Proposal: (a -> Bool) -> a -> f a
Andreas Abel
abela at chalmers.se
Sun Dec 21 11:33:36 UTC 2014
On 21.12.2014 12:28, Oliver Charles wrote:
> I tend to use `pure` and `mfilter` for this case:
>
> \x -> fromMaybe 0 $ mfilter (> 0) $ pure (x - 10)
That's what I thought as well.
'Alternative' seems like a bit of an overkill. cond works for
everything that has 'empty' and is pointed (accommodates singletons).
import Agda.Utils.Null
import Agda.Utils.Singleton
singletonWhen p a = if p a then singleton a else empty
Cheers,
Andreas
> On Sun, Dec 21, 2014 at 10:54 AM, Nikita Volkov
> <nikita.y.volkov at gmail.com <mailto:nikita.y.volkov at gmail.com>> wrote:
>
> I propose to add the following utility function into
> |Control.Applicative|:
>
> |cond :: (Alternative f) => (a -> Bool) -> a -> f a
> cond p a = if p a then pure a else empty
> |
>
> Following is a typical use case:
>
> |\x -> fromMaybe 0 $ cond (> 0) $ x - 10
> |
>
> which is the same as
>
> |\x -> let y = x - 10 in if y > 0 then y else 0
> |
>
> Why the first one is better:
>
> 1.
>
> The control flow is evident and goes from right to left step by
> step.
> It’s not scattered around like in the second example.
>
> 2.
>
> No need to interrupt to imagine a name for a temporary variable.
>
> 3.
>
> Less noise.
>
> Now, since it’s generalised to |Alternative| one can imagine tons of
> other useful cases for this.
>
> Alternative titles:
>
> *
>
> |conditional|
>
> *
>
> |partial|
>
> Best regards,
> Nikita
>
>
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org <mailto:Libraries at haskell.org>
> http://www.haskell.org/mailman/listinfo/libraries
>
>
>
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries
>
--
Andreas Abel <>< Du bist der geliebte Mensch.
Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden
andreas.abel at gu.se
http://www2.tcs.ifi.lmu.de/~abel/
More information about the Libraries
mailing list