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either.js remove destructuring Apr 19, 2016
io.js remove destructuring Apr 19, 2016

Collection of Free monads


npm install freeky


Video series on Free Monads here:

Walking through our example.js

Run with node --harmony_destructuring example.js

// either Example
const gtZero = x =>
  (x > 0) ? Right(x) : Left("it was less than zero")

// cont example (cont just wraps task. prob a misnomer)
const asyncGet = n =>
  Cont((rej, res) => setTimeout(() => res(n), 100))

// do syntax works for any 1 monad. Since it's all in Free, we can use multiple
const app = *() {
  const ioNumber = yield IO(() => 1)
  const maybeNumber = yield Just(2)
  const contNumber = yield asyncGet(3)
  const eitherNumber = yield gtZero(4)
  return Monad.of(ioNumber + maybeNumber + eitherNumber + contNumber)

// some nt's to Task
const maybeToTask = m => m.fold(Task.of, Task.rejected)
const contToTask = c => c.t
const eitherToTask = m => m.fold(Task.rejected, Task.of)
const ioToTask = i => new Task((rej, res) => res(i.f()))

// this tells our Free.foldMap how to dispatch. We need all of them to turn into a target monad (in this case Task)
const runApp = dispatch([ [IOType, ioToTask],
                          [ContType, contToTask],
                          [Either, eitherToTask],
                          [Maybe, maybeToTask]

app.foldMap(runApp, Task.of).fork(console.error, console.log)

Custom Types

You can define your own types in Free and use them as monads. This is useful if you want to define a composable dsl or multiple interpreters for an action:

const Http = daggy.taggedSum({ Get: ['url'], Post: ['url', 'data'] })
const {Get, Post} = Http

const get = url => liftF(Get(url))
const post = (url, data) => liftF(Post(url, data))

var myFn = get('/home').chain(html => post('save', {markup: html})

const httpToTask = m =>
    Get: url => new Task((rej, res) => $.getJSON(url).then(res).error(rej),
    Post: (url, data) => new Task((rej, res) => $.post(url, data).then(res).error(rej))

myFn.foldMap(httpToTask, Task.of)

or fake it for testing:

const httpToId = m =>
    Get: url => Id(`getting ${url}`),
    Post: (url, data) => Id(`posting ${url} with ${data}`)

myFn.foldMap(httpToId, Id.of)

use it with other monads

const app = *() {
  const page = yield get('/myUrl')
  const maybeHeader = yield Just($('#header', page))
  return maybeHeader.chain(h => IO(() => console.log(h)))

const runApp = dispatch([ [IOType, ioToTask],
                          [Http, httpToTask],
                          [Maybe, maybeToTask]

runApp.foldMap(httpToTask, Task.of)
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