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

[David Luposchainsky]

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On 21.12.2014 11:54, Nikita Volkov 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

This reminds me of the commonly recommended, but wacky looking function

pureWhen :: MonadPlus m => Bool -> a -> m a
pureWhen = \p x -> x <$ guard p

which is (pure x) when p holds, and mzero otherwise. This is of course
easily generalizable to using Alternative.

Using your `cond`, one could express this as

pureWhen p = cond (const p)

which looks reasonable to me. I think this is an often used function in
one form or another, and would welcome it in the libraries.

+1

Greetings,
David/quchen

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