index.js

const State = require('crocks/State')
const { get } = State

const assign = require('crocks/helpers/assign')
const concat = require('crocks/pointfree/concat')
const constant = require('crocks/combinators/constant')
const flip = require('crocks/combinators/flip')
const isString = require('crocks/predicates/isString')
const liftA2 = require('crocks/helpers/liftA2')
const mapReduce = require('crocks/helpers/mapReduce')
const objOf = require('crocks/helpers/objOf')
const propPathOr = require('crocks/helpers/propPathOr')
const unless = require('crocks/logic/unless')

const combineState =
  liftA2(assign)

const StateProp = s => ({
  s,
  concat: ({ s: t }) => StateProp(combineState(s, t)),
  fold: f => f(s),
  evalWith: s.evalWith
})

StateProp.Empty =
  StateProp(get(constant({})))

const composeState = (...readers) =>
  mapReduce(StateProp, flip(concat), StateProp.Empty, readers)

const getPropInner =
  def =>
  (isValid = constant(true)) =>
  paths =>
  (name = paths[paths.length - 1]) =>
    get(propPathOr(def, paths))
      .map(unless(isValid, constant(def)))
      .map(objOf(name))

const getProp =
  (name, path, def = 'Prop not found!', allowFalsey = true) =>
    getPropInner(def)(x => allowFalsey && !!x )(path)(name)

const getPropAsString =
  getPropInner('')(isString)

const getInvoice =
  getProp('invoice', [ 'Invoice', 'summary' ], 'default')

const getHeader =
  getProp('header', [ 'context', 'header' ], 'Default Header')

const data = {
  Invoice: { summary: 'summed up nice' },
  context: {
    header: 'dat header'
  }
}

composeState(getInvoice, getHeader)
  .evalWith(data)
//=> { header: 'dat header', invoice: 'summed up nice' }