Ramda Adjunct 2.30.0

reduceRightP.js

  1. import { curryN, pipe, equals, reduceRight, length, concat } from 'ramda';
  3. import isUndefined from './isUndefined';
  4. import resolveP from './resolveP';
  5. import allP from './allP';
  7. // in older ramda versions the order of the arguments is flipped
  8. const flipArgs = pipe(reduceRight(concat, ''), equals('ba'))(['a', 'b']);
  10. /* eslint-disable max-len */
  11. /**
  12.  * Given an `Iterable`(arrays are `Iterable`), or a promise of an `Iterable`,
  13.  * which produces promises (or a mix of promises and values),
  14.  * iterate over all the values in the `Iterable` into an array and
  15.  * reduce the array to a value using the given iterator function.
  16.  *
  17.  * Similar to {@link RA.reduceP|reduceP} except moves through the input list from the right to the left.
  18.  * The iterator function receives two values: (value, acc),
  19.  * while the arguments' order of reduceP's iterator function is (acc, value).
  20.  *
  21.  * @func reduceRightP
  22.  * @memberOf RA
  23.  * @since {@link https://char0n.github.io/ramda-adjunct/1.13.0|v1.13.0}
  24.  * @category List
  25.  * @typedef MaybePromise = Promise.<*> | *
  26.  * @sig ((MaybePromise b, Promise a) -> Promise a) -> MaybePromise a -> MaybePromise [MaybePromise b] -> Promise a
  27.  * @param {Function} fn The iterator function. Receives two values, the current element from the list and the accumulator
  28.  * @param {*|Promise.<*>} acc The accumulator value
  29.  * @param {Array.<*>|Promise.<Array<*|Promise.<*>>>} list The list to iterate over
  30.  * @return {Promise} The final, accumulated value
  31.  * @see {@link RA.reduceP|reduceP}, {@link http://bluebirdjs.com/docs/api/promise.reduce.html|bluebird.reduce}
  32.  * @example
  33.  *
  34.  * RA.reduceRightP(
  35.  *   (fileName, total) => fs
  36.  *     .readFileAsync(fileName, 'utf8')
  37.  *     .then(contents => total + parseInt(contents, 10)),
  38.  *   0,
  39.  *   ['file1.txt', 'file2.txt', 'file3.txt']
  40.  * ); // => Promise(10)
  41.  *
  42.  * RA.reduceRightP(
  43.  *   (fileName, total) => fs
  44.  *     .readFileAsync(fileName, 'utf8')
  45.  *     .then(contents => total + parseInt(contents, 10)),
  46.  *   Promise.resolve(0),
  47.  *   ['file1.txt', 'file2.txt', 'file3.txt']
  48.  * ); // => Promise(10)
  49.  *
  50.  * RA.reduceRightP(
  51.  *   (fileName, total) => fs
  52.  *     .readFileAsync(fileName, 'utf8')
  53.  *     .then(contents => total + parseInt(contents, 10)),
  54.  *   0,
  55.  *   [Promise.resolve('file1.txt'), 'file2.txt', 'file3.txt']
  56.  * ); // => Promise(10)
  57.  *
  58.  * RA.reduceRightP(
  59.  *   (fileName, total) => fs
  60.  *     .readFileAsync(fileName, 'utf8')
  61.  *     .then(contents => total + parseInt(contents, 10)),
  62.  *   0,
  63.  *   Promise.resolve([Promise.resolve('file1.txt'), 'file2.txt', 'file3.txt'])
  64.  * ); // => Promise(10)
  65.  *
  66.  */
  67. /* esline-enable max-len */
  68. const reduceRightP = curryN(3, (fn, acc, list) =>
  69.   resolveP(list).then((iterable) => {
  70.     const listLength = length(iterable);
  72.     if (listLength === 0) {
  73.       return acc;
  74.     }
  76.     const reducer = reduceRight((arg1, arg2) => {
  77.       let accP;
  78.       let currentValueP;
  80.       if (flipArgs) {
  81.         [accP, currentValueP] = [arg1, arg2];
  82.       } else {
  83.         [accP, currentValueP] = [arg2, arg1];
  84.       }
  86.       return accP
  87.         .then((previousValue) => allP([previousValue, currentValueP]))
  88.         .then(([previousValue, currentValue]) => {
  89.           if (isUndefined(previousValue) && listLength === 1) {
  90.             return currentValue;
  91.           }
  93.           return fn(currentValue, previousValue);
  94.         });
  95.     });
  97.     return reducer(resolveP(acc), iterable);
  98.   })
  99. );
  101. export default reduceRightP;