blog/content/post/2008/10/09/2008-10-09-00001049.md

---
title: 高階関数の練習をするよ
author: kazu634
date: 2008-10-09
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categories:
  - gauche
  - Lisp

---
<div class="section">
<p>
    ちょっと前に自分なりにmaxを定義してみました。同じようにすれば、minも定義できます。
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>max-number lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span>fold
<span class="synSpecial">(</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>a b<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">&#60;</span> a b<span class="synSpecial">)</span>
b
a<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cdr</span> lis<span class="synSpecial">)))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>my_max<span class="synStatement"> . </span>args<span class="synSpecial">)</span>
<span class="synSpecial">(</span>max-number args<span class="synSpecial">))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>min-number lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span>fold
<span class="synSpecial">(</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>a b<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">&#62;</span> a b<span class="synSpecial">)</span>
b
a<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cdr</span> lis<span class="synSpecial">)))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>my_min<span class="synStatement"> . </span>args<span class="synSpecial">)</span>
<span class="synSpecial">(</span>min-number args<span class="synSpecial">))</span>
</pre>

<p>
    この中から共通の構造を抜き出して、中間的な手続きを作成してみます。この中間的な手続きは「<」や「>」を受け取って、「引数を一つ取る手続き」を戻り値とします。
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>pickup proc<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span>fold
<span class="synSpecial">(</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>a b<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span>proc a b<span class="synSpecial">)</span>
b
a<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cdr</span> lis<span class="synSpecial">))))</span>
</pre>

<p>
    これを使うと、
</p>

<pre class="syntax-highlight">
gosh&#62; <span class="synSpecial">(</span>define <span class="synSpecial">(</span>my_max<span class="synStatement"> . </span>args<span class="synSpecial">)</span>
<span class="synSpecial">((</span>pickup <span class="synStatement">&#60;</span><span class="synSpecial">)</span> args<span class="synSpecial">))</span>
my_max
gosh&#62; <span class="synSpecial">(</span>my_max <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="synConstant">6</span> <span class="synConstant">7</span><span class="synSpecial">)</span>
<span class="synConstant">7</span>
gosh&#62; <span class="synSpecial">(</span>define <span class="synSpecial">(</span>my_min<span class="synStatement"> . </span>args<span class="synSpecial">)</span>
<span class="synSpecial">((</span>pickup <span class="synStatement">&#62;</span><span class="synSpecial">)</span> args<span class="synSpecial">))</span>
my_min
gosh&#62; <span class="synSpecial">(</span>my_min <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="synConstant">6</span> <span class="synConstant"></span> <span class="synConstant">34</span><span class="synSpecial">)</span>
<span class="synConstant"></span>
</pre>

<p>
    というような書き方になります。
</p>

<p>
    感想: だいぶ理解してきた気がする。
</p>
</div>