[Haskell-cafe] Generalized null / zero

[João Cristóvão]

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 <nda at informatik.uni-kiel.de>:
> 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 at 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,
>>
>> Alvaro
>> http://alva.ro
>>
>> _______________________________________________
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>>
>
>
>
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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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