halt() generates a HaltException

[Warren Strange]

Which I now know  is completely obvious but I would suggest the book mention this :-)

As a newbie I had a halt within a try/catch block and couldn't figure out why the halt never took effect.... 

[Ross A. Baker]

That's an implementation detail that we hope to to hide from
applications, but it leaks out in that particular case. Likewise with
pass and redirect. I will add these to the book, thanks.

--
Ross A. Baker
ba...@alumni.indiana.edu
Indianapolis, IN, USA

[Ruslan Shevchenko]

Btw, it is possible to inherit messenger-s from Throwable, not from Exception
which will reduce such side effects. [Usually user code does not
catch Throwables,
only Exceptions]. So, this can be suggestion for small refactoring in
next version.

[Ross A. Baker]

Good idea. The proper parent is scala.util.control.ControlThrowable,
which I just checked in. Thanks to Stefan on IRC for the link:

http://www.tzavellas.com/techblog/2010/09/20/catching-throwable-in-scala/

Key point for application developers: { catch e => ... } is usually
wrong. { catch e: Exception => ... } is about as general as you want
to go unless you have a good reason.