crocks/helpers/composeP.js

A collection of well known Algebraic Datatypes for your utter enjoyment.

/** @license ISC License (c) copyright 2017 original and current authors */
/** @author Ian Hofmann-Hicks (evil) */

var isFunction = require('../core/isFunction')
var isPromise = require('../core/isPromise')

var err = 'composeP: Promise returning functions required'

function applyPipe(f, g) {
  if(!isFunction(g)) {
    throw new TypeError(err)
  }

  return function() {
    var p = f.apply(null, arguments)

    if(!isPromise(p)) {
      throw new TypeError(err)
    }

    return p.then(g)
  }
}

function composeP() {
  var args = [], len = arguments.length;
  while ( len-- ) args[ len ] = arguments[ len ];

  if(!arguments.length) {
    throw new TypeError(err)
  }

  var fns =
    args.reverse()

  var head =
    fns[0]

  if(!isFunction(head)) {
    throw new TypeError(err)
  }

  var tail =
    fns.slice(1).concat(function (x) { return x; })

  return tail.reduce(applyPipe, head)
}

module.exports = composeP