crocks/First/eitherToFirst.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 First = require('.')
var Either = require('../core/types').proxy('Either')

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

var applyTransform = function (either) { return either.either(First.empty, First); }

// eitherToFirst : Either b a -> First a
// eitherToFirst : (a -> Either c b) -> a -> First b
function eitherToFirst(either) {
  if(isFunction(either)) {
    return function(x) {
      var m = either(x)

      if(!isSameType(Either, m)) {
        throw new TypeError('eitherToFirst: Either returning function required')
      }

      return applyTransform(m)
    }
  }

  if(isSameType(Either, either)) {
    return applyTransform(either)
  }

  throw new TypeError('eitherToFirst: Either or Either returning function required')
}

module.exports = curry(eitherToFirst)