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@i-am-tom i-am-tom/db-alt.js
Last active Oct 19, 2018

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Database failover modelled with the `alt` typeclass.
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!')))
@DrBoolean

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commented Mar 31, 2017

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
@i-am-tom

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commented Apr 4, 2017

Aha, a sneaky semigroup!

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