JavaScriptで学ぶ関数型プログラミング

functional.js

var assert = require("assert");
//var _ = require("/Users/kakim/scoop/apps/nvm/current/nodejs/v12.21.0/node_modules/underscore");
//var _ = require("/Users/kakim/scoop/apps/nvm/current/nodejs/v16.3.0/node_modules/underscore");
var _ = require("/usr/local/lib/node_modules/underscore");

var lyrics = [];

for (var bottles = 99; bottles > 0; bottles--) {
  lyrics.push(bottles + " 本のビールが残っている");
  lyrics.push(bottles + " 本のビールが");
  lyrics.push("ひとつ取って、隣に回せ");
  if (bottles > 1) {
    lyrics.push(bottles - 1 + " 本のビールが残っている");
  } else {
    lyrics.push("もうビールは残っていない");
  }
}

function lyricSegment(n) {
  return _.chain([])
    .push(n + " 本のビールが残っている")
    .push(n + " 本のビール")
    .push("ひとつ取って、隣に回せ")
    .tap(function (lyrics) {
      if (n > 1) lyrics.push(n - 1 + " 本のビールが残っている");
      else lyrics.push("もうビールは残っていない");
    })
    .value();
}

lyricSegment(9);

function song(start, end, lyricGen) {
  return _.reduce(
    _.range(start, end, -1),
    function (acc, n) {
      return acc.concat(lyricGen(n));
    },
    []
  );
}

song(99, 0, lyricSegment);

var nums = [100, 2, 25];
function div(x, y) {
  return x / y;
}

assert.equal(_.reduce(nums, div), 2);

assert.equal(_.reduceRight(nums, div), 0.125);

function allOf(/*1つ以上の関数*/) {
  return _.reduceRight(
    arguments,
    function (truth, f) {
      return truth && f();
    },
    true
  );
}
function anyOf(/*1つ以上の関数*/) {
  return _.reduceRight(
    arguments,
    function (truth, f) {
      return truth || f();
    },
    false
  );
}
function T() {
  return true;
}
function F() {
  return false;
}
assert.equal(allOf(), true);
assert.equal(allOf(T, T), true);
assert.equal(allOf(T, T, T, T, F), false);
assert.equal(anyOf(T, T, F), true);
assert.equal(anyOf(F, F, F, F), false);
assert.equal(anyOf(), false);

let result = _.find(["a", "b", 3, "d"], _.isNumber);
assert.equal(result, 3);

result = _.reject(["a", "b", 3, "d"], _.isNumber);
assert.deepEqual(result, ["a", "b", "d"]);

function complemet(pred) {
  return function () {
    return !pred.apply(null, arguments);
  };
}
result = _.filter(["a", "b", 3, "d"], complemet(_.isNumber));
assert.deepEqual(result, ["a", "b", "d"]);

result = _.all([1, 2, 3, 4], _.isNumber);
assert.equal(result, true);

result = _.any([1, 2, "c", 4], _.isString);
assert.equal(result, true);

var people = [
  { name: "Rick", age: 30 },
  { name: "Jaka", age: 24 },
];
result = _.sortBy(people, function (p) {
  return p.age;
});
assert.deepEqual(result, [
  { name: "Jaka", age: 24 },
  { name: "Rick", age: 30 },
]);

var albums = [
  { title: "Sabbath Bloody Sabbath", genre: "Metal" },
  { title: "Scientist", genre: "Dub" },
  { title: "Undertow", genre: "Metal" },
];

result = _.groupBy(albums, function (a) {
  return a.genre;
});
assert.deepEqual(result, {
  Metal: [
    { title: "Sabbath Bloody Sabbath", genre: "Metal" },
    { title: "Undertow", genre: "Metal" },
  ],
  Dub: [{ title: "Scientist", genre: "Dub" }],
});

result = _.countBy(albums, function (a) {
  return a.genre;
});
assert.deepEqual(result, { Metal: 2, Dub: 1 });