06:27 Thu, 13 Mar 2014 PDT -0700

No Exceptions: Checked Exceptions for Scala

I decided to sit down and solve a problem that has troubled me since I first started working with Scala almost 6 years ago: Checked exceptions, and specifically, the lack thereof.

The result is NX ("No Exceptions") a Scala compiler plugin (compatible with 2.10 and 2.11) that provides support for Java-style checked exceptions:

test.scala:3: error: Unhandled exception type java.net.UnknownHostException; must be caught or declared
to be thrown. Consider the use of monadic error handling, such as scala.util.Either.
one error found

There are some very strong opinions about checked exceptions; Personally, I do not want to see exceptions, checked or otherwise, become a standard Scala idiom. The double-meaning of the project name is intentional; in my book, the only thing worse than checked exceptions are unchecked exceptions, and I'd rather have no exceptions:

That said, checked exception support has already found a number of bugs in our own code where we interface with Java APIs, and the impetus to finally tackle this issue was hitting a trivially machine-detectable bug in the Scala standard library; an issue that would have been flagged immediately by the Java compiler.

If Scala is going to support exceptions at all, I believe that compiler-checked exceptions are a net gain over unchecked exceptions. You don't have to take my word for it, though; the lack of compiler-checked exceptions in Scala provides a a unique opportunity to see what sort of bugs are discovered once automated validation is enabled -- especially when coupled with existing exception-checked Java code.

If you want to give NX a try, full installation and usage instructions are available via the NX home page.

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