prevent usage of Nullable objects. Result is an expansion of Maybe which can additionally carry the reason of unavailability. It is mainly utilized to represent the output of an operation which might fail since Result is also capable of containing an error message.

Both of them implements a fraction of Promise's functionality. It allows us to model them via Promise. Therefore by the implementation of PromiseLike interface, they became compatible with async/await syntax.

Get started

The package is available via npm. It has to be installed as a local dependency:

npm install amonad

Each of them can be represented by values of two types: Just and None for Maybe, Success and Failure for Result. The values have dedicated factory functions with correspondent names. They can also be created by a smart factories which are just Maybe and Result. They correctly create the monads based on the provided argument.

A primary way to access inclosed values is then() method. It expects two optional arguments which are represented by functions which perform operations over the internal state of the containers. The first one processes the data, and the second is used for handling of its absents.

Also, there is an API for checking of an object type. It consist of isMaybe, isJust, isNone, isResult, isSuccess and isFailure functions which accept any object as an argument and returns result of the assertion as boolean value. Moreover, there are isJust and isNone methods for Maybe. Correspondingly, there are isSuccess and isFailure methods for Result.

The carried information can be accessed via get and getOrElse methods. The first one returns the value inclosed inside of the container. The second one returns only the primary value from the monad and falls back to the provided argument if the primary value is not available.


Typical usage of Maybe and Result is very similar. Sometimes it is hardly possible to make a choice, but there is a clear semantic difference in the intention behind each of them.

Maybe, primary, should represent values which might not be available by design. The most obvious example is return type of Dictionary:

interface Dictionary<K, V> {
    set(key: K, value: V): void
    get(key: K): Maybe<V>

It can also be used as a representation of optional value. The following example shows the way to model a User interface with Maybe. Some nationalities have a second name as an essential part of their identity other not. Therefore the value can nicely be treated as Maybe.

interface Client {
    name: string
    secondName: Maybe<string>
    lastName: string

Computations which might fail due to obvious reason are also a good application for Maybe. Lowest common denominator might be unavailable. That is why the signature makes perfect sense for getLCD() function:

getLCD(num1number, num2number)Maybe<number>

Result is mainly used for the representation of value which might unavailable for an uncertain reason or for tagging of a data which absents can significantly affect execution flow. For example, some piece of class’s state, required for computation, might be configured via an input provided during life-circle of the object. In this case, the default status of the property can be represented by Failure which would clarify, that computation is not possible until the state is not initialized. Following example demonstrates the described scenario. The method will return the result of the calculation as Success or “Data is not initialized” error message as Failure.

class ResultExample {
  value: Result<Value, string> = Failure(Data is not initialized)
  init( value: Value ) {
    this.value = Success(value) 
    return this.value.then( value =>
        someValueBasedComputation( value, otherArgs)

Moreover, monadic error handling is able to replace exceptions as the primary solution for error propagation. Following example presents a possible type signature for a parsing function which utilizes Result as a return type.

parseData( strstring )Result<Data>

The output of such a function might contain processed value as Success or Throwable with an explanation of an error as Failure.


As I already said it is possible to instantiate Maybe and Result via factory functions. Different ways to create the monads is presented in the following code snippet.

const just = Just(3.14159265)
const none = None<number>()
const success = Success<string, Error>("Iron Man")
const failure: Failure<string, Error> = Failure( new Error("Does not exist."))

NaN safe division function can be created using this library in the way demonstrated below.

const divide = (numerator: number, quotient: number ): Result<number, string> => 
    quotient !== 0 ?
        Success( numerator/quotient )
        Failure("It is not possible to divide by 0")

Smart Maybe factory is handy if there is Nullable object which has to be wrapped by Maybe.

const maybe = Maybe(map.get())

Smart Result factory expects a function () => T | Success<T, E> | Failure<T, E> which might throw object of E type as exception. Result wraps around the output accordingly.

const data = Result( () => JSON.parse(inputStr) )

Data handling

The first argument of then() method is handler responsible for processing of expected value. It accepts two kinds of output values: value of arbitrary, monad of the same type.

