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    <p>The ones I can think of are</p> <h3>Existential types</h3> <pre><code>def foo(l: List[Option[_]]) = ... </code></pre> <h3>Higher kinded type parameters</h3> <pre><code>case class A[K[_],T](a: K[T]) </code></pre> <h3>Ignored variables</h3> <pre><code>val _ = 5 </code></pre> <h3>Ignored parameters</h3> <pre><code>List(1, 2, 3) foreach { _ =&gt; println("Hi") } </code></pre> <h3>Ignored names of self types</h3> <pre><code>trait MySeq { _: Seq[_] =&gt; } </code></pre> <h3>Wildcard patterns</h3> <pre><code>Some(5) match { case Some(_) =&gt; println("Yes") } </code></pre> <h3>Wildcard imports</h3> <pre><code>import java.util._ </code></pre> <h3>Hiding imports</h3> <pre><code>import java.util.{ArrayList =&gt; _, _} </code></pre> <h3>Joining letters to punctuation</h3> <pre><code>def bang_!(x: Int) = 5 </code></pre> <h3>Assignment operators</h3> <pre><code>def foo_=(x: Int) { ... } </code></pre> <h3>Placeholder syntax</h3> <pre><code>List(1, 2, 3) map (_ + 2) </code></pre> <h3>Partially applied functions</h3> <pre><code>List(1, 2, 3) foreach println _ </code></pre> <h3>Converting call-by-name parameters to functions</h3> <pre><code>def toFunction(callByName: =&gt; Int): () =&gt; Int = callByName _ </code></pre> <p>There may be others I have forgotten!</p> <hr> <p>Example showing why <code>foo(_)</code> and <code>foo _</code> are different:</p> <p>This example <a href="https://stackoverflow.com/questions/9610736/strange-type-mismatch-when-using-member-access-instead-of-extractor/9610961">comes from 0__</a>:</p> <pre><code>trait PlaceholderExample { def process[A](f: A =&gt; Unit) val set: Set[_ =&gt; Unit] set.foreach(process _) // Error set.foreach(process(_)) // No Error } </code></pre> <p>In the first case, <code>process _</code> represents a method; Scala takes the polymorphic method and attempts to make it monomorphic by filling in the type parameter, but realizes that there is no <em>type</em> that can be filled in for <code>A</code> that will give the type <code>(_ =&gt; Unit) =&gt; ?</code> (Existential <code>_</code> is not a type).</p> <p>In the second case, <code>process(_)</code> is a lambda; when writing a lambda with no explicit argument type, Scala infers the type from the argument that <code>foreach</code> expects, and <code>_ =&gt; Unit</code> <em>is</em> a type (whereas just plain <code>_</code> isn't), so it can be substituted and inferred.</p> <p>This may well be the trickiest gotcha in Scala I have ever encountered.</p>
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    1. POWhat are all the uses of an underscore in Scala?
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    1. USCommunity
    1. USOwen
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    1. COI think there are two or three that all fit under underscore usage in pattern matching, but +1 for joining letters to punctuation! :-)
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      1. USDaniel C. Sobral
    2. CO`val x: Any = _`
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      1. USGiovanni Botta
 

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