Pattern matching with conjunctions (PatternA AND PatternB)

I really like this trick. I do not know of any existing way to do this, and I don't foresee any problem with it -- which doesn't mean much, though. I can't think of any way to create a Not.

As for adding it to the standard library... perhaps. But I think it's a bit hard. On the other hand, how about talking Scalaz people into including it? It looks much more like their own bailiwick.

A possible problem with this is the bloated translation that the pattern matcher generates. Here is the translation of the sample program, generated with scalac -print. Even -optimise fails to simplify the if (true) "_" else throw new MatchError() expressions.

Large pattern matches already generate more bytecode than is legal for a single method, and use of this combinator may amplify that problem.

If && was built into the language, perhaps the translation could be smarter. Alternatively, small improvements to -optimise could help.