// converts number value to string
const eNumberStr: Maybe<string> = Just(2.7182818284)
        eNumber => `E number is: ${eNumber}` 
// checks if string is valid and turns the monad to None if not
const validValue = Just<string>(inputStr)
    .then( str => 
        isValid(inputStr) ?

The content can also be access by get(), getOrElse() and getOrThrow() methods. get() output a union type of the value type and the error one for Result and the union type of the value and undefined for Maybe. The issue can be resolved by validation of the monad type by isJust() and isSuccess() methods or functions.

if(maybe.isJust()) { // it is also possible to write it via isJust(maybe)
    const value = maybe.get(); // return the value here
    // Some other actions...
} else {
    // it does not make sense to call get() here since the output is going to be undefined
    // Some other actions...
if(result.isSuccess()) { // it is also possible to write it via isSuccess(result)
    const error = result.get(); // return the value here
    // Some other actions...
} else {
    const value = result.get(); // return the error here
    // Some other actions...

The main advantage of the library against other existing implementations of Maybe/Option and Result/Try monads for JavaScript is compatibility with async/await syntax. It is possible to await Result and Maybe inside of async functions. Anyway, the output is going to be wrapped by Promise.

const someStrangeMeaninglessComputations = async (num1: number, num2: number, num3: number ): Promise<number> => {
    const lcd = await getLCD(num1, num2) // will throw undefined in case LCD does not exist
    return await divide(lcd, num3)

Error handling

The second argument of the then() method is a callback responsible for the handling of unexpected behavior. It works a bit differently for Result and Maybe. None has no value, that's why its callback doesn't have an argument. Additionally, it doesn't accept mapping to the value, since it should produce another None which also cannot contain any data. But returning of Just might be utilized to recovery Maybe. It is also possible to pass a void procedure to perform some side effect, for example, logging. Failure oriented handler works a bit more similar to the first one. It accepts two kinds of output values: the value of Throwable and monad of the Result type.

// tries to divide number e by n, recoveries to Infinity if division is not possible
const eDividedByN: Failure<string, string> = divide(2.7182818284, n)
        eNumber => `E number divided by n is: ${eNumber}`,
        error => Success(Infinity)
// looks up color from a dictionary by key, if color is not available falls back to black
const valueFrom = colorDictionary.get(key)
        () => Just("#000000")

It is also possible to verify if the monads are Failure or None, it can be done via isNone() and isFailure() methods and functions.

if(maybe.isNone()) { // it is also possible to write it via isNone(maybe)
    // it does not make sense to call get() here since the output is going to be undefined
    // Some other actions...
} else {
    const value = maybe.get(); // return the value here
    // Some other actions...
if(result.isFailure()) { // it is also possible to write it via isFailure(result)
    const value = result.get(); // return the error here
    // Some other actions...
} else {
    const error = result.get(); // return the value here
    // Some other actions...

Awaiting of None and Failure led to throwing of an exception inside of async function. None doesn't have inclosed value. Therefore undefined is thrown. Failure conveniently store Throwable object which fulfils its purpose in such an occasion. Beyond async function the error is propagated as rejected Promise.

const someStrangeMeaninglessComputations = async (num1: number, num2: number, num3: number ): Promise<number> => {
    try {
        const lcd = await getLCD(num1, num2) // will throw undefined in case LCD is not available for the values
        return await divide(lcd, num3) // throws "It is not possible to divide by 0" in case num3 is 0
    } catch(e) {
        // it is possible to recovery normal workflow here
        return someFallBackValue


The interfaces contains an up to a certain degree shared API as well as specific ones for Maybe and Result.



