PatternMatching

The Gist

Pattern Matching gives a very gentle introduction to Scala’s match and case keywords

My Interpretation

Scala’s match keyword is akin to Java’s switch statement, however it is far more powerful.

Suppose you wish to examine a URL to determine, by extension, how to render an object you’ve looked up. So a url like /people/dave.xml would find a person named “dave” and return the data as XML. Similarly, /people/dave.json would return the data as JSON.

In Java, you would do something like this:

Now, you can certainly factor that horrible if-then-else-if structure out, but it seems a shame to have to do that. Also, the null checking and temporary variables are a bit noisy. Here’s the same thing in Scala:

We are using two case statements here. In the first one, we combine the search for the '.' character with the substring to get the extension. The result is an instance of Scala’s Option class, which is better way of encapsulating “no value” than null is.

Essentially what this statement means is "if the rest of lastIndexOf was 1, set extension to None , otherwise, set it to the substring following the '.'“.
The next case section decides which rendering to use. This is a bit easier to grok, as it’s basically a compact if-then-else-if construct. The thing to note here is that the expression Some("xml") means”if our extension is not None and it’s string is “xml”, we match. This much more compact than if ( (ext != null) && (ext.equals("xml") ) from the Java code.
Finally, note that the expression case _ is akin to default in a Java switch statement.
This just touches the surface of Scala’s pattern matching.
Suppose you wish to add rendering of your object’s encoding after the handleOutput call. Suppose that you wish to handle anything, not just instances of Entity, and you also wish to hide the names of any Person who is under 18 years old.
<script src=“https://gist.github.com/154843.js”></script>
Note that we aren’t just switching based on the value of obj, but also on it’s type, and it’s contents; the guard condition in the first case statement allows us to refine our match. Anyone under 18 will match this; anyone else who is a Person will match the second case, with any other Entity instance matching the third. Finally, we just toString nonEntity objects that we receive.

My Thoughts on this Feature

I almost never use switch statements in Java, because they are essentially useless. Even if all it did here is get rid of annoying if-then-else blocks, I’d call it a win. As it stands, this is one of the coolest features of Scala and a really cool thing to have that takes advantage of the type information we get via static typing.