crocks/Either/lastToEither.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 Either = require('.')
var Last = require('../core/types').proxy('Last')

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

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

var applyTransform = function (left, last) { return last.valueOf().either(
    constant(Either.Left(left)),
    Either.Right
  ); }

// lastToEither : c -> Last a -> Either c a
// lastToEither : c -> (a -> Last b) -> a -> Either c b
function lastToEither(left, last) {
  if(isFunction(last)) {
    return function(x) {
      var m = last(x)

      if(!isSameType(Last, m)) {
        throw new TypeError('lastToEither: Last returning function required for second argument')
      }

      return applyTransform(left, m)
    }
  }

  if(isSameType(Last, last)) {
    return applyTransform(left, last)
  }

  throw new TypeError('lastToEither: Last or Last returning function required for second argument')
}

module.exports = curry(lastToEither)