package.json

{
  "name": "state-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "Mark",
  "license": "ISC",
  "dependencies": {
    "fantasy-states": "^0.2.1",
    "ramda": "^0.19.1",
    "ramda-fantasy": "^0.4.1"
  },
  "devDependencies": {
    "babel": "^5.8.23"
  }
}

stateT.js

import {StateT} from "fantasy-states"
import {Future} from "ramda-fantasy"
import {compose, composeK, merge, always, curry} from "ramda"

// Make a new monad from Future and StateT (State Transformer)
const StateF = StateT(Future)
const {get, modify} = StateF

const babies = [
  {id: 2, name: "Alice", sex: "F"},
  {id: 3, name: "Bob", sex: "M"},
]

const isFemale = baby => baby.sex === "F"

// Simulate async operation to fetch data
const findBaby = i => new Future((reject, resolve) => babies[i]
  ? resolve(babies[i])
  : reject("None Found")
)

// Update metadata in 'state' to reflect preferred colours
const updatePrefs = baby => modify(merge({bgColor: isFemale(baby) ? "pink" : "blue"}))
  .map(always(baby))

const html = (baby, prefs) => `
  <div style={background-color: ${prefs.bgColor}, font: ${prefs.font} }>
    ${baby.name}
  </div>
`

const drawPage = baby => {
  console.log("side effect")
  return get.map(prefs => html(baby, prefs))
}

const tapS = curry((fn, arg) => get
  .map(state => fn(state, arg))
  .map(always(arg))
)

const drawBaby = composeK(
  drawPage,
  tapS(console.log),
  updatePrefs,
  tapS(console.log),
  compose(StateF.lift, findBaby) // lift a Future to a StateF
)

drawBaby(StateF.of(0))            // use stateF.of to generate monadic value
  .evalState({font: "cursive"})   // initialise the state and start the processing
  .fork(console.log, console.log) // like 'then' and 'catch' from a Promise