The current proposal is not trying to fix the fail situation, so this is a bit off topic, but to answer your question:
So I prefer 'case xyz of { pat -> mzero/throw/whatever }'. It's more wordy but clearer. Are there uses out there that really rely on the conciseness afforded by refutable patterns?
The stand-alone alternative is more like do { p1 <- return x; return e1 } vs case x of { p1 -> return e1; _ -> mzero} which doesn't look much worse until you realize that the former embeds nicely into existing monadic code, the latter doesn't. So it becomes do { ..; p1 <- return x; .. } vs do { ..; case x of { p1 -> ..; _ -> mzero } } and so on for every added pattern. When I embed Haskell-Coloured Petri Nets in Haskell, the former style can be made to resemble the input language quite closely (changing from graphical to textual representation, but with easily recognizable structure). When writing combinator parsers, type system rules, or other rule-based embedded languages that embed into a MonadPlus, it can be similarly helpful to write down only the interesting success cases, without messing up the code with standard failure cases. Claus