[Haskell-cafe] Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a)

[Tom Ellis]

On Tue, Nov 11, 2014 at 08:57:48AM -0800, Evan Laforge wrote:
> try :: Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a)
> try action alternative = maybe alternative (return . Just) =<< action

Looks like the MonadPlus instance for MaybeT to me

    runMaybeT $ MaybeT (print "first" >> return (Just 1))
                `mplus` MaybeT (print "second" >> return (Just 2))

    Just 1

    runMaybeT $ MaybeT (print "first" >> return Nothing)
                `mplus` MaybeT (print "second" >> return (Just 2))

    "first"
    "second"
    Just 2

Tom