crocks/pointfree/head.js

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

/** @license ISC License (c) copyright 2016 original and current authors */
/** @author Ian Hofmann-Hicks (evil) */

var cloneIterable = require('../core/cloneIterable')
var isArray = require('../core/isArray')
var isFunction = require('../core/isFunction')
var isIterable = require('../core/isIterable')
var isString = require('../core/isString')

var ref = require('../core/Maybe');
var Nothing = ref.Nothing;
var Just = ref.Just;

function head(m) {
  if(m && isFunction(m.head)) {
    return m.head()
  }

  if(isArray(m) || isString(m)) {
    return !m.length ? Nothing() : Just(m[0])
  }

  if(isIterable(m)) {
    var cloned = cloneIterable(m)
    var iterator = cloned[Symbol.iterator]()
    var head = iterator.next()

    return head.done ? Nothing() : Just(head.value)
  }

  throw new TypeError('head: List or iterable required')
}

module.exports = head