Re: [Haskell-cafe] Generalized null / zero

Hi Alvaro, Not long ago I faced the same question, and ended up developing a very simplistic library IsNull: https://github.com/jcristovao/IsNull To be honest, its a very stripped down version of Mono-traversable referenced by Michael, which I highly recomend, but in my case specialized to provide some extra functions, namely nested null: isNullN (Just "abc") == False isNullN (Just "" ) == True isNullN (Nothing ) == True Now, a small note regarding: zero :: (Monoid m, Eq m) => m -> Bool zero = m == mempty This is dangerous. For example: -- | Monoid under multiplication. newtype Product a = Product { getProduct :: a } deriving (Eq, Ord, Read, Show, Bounded) instance Num a => Monoid (Product a) where mempty = Product 1 Product x `mappend` Product y = Product (x * y) This leads to: zero (Product 1) = True. Is this what you wanted? I had this in my library at first, and ended up removing it, because it might not always be what is expected. Cheers, Joao 2014/1/29 Nikita Danilenko :

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...

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

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

On Wed, Jan 29, 2014 at 4:25 AM, Alvaro J. Genial 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-MonoTr...

...

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,

Alvaro http://alva.ro

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-- Dipl.-Math. Nikita Danilenko Research group: Computer Aided Program Development Kiel University Olshausenstr. 40, D-24098 Kiel Phone: +49 431 880 7275 URL: https://www.informatik.uni-kiel.de/index.php?id=nikita

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