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Convert (maybe a, b) to Maybe (a, b)

Is there a simple (no wheel reuse) to convert from (Maybe a, b)

to Maybe (a,b)

. I have looked at Traversable

but cannot get it to work.

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4 answers


You can also use Bitraversable

:

bitraverse id pure :: (Bitraversable t, Applicative f) => t (f c) d -> f (t c d)


which specializes in

bitraverse id pure :: (Maybe a, b) -> Maybe (a, b)
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Yes. Functors.

solution :: (Maybe a, b) -> Maybe (a, b)
solution (a, b) = fmap (\q -> (q, b)) a

Or, in a style without spaces:

solution = uncurry $ flip $ fmap . flip (,)


Also, as @Bakuriu said, if you include TupleSections

and Control.Applicative

, it becomes very simple:

solution (a, b) = (, b) <$> a

More details here .

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The following works with lens

:

> import Control.Lens
> _1 id (Just 1, 2)
Just (1, 2)
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What do you mean by "without inventing the wheel"? The easiest way to create such a function is as follows:

f :: Maybe a -> b -> Maybe (a, b)
f Nothing  _ = Nothing
f (Just a) b = Just (a, b)

g :: (Maybe a, b) -> Maybe (a, b)
g = uncurry f

Hope it helps.

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