January 2014
Intermediate to advanced
352 pages
6h 34m
Japanese
Content preview from JavaScriptで学ぶ関数型プログラミング
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155
6
章
再帰
本書は、ただ関数について考えることから、より深い関数型スタイルの理解へ発想
をスムーズに転換するための章です。関数を終了すべき時はいつなのか、そしてその
時になぜそうすることが良いのかを学びます。本章では主に、自分自身を直接、もし
くは他の関数を介して呼び出す再帰関数を紹介します。
6.1
自身を呼ぶ関数
歴史的に、再帰（
recursion
）と関数型プログラミングは関連したものと捉えられ、
学校でもそのように教えられてきました。この章では、これらの関連性を説明しま
す。まず、再帰が関数型プログラミングにとって重要である
3
つの理由をここで述
べておきます。
●
再帰関数をつかったソリューションは、共通の問題のサブセットに単一の抽
象を使用する。
●
再帰は可変の状態を隠蔽することができる。
●
再帰は遅延評価や、無限データ構造の処理を行う手段のひとつである。
例えば、配列を引数としてとり、その長さ（つまり、要素の数）を返す
myLength
という関数を構築しようとする場合、その基本的特性を考えると、次のような説明に
たどり着くかもしれません
†
。
†
「配列の
length
プロパティを使えばよいのでは？」と思うかもしれません。このような実践的な考え方は
素晴らしいシステムを構築するためには役立ちますが、再帰を学ぶためにはあまり役に立ちません。
156
6 章 再帰
1.
サイズ
0
から開始
2.
配列をループで走査し、それぞれの要素を走査する度に
1
を加える。
3.
最後まで走査したら、そのサイズが長さである。 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.