blog/content/post/2008/10/02/2008-10-02-00001044.md

---
title: applyの使い方がわかったかもしれない！
author: kazu634
date: 2008-10-02
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categories:
  - gauche
  - Lisp

---
<div class="section">
<p>
<a href="http://d.hatena.ne.jp/SaitoAtsushi/" onclick="__gaTracker('send', 'event', 'outbound-article', 'http://d.hatena.ne.jp/SaitoAtsushi/', 'id:SaitoAtsushi');">id:SaitoAtsushi</a>さんとShiroさんに教えていただいて、なんとなくわかった瞬間がきました！とりあえずその全過程をご覧ください。
</p>

<pre class="syntax-highlight">
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">cons</span> <span class="synConstant">1</span> <span class="synConstant">2</span><span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synConstant">1</span><span class="synStatement"> . </span><span class="synConstant">2</span><span class="synSpecial">)</span>
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> <span class="synStatement">cons</span> <span class="synSpecial">'(</span>1 2<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synConstant">1</span><span class="synStatement"> . </span><span class="synConstant">2</span><span class="synSpecial">)</span>
gosh&#62; <span class="synSpecial">(</span>define p <span class="synSpecial">'(</span>a b c<span class="synSpecial">))</span>
p
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> <span class="synStatement">car</span> <span class="synSpecial">(</span><span class="synStatement">list</span> p<span class="synSpecial">))</span>
a
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">car</span> p<span class="synSpecial">)</span>
a
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span><span class="synStatement">list</span> p<span class="synSpecial">))</span>
<span class="synSpecial">(</span>a b c<span class="synSpecial">)</span>
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> <span class="synStatement">cdr</span> <span class="synSpecial">(</span><span class="synStatement">list</span> p<span class="synSpecial">))</span>
<span class="synSpecial">(</span>b c<span class="synSpecial">)</span>
</pre>

<p>
    ここら辺の段階ではまだよくわかっていない。。。
</p>

<pre class="syntax-highlight">
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> list-ref <span class="synSpecial">(</span><span class="synStatement">list</span> p <span class="synConstant"></span><span class="synSpecial">))</span>
a
gosh&#62; <span class="synSpecial">(</span>list-ref p <span class="synConstant"></span><span class="synSpecial">)</span>
a
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> list-ref <span class="synSpecial">(</span><span class="synStatement">list</span> p <span class="synConstant">1</span><span class="synSpecial">))</span>
b
gosh&#62; <span class="synSpecial">(</span>list-ref p <span class="synConstant">1</span><span class="synSpecial">)</span>
b
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> list-ref <span class="synSpecial">(</span><span class="synStatement">list</span> p <span class="synConstant">2</span><span class="synSpecial">))</span>
c
gosh&#62; <span class="synSpecial">(</span>list-ref p <span class="synConstant">2</span><span class="synSpecial">)</span>
c
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> <span class="synStatement">length</span> <span class="synSpecial">(</span><span class="synStatement">list</span> p<span class="synSpecial">))</span>
<span class="synConstant">3</span>
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">length</span> p<span class="synSpecial">)</span>
<span class="synConstant">3</span>
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> list-ref <span class="synSpecial">'(</span>1<span class="synSpecial">))</span>
<span class="synStatement">***</span> ERROR: wrong <span class="synStatement">number</span> of arguments for #&#60;subr list-ref&#62; <span class="synSpecial">(</span>required <span class="synConstant">2</span>, got <span class="synConstant">1</span><span class="synSpecial">)</span>
Stack Trace:
_______________________________________
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">list</span> p <span class="synConstant">2</span><span class="synSpecial">)</span>
<span class="synSpecial">((</span>a b c<span class="synSpecial">)</span> <span class="synConstant">2</span><span class="synSpecial">)</span>
</pre>

<p>
    ここら辺でもまだ事の重要性に気づいていなかった。。。
</p>

<pre class="syntax-highlight">
gosh&#62; <span class="synSpecial">(</span>list-ref p <span class="synConstant">1</span><span class="synSpecial">)</span>
b
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">list</span> p <span class="synConstant"></span><span class="synSpecial">)</span>
<span class="synSpecial">((</span>a b c<span class="synSpecial">)</span> <span class="synConstant"></span><span class="synSpecial">)</span>
</pre>

<p>
    もしや…applyって「一番外側の()を外して手続きに引数として渡している」んじゃあ…そうじゃないと、
</p>

<pre class="syntax-highlight">
gosh&#62; <span class="synSpecial">(</span><span class="synStatement">apply</span> list-ref <span class="synSpecial">(</span><span class="synStatement">list</span> p <span class="synConstant">2</span><span class="synSpecial">))</span>
c
</pre>

<p>
    とかになる理由が理解できない。これは結局
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>list-ref <span class="synSpecial">'(</span>a b c<span class="synSpecial">)</span> <span class="synConstant"></span><span class="synSpecial">)</span>
</pre>

<p>
    となっているはず。そうじゃなければ、こんな挙動になるはずがない。。。
</p>

<p>
</p>

<p>
    Shiro様、ありがとうございました(__)理解できました。
</p>
</div>