MM

A few Missing Monads for Javascript and Typescript.

• Option (aka Maybe): represents optional values and avoids ugly tests for null, undefined or NaN
• Seq: an ordered sequence of values with numerous methods to manipulate it
• Range: numeric values in range [start;end) with non-zero step value step
• Try: a computation that may either result in an error, or return a successfully computed value and that may be easily chained with other computations

A small library with no external dependencies that implements a few useful Monads for functional programming.

• no external dependencies
• works with ES5
• based on iterators with lazy evaluation
• largely inspired from Scala
• typings automatically loaded when programming in Typescript
• excellent tests coverage

Write shorter immutable code

Instead of

let value;
if (typeof input === 'undefined' || input === null) {
    value = getDefaultValue()
}
else {
   value = calculateFromInput(input)
}

write

const value = option(input).map(calculateFromInput).getOrElse(getDefaultValue) 

Write lazy evaluated code

const mySeq = seq(hugeArray).drop(2).map(expensiveOp).takeFirst(3)
mySeq.toArray
  

mySeq is not evaluated (iterated) until toArray is called on it, and when it is, only five iterations on the Seq and 3 calls to map are performed

Usage

Typescript / ES6: import { some, seq, .. } from 'm.m'

ES5 var MM = require('m.m')

License

MIT

TODO, Contributing, etc..

Most Wanted: Either,...

You are most welcome to contribute by opening new Pull Requests. For new Monads, please get inspiration from the Scala definitions and extend Collection for the implementation

API

Option

Represents optional values.

/**
 * Creates a None, i.e. an empty Option
 */
function none(): Option<A>

/**
 * Create a Some i.e. an Option holding a value
 */
function some<A>( value?: A ): Option<A>

/**
 * Create a None if value is undefined or null or NaN
 * otherwise create a Some holding that value
 */
function option<A>(value?: A): Option<A>

The most idiomatic way to use an Option is to treat it as a collection or monad and use map, flatMap, filter, or foreach:

const name = option( request.getParameter('name') )
const upper = name.map( n => n.trim() ).filter( n => n.length !== 0 ).map( n => n.toUpperCase() )
console.log(upper.getOrElse( () =>  "" ) )

Seq

A Seq is an ordered sequence of values

/**
 * Create a Seq
 */
function seq<A>( ...values: any[]  ): Seq<A>

A Seq can be created from

• a list of disctrete values e.g. seq( 1, 2, 3 )
• any Iterable including an Array, a String, another Seq, an ES6 Map, etc... e.g. seq( [1 ,2, 3] ), seq('abcd')

Examples:

seq('dlroW olleH').reverse.mkString(' ')    // 'H e l l o   W o r l d'

seq(1, 2, 'a', 3, 'b', 4).collect( n => !isNaN(n) )( n => n*2 ).mkString('[', ',', ']')    // '[2,4,6,8]'

seq( seq( [1, 2] ), 3, seq( 4, 5 ) ).flatten().toArray // [ 1, 2, 3, 4, 5 ]

Range

The Range class represents numeric values in range with non-zero step value step. A Range is a special case of Seq.

/**
 * Create a range of integers of the specified length starting at 0
 */
function range( length: number ): Range;

/**
 * Create a range from the specified start to the element (end - 1) included in steps of 1
 */
function range( start: number, end: number ): Range

/**
 * Create a range from start to (end - step) included
 */
function range( start: number, end: number, step: number ): Range

Ranges are not indexed but based on (lazy) iterators; it is therefore possible to manipulate Infinity in Ranges:

range(0, Infinity).take(10).toArray   //[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

Try

A Try represents a computation that may either result in an error, or return a successfully computed value.

A computation can be created by passing function without arguments to tri().

/**
 * Wraps this computation in a Try
 */
function tri<A>( computation: ( ) => A ): Try<A>

Please note that the function is spelled with an 'i' to avoid a conflict with the reserved keyword try

As with other monads of this library, Try is lazy and the computation will only be performed when attempting to "extract" a result.

What is interesting with Try is the ability to chain error throwing functions with a fail fast behavior, and potentially recover from these error(s)

function divide( numerator: any, denominator: any ): Try<number> {

    const parseNumerator = () => option( parseFloat( numerator ) ).orThrow( () => "Invalid numerator" );
    const parseDenominator = () => option( parseFloat( denominator ) ).orThrow( () => "Invalid denominator" );

    //chain error throwing functions
    return tri( parseNumerator ).flatMap( num => tri( parseDenominator ).map( den => num / den ) )
}

//now, divide(), can itself be chained
divide(num, den).map(x => x*2)

//or can be "recovered" from

divide( num, den )
    .recover( ( e: Error ) => {
                console.log('Divide failed. '+e.message+'. Returning Infinity')
                return Infinity
            } )
   .get

//divide('blah',3) -> Infinity