blog/content/post/2008/08/20/2008-08-20-00001007.md

---
title: なんかもっと簡単なところからやりたい
author: kazu634
date: 2008-08-20
wordtwit_post_info:
  - 'O:8:"stdClass":13:{s:6:"manual";b:0;s:11:"tweet_times";i:1;s:5:"delay";i:0;s:7:"enabled";i:1;s:10:"separation";s:2:"60";s:7:"version";s:3:"3.7";s:14:"tweet_template";b:0;s:6:"status";i:2;s:6:"result";a:0:{}s:13:"tweet_counter";i:2;s:13:"tweet_log_ids";a:1:{i:0;i:4225;}s:9:"hash_tags";a:0:{}s:8:"accounts";a:1:{i:0;s:7:"kazu634";}}'
categories:
  - gauche
  - Lisp

---
<div class="section">
<p>
    ちょっぴり置いてけぼりな感じを味わっているので、もう少し時間をかけてやっていこうと思う。というわけで、<a href="http://d.hatena.ne.jp/hayamiz/" onclick="__gaTracker('send', 'event', 'outbound-article', 'http://d.hatena.ne.jp/hayamiz/', 'id:hayamiz');">id:hayamiz</a>がEmacs勉強会でお勧めしてた<a href="http://www.sampou.org/scheme/t-y-scheme/" onclick="__gaTracker('send', 'event', 'outbound-article', 'http://www.sampou.org/scheme/t-y-scheme/', 'Index of /scheme/t-y-scheme');" target="_blank">Index of /scheme/t-y-scheme</a>を読み始める。これをやってから、もういちど『<a href="http://d.hatena.ne.jp/asin/4873113482" onclick="__gaTracker('send', 'event', 'outbound-article', 'http://d.hatena.ne.jp/asin/4873113482', 'プログラミングGauche');">プログラミングGauche</a>』に挑戦しようと思う。
</p>

<h4>
    letrecとか完全に誤解していた気がする
</h4>

<p>
    わかったこと:
</p>

<p>
    letはlambdaを言い換えている構文。だからブロックを作って、そのブロックの中で通用するローカル変数を持って結果を返しているイメージ…なのかな？たぶん。lispとperlで比較:
</p>

<pre class="syntax-highlight">
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">let</span> <span class="synSpecial">((</span>a <span class="synConstant">3</span><span class="synSpecial">)</span>
<span class="synSpecial">(</span>b <span class="synConstant">5</span><span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">+</span> <span class="synSpecial">(</span><span class="synStatement">*</span> a a<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">*</span> b b<span class="synSpecial">)))</span>
<span class="synConstant">34</span>
</pre>

<pre class="syntax-highlight">
<span class="synComment"># === use ===</span>
<span class="synStatement">use  strict</span>;
<span class="synStatement">use warnings</span>;
<span class="synComment"># === main ===</span>
{
<span class="synStatement">my</span> <span class="synIdentifier">$a</span> = <span class="synConstant">3</span>;
<span class="synStatement">my</span> <span class="synIdentifier">$b</span> = <span class="synConstant">5</span>;
<span class="synStatement">print</span>((<span class="synIdentifier">$a</span> * <span class="synIdentifier">$a</span>) + (<span class="synIdentifier">$b</span> + <span class="synIdentifier">$b</span>), <span class="synConstant">&#34;</span><span class="synSpecial">\n</span><span class="synConstant">&#34;</span>);
}
<span class="synComment"># print ($a);                             # これはエラーになる</span>
</pre>

<p>
    letrecはlambdaで再帰的な手続きを作るときに用いるイメージなのかな？うでも、perlだとforeachとかすごくしたくなるかも（数字かどうかのチェックは省略）。
</p>

<pre class="syntax-highlight">
gosh&#62; <span class="synSpecial">(</span>letrec <span class="synSpecial">((</span>sum <span class="synSpecial">(</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">cond</span> [<span class="synSpecial">(</span><span class="synStatement">null</span>? lis<span class="synSpecial">)</span> <span class="synConstant"></span>]
[<span class="synSpecial">(</span><span class="synStatement">number</span>? <span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">+</span> <span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span>sum <span class="synSpecial">(</span><span class="synStatement">cdr</span> lis<span class="synSpecial">)))</span>]
[else <span class="synSpecial">(</span>sum <span class="synSpecial">(</span><span class="synStatement">cdr</span> lis<span class="synSpecial">))</span>]<span class="synSpecial">))))</span>
<span class="synSpecial">(</span>sum <span class="synSpecial">'(</span>1 2 3 4 5<span class="synSpecial">)))</span>
<span class="synConstant">15</span>
</pre>

<pre class="syntax-highlight">
<span class="synComment"># === use ===</span>
<span class="synStatement">use  strict</span>;
<span class="synStatement">use warnings</span>;
<span class="synStatement">my</span> $ res = <span class="synConstant"></span>;
<span class="synComment"># === main ===</span>
<span class="synStatement">my</span> <span class="synIdentifier">@i</span> = (<span class="synConstant">1</span>, <span class="synConstant">2</span>, <span class="synConstant">3</span>, <span class="synConstant">4</span>, <span class="synConstant">5</span>);
<span class="synStatement">print</span> (<span class="synIdentifier">&#38;sub_sum</span>(<span class="synIdentifier">@i</span>), <span class="synConstant">&#34;</span><span class="synSpecial">\n</span><span class="synConstant">&#34;</span>);
<span class="synComment"># === sub ===</span>
<span class="synStatement">sub</span><span class="synIdentifier"> sub_sum</span>{
<span class="synStatement">my</span> <span class="synIdentifier">$x</span>;
<span class="synStatement">my</span> <span class="synIdentifier">@y</span>;
(<span class="synIdentifier">$x</span>, <span class="synIdentifier">@y</span>) = <span class="synIdentifier">@_</span>;
<span class="synStatement">if</span> (<span class="synConstant"></span> == <span class="synIdentifier">@y</span>){
<span class="synStatement">return</span> <span class="synConstant"></span>;
} <span class="synStatement">else</span> {
<span class="synIdentifier">$res</span> = <span class="synIdentifier">$x</span> + <span class="synIdentifier">&#38;sub_sum</span>(<span class="synIdentifier">@y</span>);
}
<span class="synStatement">return</span> <span class="synIdentifier">$res</span>;
}
</pre>
</div>