List Monad and Monad Transformer for Akh Javascript Monad Library

The list transformer ListT and the base list type, List. This uses Javascript arrays which are both finite and not lazy. For working with lazy, potentially infinite lists of values see Stream-m.

# To use as standalone package
$ npm install --save akh.either

# To use as part of akh library
$ npm install --save akh

Usage

ListT and List implement the Fantasy Land a monad, monoid, functor, and applicative functor interfaces.

// List monad
require('akh.list').List
require('akh').List

// List monad tranformer
require('akh.list').ListT
require('akh').ListT

List.of(x)

Create a list consisting of a single element x.

const List = require('akh.list').List

List.run(List.of(1)) === [1]

List.run(List.of([1,2,3])) === [[1, 2, 3]]

List.zero

Empty list.

List.run(List.zero) === []
List.zero.run() === []

a.concat(b)

Concatenate list b onto list a.

List.run(
    List.zero
        .concat(List.of(1))
        .concat(List.of([2, 3]))
        .concat(List.of(4))) === [1, [2, 3], 4]

a.chain(f)

Flatmap list a with function f. f is mapped over a, returning a new List for each element of a. The resulting list is then flattened.

List.run(
    List.of(1)
        .chain(x => List.of([x, x + 1]))
        .chain(x => List.of([x, x * 2]))) === [1, 2, 2, 4]

a.map(f)

Regular mapping of list a with function f. f is maps current values to new values.

List.run(
    List.zero
        .concat(List.of(1))
        .concat(List.of(2))
        .concat(List.of(3))
        .map(x -> x * x)) === [1, 4, 9]

f.ap(m)

Map the functors in list f over list m.

List.run(
    List.of(x => x * x)
        .ap(List.zero
            .concat(List.of(1))
            .concat(List.of(2))
            .concat(List.of(3)))) === [1, 4, 9]

List.run(List
    .of(x => x * x)
    .concat(List.of(x => x + 10))
    .concat(List.of(x => x * x))
        .ap(List.zero
            .concat(List.of(1))
            .concat(List.of(2))
            .concat(List.of(3)))) === [2, 4, 6, 11, 12, 13, 1, 4, 9]

Running

List.run(m), m.run()

Run list computation m, getting the resulting list.

ListT.run(t), t.run()

Same as List.run but for transformed types.

Contributing

Contributions are welcome.

To get started:

$ cd akh-list
$ npm install # install dev packages
$ npm test # run tests