David Menendez
I wonder how often people rely on the use of fail in pattern matching. Could we get by without fail or unfailable patterns?
ensureCons :: MonadPlus m => [a] -> m [a] ensureCons x@(_:_) = return x ensureCons _ = mzero
do ... x:xs <- ensureCons $ some_compuation
This is more flexible than the current situation (you can easily adapt it to throw custom exceptions in ErrorT), but gets cumbersome when you're doing nested patterns. Also, it does the match twice, but presumably the optimizer can be improved to catch that if the idiom became popular.
Well, any time you have a do-block like this you're using failable patterns: maybeAdd :: Maybe Int -> Maybe Int -> Maybe Int maybeAdd mx my = do x <- mx y <- my return $ x + y -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com