1. Is there a more general version of `null`? (e.g. for a Monad, Functor, Applicative, Traversable or the like.) The closest I can come up with is, in decreasing clunkiness:
zero :: (MonadPlus m, Eq (m a)) => m a -> Bool
zero = m == mzero
zero :: (Alternative f, Eq (f a)) => f a -> Bool
zero = m == empty
zero :: (Monoid m, Eq m) => m -> Bool
zero = m == mempty
Though requiring Eq seems ugly and unnecessary, in theory.
2. In that vein, is there an existing function for "a value or a default if it's zero"? E.g.:
orElse :: (Monoid m) => m -> m -> m
a `orElse` b = if zero a then b else a
Thank you,