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    <p><code>_</code> is a valid identifier. Identifiers can't just contain underscores, they can also <em>be</em> an underscore.</p> <pre><code>_ = o = Object.new _.object_id == o.object_id # =&gt; true </code></pre> <p>You can also use it as method names:</p> <pre><code>def o._; :_ end o._ # =&gt; :_ </code></pre> <p>Of course, it is not exactly a readable name, nor does it pass any information to the reader about what the variable refers to or what the method does.</p> <p><code>IRB</code>, in particular, sets <code>_</code> to the value of the last expression:</p> <pre><code>$ irb &gt; 'asd' # =&gt; "asd" &gt; _ # =&gt; "asd" </code></pre> <p>As it is <a href="https://github.com/ruby/ruby/blob/trunk/lib/irb/context.rb#L167" rel="noreferrer">in the source code</a>, it simply sets <code>_</code> to the last value:</p> <pre><code>@workspace.evaluate self, "_ = IRB.CurrentContext.last_value" </code></pre> <hr> <p>Did some repository exploring. Here's what I found:</p> <p>On the last lines of the file <a href="https://github.com/ruby/ruby/blob/trunk/id.c#L50" rel="noreferrer"><code>id.c</code></a>, there is the call:</p> <pre><code>REGISTER_SYMID(idUScore, "_"); </code></pre> <p><code>grep</code>ing the source for <code>idUScore</code> gave me two seemingly relevant results:</p> <ul> <li>In the <a href="https://github.com/ruby/ruby/blob/trunk/parse.y#L8993" rel="noreferrer"><code>shadowing_lvar_gen</code> function</a></li> <li>In the <a href="https://github.com/ruby/ruby/blob/trunk/parse.y#L9692" rel="noreferrer"><code>warn_unused_var</code> function</a></li> </ul> <p><code>shadowing_lvar_gen</code> seems to be the mechanism through which the formal parameter of a block replaces a variable of the same name that exists in another scope. It is the function that seems to raise "duplicated argument name" <code>SyntaxError</code> and the "shadowing outer local variable" warning.</p> <p>After <code>grep</code>ing the source for <code>shadowing_lvar_gen</code>, I found the following <a href="https://raw.github.com/ruby/ruby/trunk/doc/ChangeLog-1.9.3" rel="noreferrer">on the changelog for Ruby 1.9.3</a>:</p> <blockquote> <p>Tue Dec 11 01:21:21 2007 Yukihiro Matsumoto </p> <ul> <li>parse.y (shadowing_lvar_gen): no duplicate error for "_".</li> </ul> </blockquote> <p>Which is likely to be the origin of <a href="https://github.com/ruby/ruby/blob/trunk/parse.y#L8993" rel="noreferrer">this line</a>:</p> <pre><code>if (idUScore == name) return name; </code></pre> <p>From this, I deduce that in a situation such as <code>proc { |_, _| :x }.call :a, :b</code>, one <code>_</code> variable simply shadows the other.</p> <hr> <p>Here's <a href="https://github.com/ruby/ruby/commit/6bd65de203c7aaffc8d4241a771491356002ed75" rel="noreferrer">the commit in question</a>. It basically introduced these two lines:</p> <pre><code>if (!uscore) uscore = rb_intern("_"); if (uscore == name) return; </code></pre> <p>From a time when <code>idUScore</code> did not even exist, apparently.</p>
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    1. POWhere and how is the _ (underscore) variable specified?
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    1. COThis doesn't explain *at all* why `lambda { |_, _| 42 }` works while `lambda { |x, x| 42 }` doesn't.
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      1. USAndrew Marshall
    2. CO@AndrewMarshall, it appears you are right. `|_, _|` does work, but `|__, __|` doesn't. `_` does seem to have some special meaning, I will see if I can dig any information from the Ruby source.
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    3. COBy the way, your update, while informative, is not relevant as it only applies to IRb, which I specifically stated in my question I was not asking about.
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