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copied!<p><code>collect</code> (defined on <a href="http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/scala/collection/TraversableLike.html" rel="noreferrer">TraversableLike</a> and available in all subclasses) works with a collection and a <code>PartialFunction</code>. It also just so happens that a bunch of case clauses defined inside braces are a partial function (See section 8.5 of the <a href="http://www.scala-lang.org/sites/default/files/linuxsoft_archives/docu/files/ScalaReference.pdf" rel="noreferrer">Scala Language Specification</a> <em>[warning - PDF]</em>)</p> <p>As in exception handling:</p> <pre><code>try { ... do something risky ... } catch { //The contents of this catch block are a partial function case e: IOException => ... case e: OtherException => ... } </code></pre> <p>It's a handy way to define a function that will only accept some values of a given type.</p> <p>Consider using it on a list of mixed values:</p> <pre><code>val mixedList = List("a", 1, 2, "b", 19, 42.0) //this is a List[Any] val results = mixedList collect { case s: String => "String:" + s case i: Int => "Int:" + i.toString } </code></pre> <p>The argument to to <code>collect</code> method is a <code>PartialFunction[Any,String]</code>. <code>PartialFunction</code> because it's not defined for all possible inputs of type <code>Any</code> (that being the type of the <code>List</code>) and <code>String</code> because that's what all the clauses return.</p> <p>If you tried to use <code>map</code> instead of <code>collect</code>, the the double value at the end of <code>mixedList</code> would cause a <code>MatchError</code>. Using <code>collect</code> just discards this, as well as any other value for which the PartialFunction is not defined.</p> <p>One possible use would be to apply different logic to elements of the list:</p> <pre><code>var strings = List.empty[String] var ints = List.empty[Int] mixedList collect { case s: String => strings :+= s case i: Int => ints :+= i } </code></pre> <p>Although this is just an example, using mutable variables like this is considered by many to be a war crime - So please don't do it!</p> <p>A <em>much</em> better solution is to use collect twice:</p> <pre><code>val strings = mixedList collect { case s: String => s } val ints = mixedList collect { case i: Int => i } </code></pre> <p>Or if you know for certain that the list only contains two types of values, you can use <code>partition</code>, which splits a collections into values depending on whether or not they match some predicate:</p> <pre><code>//if the list only contains Strings and Ints: val (strings, ints) = mixedList partition { case s: String => true; case _ => false } </code></pre> <p>The catch here is that both <code>strings</code> and <code>ints</code> are of type <code>List[Any]</code>, though you can easily coerce them back to something more typesafe (perhaps by using <code>collect</code>...)</p> <p>If you already have a type-safe collection and want to split on some other property of the elements, then things are a bit easier for you:</p> <pre><code>val intList = List(2,7,9,1,6,5,8,2,4,6,2,9,8) val (big,small) = intList partition (_ > 5) //big and small are both now List[Int]s </code></pre> <p>Hope that sums up how the two methods can help you out here!</p>

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