crocks/Either/maybeToEither.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 Maybe = require('../core/types').proxy('Maybe')

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, maybe) { return maybe.either(
    constant(Either.Left(left)),
    Either.Right
  ); }

// maybeToEither : c -> Maybe a -> Either c a
// maybeToEither : c -> (a -> Maybe b) -> a -> Either c b
function maybeToEither(left, maybe) {
  if(isFunction(maybe)) {
    return function(x) {
      var m = maybe(x)

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

      return applyTransform(left, m)
    }
  }

  if(isSameType(Maybe, maybe)) {
    return applyTransform(left, maybe)
  }

  throw new TypeError('maybeToEither: Maybe or Maybe returning function required for second argument')
}

module.exports = curry(maybeToEither)