funfix-effect/src/internals.ts

Sub-package of Funfix defining monadic data types for dealing with laziness and side effects

/*!
 * Copyright (c) 2017-2018 by The Funfix Project Developers.
 * Some rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * We don't need the full power of JS's iterators, just a way
 * to traverse data structures.
 *
 * @hidden
 */
export interface IteratorLike<A> {
  next(): { done: boolean, value?: A }
}

/**
 * Reusable empty `IteratorLike` reference.
 *
 * @hidden
 */
export const emptyIteratorRef: IteratorLike<never> =
  { next: () => ({ done: true }) }

/**
 * Given an array or an `Iterable`, returns a simple iterator type
 * that we can use to traverse the given list lazily.
 *
 * @hidden
 */
export function iteratorOf<A>(list: A[] | Iterable<A>): IteratorLike<A> {
  if (!list) return emptyIteratorRef
  if (Object.prototype.toString.call(list) !== "[object Array]")
    return list[Symbol.iterator]()

  const array = list as A[]
  if (array.length === 0) return emptyIteratorRef

  let cursor = 0
  const next = () => {
    const value = array[cursor++]
    const done = cursor >= array.length
    return { done, value }
  }

  return { next }
}