[Haskell-cafe] IO (Either a Error) question

[Ivan Lazar Miljenovic]

David Menendez <dave at zednenem.com> writes:
>
> 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 at gmail.com
IvanMiljenovic.wordpress.com