On Tue, Jul 27, 2010 at 6:29 PM, Dietrich Epp
I'll say yes, a pattern match failure is a bug. This is one of the great debates in the language: whether all pattern matching code should be guaranteed complete at compile time or not. However, any function you call which returns a result in your monad could theoretically call "fail" if it was written that way. Data.Map.lookup used to call "fail" when it could not find a key, but that got changed.
I've always thought that being able to write:
catMaybes :: [Maybe a] -> [a] catMaybes xs = [ x | Just x <- xs ]
is really cool, which relies on:
fail _ = []
being in the Monad instance for List. But I would give that up for getting "fail" out of Monad. We can alway re-implement "catMaybes." Antoine