Hi Alvaro,

as for your second question

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

There is the function orElse from the syb package [1] that works on (Maybe a) values. It can be considered a particular instance of the above with mempty = Nothing.

Alternatively, the function fromMaybe from Data.Maybe [2] provides a similar functionality, but with the heterogeneous type

fromMaybe :: a -> Maybe a -> a

Essentially, in both cases the zero predicate is specialised to a pattern matching for Nothing, which doesn't require an Eq instance. Also, there is no need for a mappend function, which may be more convenient.

Best regards,

Nikita

[1] http://hackage.haskell.org/package/syb-0.4.1/docs/Data-Generics-Aliases.html#v:orElse

[2] http://hackage.haskell.org/package/base-4.6.0.1/docs/Data-Maybe.html#v:fromMaybe

On 29/01/14 07:46, Michael Snoyman wrote:



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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-- 
Dipl.-Math. Nikita Danilenko
Research group:
Computer Aided Program Development
Kiel University
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Phone: +49 431 880 7275
URL: https://www.informatik.uni-kiel.de/index.php?id=nikita