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POConfusion with `nil` output when using `Symbol#<=>` in Ruby
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  1. POConfusion with `nil` output when using `Symbol#<=>` in Ruby
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    copied!<p><a href="http://www.ruby-doc.org/core-2.0/Symbol.html#method-i-3C-3D-3E" rel="nofollow"><strong><em><code>Symbol#&lt;=&gt;</code></em></strong></a> </p> <blockquote> <p>simply says: Compares symbol with other_symbol after calling to_s on each of the symbols. Returns -1, 0, +1 or nil depending on whether symbol is less than, equal to, or greater than other_symbol. <strong><code>nil</code> is returned if the two values are incomparable.</strong></p> </blockquote> <p>I was trying to understand how <code>Symbol#&lt;=&gt;</code> works when returning <code>nil</code>. Doing so I played with the code:</p> <pre><code>&gt;&gt; :x.to_s =&gt; "x" &gt;&gt; 'x'.to_s =&gt; "x" </code></pre> <p>From the above <code>IRB</code> code I thought the return value will be <code>0</code>. But the actual is <code>nil</code>. As doc says that before using <code>&lt;=&gt;</code> operator <code>to_s</code> is applied both the <code>RHO</code> and <code>LHO</code>. But here the below code is not supporting that principle, seems to me.</p> <pre><code>&gt;&gt; :x &lt;=&gt; "x" #=&gt; nil </code></pre> <p>So I tried to see the source code,to answer myself:</p> <pre><code>static VALUE sym_cmp(VALUE sym, VALUE other) { if (!SYMBOL_P(other)) { return Qnil; } return rb_str_cmp_m(rb_sym_to_s(sym), rb_sym_to_s(other)); } </code></pre> <p>Looking at the source code it is clear that if <code>RHO</code> is not the object of class <code>Symbol</code>, <code>nil</code> will be returned. Let's see the something more in IRB:</p> <pre><code>&gt;&gt; "x" &lt;=&gt; :x #=&gt; nil </code></pre> <p>Again <code>nil</code>. The source code saying that <code>rb_str_cmp_m(rb_sym_to_s(sym),rb_sym_to_s(other))</code> will be executed now. So now I went to the see the source code of <a href="https://github.com/ruby/ruby/blob/trunk/string.c" rel="nofollow"><strong><em><code>STRING.C</code></em></strong></a>. So we are basically pasing the <code>rb_str_cmp_m(???,"x")</code>. Now the I found from github:(<code>?</code> means don't know what value)</p> <pre><code>rb_str_cmp_m(VALUE str1, VALUE str2) { int result; if (!RB_TYPE_P(str2, T_STRING)) { VALUE tmp = rb_check_funcall(str2, rb_intern("to_str"), 0, 0); if (RB_TYPE_P(tmp, T_STRING)) { result = rb_str_cmp(str1, tmp); } else { return rb_invcmp(str1, str2); } } else { result = rb_str_cmp(str1, str2); } return INT2FIX(result); } </code></pre> <p>But the above code I couldn't understand.But I beleieve it has the answer how <code>nil</code> is producing when <code>LHO</code> is not the object of class <code>Symbol</code>.</p> <p>Can anyone help me here to understand how the <code>nil</code> is coming when <code>LHO</code> is not <code>sym</code>?</p>
 

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