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Either Monad and Monad Transformer for Akh Javascript Monad Library

The EitherT transformer, EitherT, adds either values to a monad. The base type, Either, provides either computation logic on its own.

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

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

Usage

The EitherT and Either implement the Fantasy Land monad, functor, and applicative functor interfaces.

<a href="https://github.com/fantasyland/fantasy-land"> <img src="https://raw.github.com/fantasyland/fantasy-land/master/logo.png" align="right" width="82px" height="82px" alt="Fantasy Land logo" /> </a> 
// Either monad
require('akh.either').Either
require('akh').Either

// Either monad transformer
require('akh.either').EitherT
require('akh').EitherT

Either.run(m), m.run()

Perform a continuation computation m and get the result as an Either. Either has a value property plus either a left or right property to show its type

const Either = require('akh.either').Either

Either.run(Either.right(4)) === { right: true, value: 4 }
Either.run(Either.left(-1)) === { left: true, value: -1 }

EitherT.run(t), t.run()

Same as Either.run but for a monad transformer. Returns an Either value inside of the inner monad.

Either.either(m, leftCallback, rightCallback), m.either(l, r)

Perform a continuation computation m and invoke leftCallback for left results and rightCallback for right results.

const Either = require('akh.either').Either

const c = Either.of(3)
        .map((x) => -x);

Either.either(c, console.error, console.log); // logs: -3

EitherT.either(t, leftCallback, rightCallback), t.either(l, r)

Same as Either.either but for transformed types

Either Interface

Either.right(x)

EitherT(m).right(x)

Construct a right value.

This is the same as of since right values are considered the normal case.

Either.left(x)

EitherT(m).left(x)

Construct a left value.

Left values are considered the error case and do not have map or chain applied to them.

const c =
    Either.left(3)
        .map((x) => -x);

Either.either(c, console.error, console.log) // errors: 3

Contributing

Contributions are welcome.

To get started:

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