On Wed, Jan 29, 2014 at 4:25 AM, Alvaro J. Genial <genial@alva.ro> wrote:
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.


You can try out onull[1], which will work on any MonoFoldable. That allows it to work with classical Foldable instances (like a list or Maybe), but also monomorphic containers like ByteString or Text.

[1] http://hackage.haskell.org/package/mono-traversable-0.2.0.0/docs/Data-MonoTraversable.html#v:onull
 
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,


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