Database failover modelled with the `alt` typeclass.

db-alt.js

const Task = require('data.task')

Task.prototype.alt = function (that) {
  return new Task((rej, res) =>
    this.fork(_ => that.fork(rej, res), res))
}

const hosts = [
  [ 'db1.mysite.com', 'user', 'password' ],
  [ 'db2.mysite.com', 'user', 'password' ],
  [ 'db3.mysite.com', 'user', 'password' ]
]

const connect = config => new Task((rej, res) =>
  doSomeConnection(config).then(res).else(rej))

// The first available DB or an error!
// db :: Task String DB
const db = hosts.reduceRight(
  (acc, host) => connect(host).alt(acc),
  new Task((rej, res) => rej('No DBs available!')))

Interestingly, you can wrap any Alternative in Alt to make it a monoid:

const Alt = x =>
({
  x,
  concat: o => Alt(x.alt(o.x))
})

Then you can foldMap instead of manually reducing

const I = require('immutable-ext')
I.List(hosts)
.foldMap(Alt, Alt(Task.rejected('No DBs available!')))
.x

Aha, a sneaky semigroup!