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POWhy does the empty string not match as Seq.empty?
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  1. POWhy does the empty string not match as Seq.empty?
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    copied!<p><strong>EDIT: This was an old bug long since fixed in Scala 2.8 and later</strong></p> <p>During some experimentation around question <a href="https://stackoverflow.com/questions/740563/pattern-matching-a-string-as-seqchar">Pattern matching a String as Seq[Char]</a>, I ran across another weird matching phenomenon. Consider the following code that treats a string as a sequence of characters:</p> <pre><code>def %%&amp;#(input: String) : String = { val uha : Seq[Char] = input uha match { case Seq() =&gt; "Empty" case Seq(first @ _, 'o', 'o') =&gt; "Bar" case _ =&gt; "Oh" } } </code></pre> <p>Calling input on the empty String <code>""</code> correctly yields <code>"Empty"</code>.</p> <p>However, if I rewrite the first match clause as </p> <pre><code>case Seq.empty =&gt; "Empty" </code></pre> <p>the matching of <code>""</code> fails and matches the default clause instead.</p> <p>Walking through the Scala library source code (which you shouldn't have to do in an ideal world :-) ) I believe that both <code>Seq()</code> and <code>Seq.empty</code> will result in <code>RandomAccessSeq.empty</code>. Apparently, this does not concur with the phenomenon described above because only <code>Seq()</code> matches the empty String. </p> <p>UPDATE: Upon some further experimentation this question can be narrowed down to the following: </p> <pre><code>val list = List() &gt;&gt;&gt; list2: List[Nothing] = List() val emptySeq = Seq.empty list == emptySeq &gt;&gt;&gt; res1: Boolean = false </code></pre> <p>This basically means that an empty <code>Seq</code> does not automatically equal <code>Seq.empty</code> . So when matching against a constant (as opposed to using an extractor as suggested by starblue) this unequality leads to the failing match. The same is true when interpreting the empty <code>String</code> as a sequence.</p>
 

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