xform/map.js

/*
 * Copyright (c) 2017 Thomas Otterson
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
 * the Software, and to permit persons to whom the Software is furnished to do so,
 * subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
 * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
 * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
 * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

/**
 * Transducers for using functions to determine new values of collection elements.
 *
 * @module map
 * @private
 */

const { protocols } = require('../modules/protocol');
const { sequence, compose } = require('../modules/transformation');
const { isFunction } = require('../modules/util');
const { flatten } = require('./core');
const p = protocols;

/**
 * A transducer function that is returned by the `{@link module:xduce.transducers.map|map}` transducer.
 *
 * @private
 *
 * @param {function} fn A single-parameter function which is supplied each input collection element in turn. The return
 *     values of these calls become the elements of the output collection.
 * @param {module:xduce~transducerObject} xform The transducer object that the new one should be chained to.
 * @return {module:xduce~transducerObject} A new transducer object, performing no transformation and chaining to the
 *     provided transducer object.
 */
function mapTransducer(fn, xform) {
  return {
    [p.init]() {
      return xform[p.init]();
    },

    [p.step](acc, input) {
      return xform[p.step](acc, fn(input));
    },

    [p.result](value) {
      return xform[p.result](value);
    }
  };
}

/**
 * **Creates a new collection whose values are the results of mapping input collection elements over a function.**
 *
 * If no collection is provided, a function is returned that can be passed to `{@link module:xduce.sequence|sequence}`,
 * et al.
 *
 * ```
 * const result = map([1, 2, 3, 4, 5], x => x * x);
 * // result = [1, 4, 9, 16, 25]
 * ```
 *
 * @memberof module:xduce.transducers
 *
 * @param {*} [collection] An optional input collection that is to be transduced.
 * @param {function} fn A function that is supplied each input collection element in turn. The return values of this
 *     function become the elements of the output collection.
 * @param {object} [ctx] An optional context object which is set to `this` for the function `fn`. This does not work if
 *     `fn` is an arrow function, as they cannot be bound to contexts.
 * @return {(*|module:xduce~transducerFunction)} If a collection is supplied, then the function returns a new
 *     collection of the same type containing the return values of `fn` when passed those elements. If no collection is
 *     supplied, a transducer function, suitable for passing to `{@link module:xduce.sequence|sequence}`,
 *     `{@link module:xduce.into|into}`, etc. is returned.
 */
function map(collection, fn, ctx) {
  const [col, func] = isFunction(collection) ? [null, collection.bind(fn)] : [collection, fn.bind(ctx)];
  return col ? sequence(col, map(func)) : xform => mapTransducer(func, xform);
}

/**
 * **A map function that flattens any collections among the return values.**
 *
 * This is a composition of `{@link module:xduce.transducers.map|map}` and
 * `{@link module:xduce.transducers.flatten|flatten}`. In fact it could be defined by the user by using those two
 * functions with `{@link module:xduce.compose|compose}`, but the concept of a flatmap is so fundamental that it's
 * included separately.
 *
 * Because the map is followed by flattening, there are the same notes as with
 * `{@link module:xduce.transducers.flatten|flatten}`; this function doesn't make a lot of sense with functions that
 * return objects, strings, or iterators.
 *
 * If no collection is provided, a function is returned that can be passed to `{@link module:xduce.sequence|sequence}`,
 * et al.
 *
 * ```
 * const duplicate = x => [x, x];
 *
 * let result = flatMap([1, 2, 3, 4, 5], duplicate);
 * // result = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]
 *
 * // The following is equivalent
 * const fn = compose(map(duplicate), flatten());
 * result = sequence([1, 2, 3, 4, 5], fn);
 * // result = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]
 *
 * // To illustrate the difference from `map`, here's what `map` would do with
 * // the same parameters
 * result = map([1, 2, 3, 4, 5], duplicate);
 * // result = [[1, 1], [2, 2], [3, 3], [4, 4], [5, 5]]
 * ```
 *
 * @memberof module:xduce.transducers
 *
 * @param {*} [collection] An optional input collection that is to be transduced.
 * @param {function} fn A function that is supplied each input collection element in turn. The return values of this
 *     function are flattened to become the elements of the output collection.
 * @param {object} [ctx] An optional context object which is set to `this` for the function `fn`. This does not work if
 *     `fn` is an arrow function, as they cannot be bound to contexts.
 * @return {(*|module:xduce~transducerFunction)} If a collection is supplied, then the function returns a new
 *     collection of the same type containing those elements, mapped and flattened. If no collection is supplied, a
 *     transducer function, suitable for passing to `{@link module:xduce.sequence|sequence}`,
 *     `{@link module:xduce.into|into}`, etc. is returned.
 */
function flatMap(collection, fn, ctx) {
  const [col, func] = isFunction(collection) ? [null, collection.bind(fn)] : [collection, fn.bind(ctx)];
  return col ? sequence(col, compose(map(func), flatten())) : compose(map(func), flatten());
}

module.exports = {
  map,
  flatMap
};