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copied!<p>In addition to Daniel's answer, keep in mind that <code>Stream</code> is useful for short-circuiting evaluations. For example, suppose I have a huge set of functions that take <code>String</code> and return <code>Option[String]</code>, and I want to keep executing them until one of them works:</p> <pre><code>val stringOps = List( (s:String) => if (s.length>10) Some(s.length.toString) else None , (s:String) => if (s.length==0) Some("empty") else None , (s:String) => if (s.indexOf(" ")>=0) Some(s.trim) else None ); </code></pre> <p>Well, I certainly don't want to execute the <em>entire</em> list, and there isn't any handy method on <code>List</code> that says, "treat these as functions and execute them until one of them returns something other than <code>None</code>". What to do? Perhaps this:</p> <pre><code>def transform(input: String, ops: List[String=>Option[String]]) = { ops.toStream.map( _(input) ).find(_ isDefined).getOrElse(None) } </code></pre> <p>This takes a list and treats it as a <code>Stream</code> (which doesn't actually evaluate anything), then defines a new <code>Stream</code> that is a result of applying the functions (but that doesn't evaluate anything either yet), then searches for the first one which is defined--and here, magically, it looks back and realizes it has to apply the map, and get the right data from the original list--and then unwraps it from <code>Option[Option[String]]</code> to <code>Option[String]</code> using <code>getOrElse</code>.</p> <p>Here's an example:</p> <pre><code>scala> transform("This is a really long string",stringOps) res0: Option[String] = Some(28) scala> transform("",stringOps) res1: Option[String] = Some(empty) scala> transform(" hi ",stringOps) res2: Option[String] = Some(hi) scala> transform("no-match",stringOps) res3: Option[String] = None </code></pre> <p>But does it work? If we put a <code>println</code> into our functions so we can tell if they're called, we get</p> <pre><code>val stringOps = List( (s:String) => {println("1"); if (s.length>10) Some(s.length.toString) else None }, (s:String) => {println("2"); if (s.length==0) Some("empty") else None }, (s:String) => {println("3"); if (s.indexOf(" ")>=0) Some(s.trim) else None } ); // (transform is the same) scala> transform("This is a really long string",stringOps) 1 res0: Option[String] = Some(28) scala> transform("no-match",stringOps) 1 2 3 res1: Option[String] = None </code></pre> <p>(This is with Scala 2.8; 2.7's implementation will sometimes overshoot by one, unfortunately. And note that you <em>do</em> accumulate a long list of <code>None</code> as your failures accrue, but presumably this is inexpensive compared to your true computation here.)</p>

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