Re: Proposal: (a -> Bool) -> a -> f a

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 mailto:nikita.y.volkov@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

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