crocks/core/defineUnion.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 curry = require('./curry')
var isArray = require('./isArray')
var isEmpty = require('./isEmpty')
var isFunction = require('./isFunction')
var isObject = require('./isObject')
var isString = require('./isString')

var constant = function (x) { return function () { return x; }; }

var isDefinition =
  function (x) { return isString(x) && x.length; }

function caseOf(defs) {
  return function(cases, m) {
    var tag = m.tag
    var def = defs[tag()]

    var args = def.reduce(
      function (xs, x) { return xs.concat([ m[x].value() ]); },
      []
    )

    return cases[tag()].apply(null, args)
  }
}

var includes =
  function (defs) { return function (m) { return !!m && isFunction(m.tag) && Object.keys(defs).indexOf(m.tag()) !== -1; }; }

function construction(def, tag) {
  return function() {
    var args = [], len = arguments.length;
    while ( len-- ) args[ len ] = arguments[ len ];

    return def.reduce(function(obj, key, index) {
      obj[key] = { value: constant(args[index]) }
      return obj
    }, { tag: constant(tag) })
  }
}

function defineUnion(defs) {
  if(!isObject(defs) || isEmpty(defs)) {
    throw new TypeError('defineUnion: Argument must be an Object containing definition lists')
  }

  return Object.keys(defs).reduce(function(obj, tag) {
    var def = defs[tag]

    if(!isArray(def) || !def.reduce(function (x, y) { return x && isDefinition(y); }, true)) {
      throw new TypeError('defineUnion: Definitions must be a list of non-empty string identifiers')
    }

    obj[tag] = construction(def, tag)

    return obj
  }, { caseOf: curry(caseOf(defs)), includes: curry(includes(defs)) })
}

module.exports = defineUnion