Free(er) monads in JS (pt 1)

free-er.js

const daggy = require('daggy')

const compose = (f, g) => x => f(g(x))
const id = x => x
const kleisli_comp = (f, g) => x => f(x).chain(g)

//=============FREE=============
const Free = daggy.taggedSum({Impure: ['x', 'f'], Pure: ['x']})
const {Impure, Pure} = Free

Free.of = Pure

Free.prototype.chain = function(f) {
  return this.cata({
    Impure: (x, g) => Impure(x, kleisli_comp(g, f)),
    Pure: x => f(x)
  })
}

Free.prototype.map = function(f) {
  return this.chain(a => Free.of(f(a)))
}

const liftF = command => Impure(command, Pure)

//=============IO via FREE=============
const IO = daggy.taggedSum({IO: ['f']})

const io = compose(liftF, IO.IO)

const unsafePerformIO = free =>
  free.cata({
    Pure: id,
    Impure: (m, q) =>
      unsafePerformIO(q(m.f()))
  })

//=============Maybe via FREE=============
const Maybe = daggy.taggedSum({ Just: ['x'], Nothing: [] })
const {Just, Nothing} = Maybe

const just = compose(liftF, Just)
const nothing = liftF(Nothing)

const runMaybe = free =>
  free.cata({
    Pure: x => x,
    Impure: (m, q) =>
      m.cata({
        Just: x => runMaybe(q(x)),
        Nothing: () => Nothing
      })
  })


//=============Examples=============

const safeProp = (k, o) => o[k] ? just(o[k]) : nothing
const localStorage = io(() => ({preferences: 'blue'}))
const printLn = x => io(() => console.log(x))

//maybe_name :: Free (Maybe String)
const maybe_name = safeProp('user', {user: {name: 'jerry'}})
                   .chain(u => safeProp('name', u))
                   .map(n => n.toUpperCase())


runMaybe(maybe_name)
// JERRY


//i :: Free (IO ())
const i = localStorage
          .map(l => l.preferences)
          .chain(printLn)
unsafePerformIO(i)
// blue

Code for: https://www.youtube.com/watch?v=RkDtMeZXMTA

Part 1 of 2

What is the cata property of the free monad?

cata is from the daggy library