Accordingly, applying the handlers produces a new Monadic as a container for the output of called function

Signature for Maybe is:

Maybe<T>.prototype.then<TResult1 = T, TResult2 = never>(
    onJust?: (value: T) => TResult1 | Maybe<TResult1>,
    onNone?: () => Maybe<TResult2>
)Maybe<TResult1 | TResult2> 

Signature for Result is:

Result<T, E>.prototype.then<TResult1 = T, EResult1 extends Throwable = E, TResult2 = never, EResult2 extends Throwable = never >(
    onSuccess?: (value: T) => TResult1 | IResult<TResult1, EResult1>,
    onFailure?: (reason: E) => EResult2 | IResult<TResult2, EResult2>
)Result<TResult1 | TResult2, EResult1 | EResult2>


It returns the value enclosed inside a container.

Signature for Maybe is:

Maybe<T>.prototype.get()T | undefined

Signature for Result is:

Result<T, E>.prototype.get()T | E


It returns the inclosed primary value or the one provided as an argument.

Signature for Maybe is:

Maybe<T>.prototype.getOrElse( valueT )T 

Signature for Result is:

Result<T, E>.prototype.getOrElse( valueT )T  


It returns the value for Just and Success and throws the throwable for Failure, None throws an undefined.

Monad<T, E>.prototype.getOrThrow( valueT )T 

It might be useful for refactoring of a legacy codebase, since it simplifies implementation of interfaces with exceptions based error handling.


Maybe itself represents a union type of Just and None.

It is also a smart factory which turns Nullable object to Just or None accordingly.

Maybe<T>(valueT | undefined | null) => Maybe<T>


It represents the value of a specified type. It can be created via a factory which wraps the value with Just

Just<T>(valueT) => Maybe<T> 


It represents an absents of value with the specified type. It can be created via a factory with the specified type.

None<T>() => Maybe<T> 


It exists as a stand-alone function which checks whether an object of any type is Just

isJust<T>(objany)obj is Just<T>

Moreover Maybe has a method dedicated to the same goal.

Maybe<T>.prototype.isJust()obj is Just<T>


It exists as a stand-alone function which checks whether an object of any type is None

isNone<T>(objany)obj is None<T>

Moreover Maybe has a method dedicated to the same goal.

Maybe<T>.prototype.isNone()obj is Just<T>


Result<T, E> itself represents a union type of Success<T, E> and Failure<T, E>.

It is also a smart factory which calls provided function and stores its output as Success<T, E> or Failure<T, E> accordingly.

Maybe<T, E extends Throwable>(action: () => T | Result<T, E>) => Result<T, E>


It represents the value of a specified type. It can be created via a factory which wraps the value with Success<T, E>

Success<T, E extends Throwable>(valueT) => Result<T, E>


Represents an error which explains an absents of a value. It can be created via a factory with the specified type.

Failure<T, E extends Throwable>(errorE) => Result<T, E>


It exists as a stand-alone function which checks whether an object of any type is Success

isSuccess<T>(objany)obj is Success<T>

Moreover, Result has a method dedicated to the same goal.

Result<T, E>.prototype.isSuccess()obj is Success<T, E>


It exists as a stand-alone function which checks whether an object of any type is Failure

isFailure<T, E>(objany)obj is Failure<T, E>

Moreover, Result has a method dedicated to the same goal.

Result<T, E>.prototype.isFailure()obj is Failure<T>

Contribution guidelines

The project is based on npm eco-system. Therefore, development process is organized via npm scripts.

For installation of dependencies run

npm install

To build application once

npm run build

To build an application and watch for changes of files

npm run build:w

To run tslint one time for CI

npm run lint

To unit tests in a watching mode are performed by

npm run test

To execute a test suit single time

npm run test:once

To execute a test suit single time with coverage report

npm run test:c

To execute a test suit single time with coverage report submitted to coveralls

npm run test:ci

Everybody is welcome to contribute and submit pull requests. Please communicate your ideas and suggestions via issues.

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