crocks/Pair/fanout.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 Pair = require('../core/Pair')

var curry = require('../core/curry')
var isContravariant = require('../core/isContravariant')
var isFunction = require('../core/isFunction')
var isSameType = require('../core/isSameType')
var isSemigroupoid = require('../core/isSemigroupoid')

var valid = function (x, y) { return isSameType(x, y)
    && isSemigroupoid(x)
    && isContravariant(x)
    && isFunction(x.first)
    && isFunction(x.second); }

// fanout : m a b -> m a c -> m a (b, c)
function fanout(fst, snd) {
  if(isFunction(fst) && isFunction(snd)) {
    return function (x) { return Pair(fst(x), snd(x)); }
  }

  if(valid(fst, snd)) {
    return fst.first()
      .compose(snd.second())
      .contramap(function (x) { return Pair(x, x); })
  }

  throw new TypeError(
    'fanout: Arrows, Functions or Stars of the same type required for both arguments'
  )
}

module.exports = curry(fanout)