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

[Oliver Charles]

I tend to use `pure` and `mfilter` for this case:

\x -> fromMaybe 0 $ mfilter (> 0) $ pure (x - 10)

-- ocharles

On Sun, Dec 21, 2014 at 10:54 AM, Nikita Volkov <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
> ​
>
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