Add function for (Maybe a, b) -> Maybe (a, b) about extra

[ndmitchell]

Thanks for the suggestion. How often do you think these patterns occur in practice? Would be good to get a sense of how useful they would be.

[qwbarch]

Thanks for the reply! For me it's when I'm traversing a list and zip it up to return a map:

• traverse a list that returns [Maybe b]
• zip the previous list with [a]
• convert [(a, Maybe B)] to [Maybe (a, b)]
• keep only the available values: [(a, b)]

For example:

newtype UserId = UserId Int deriving (Eq, Ord)
data User

getUserById :: UserId -> m (Maybe User)
getUserById = undefined

getUsersById :: Monad m => [UserId] -> m (Map UserId User)
getUsersById userIds =
  fromList . mapMaybe maybeSnd . zip userIds <$> traverse getUserById userIds

[ndmitchell]

Would be interesting to get a sense of how often this occurs. One way would be to add an hlint rule and run it over a reasonable volume of Haskell code to show how often this shows up (or maybe just grep for it). Another would be to ask on Twitter/Reddit or similar if people have this issue.

[phadej]

strong :: Functor f => (a, f b) -> f (a, b)
strong (a, fb) = fmap (a,) fb

is common enough pattern it has a name (strong functors), and all haskell Functors are strong in respect to products. But you would write it as uncurried in Haskell

strong :: Functor f => a -> f b -> f (a, b)
strong a fb = fmap (a,) fb

and then

getUsersById :: Monad m => [UserId] -> m (Map UserId User)
getUsersById userIds =
  fromList . mapMaybe maybeSnd . zip userIds <$> traverse getUserById userIds

I'd write that as

getUsersById :: Monad m => [UserId] -> m (Map UserId User)
getUsersById usedIds = fromList . catMaybes <$> forM userIds $ \usedId ->
    user <- getUsersById
    return (strong userId user)

but here it's easy to inline fmap (userId,) user as well.

The fmap (something,) is not uncommon on Hackage: https://hackage-search.serokell.io/?q=fmap+%5C%28%5Ba-zA-Z%5D%2B%2C%5C%29, but I'm not sure it actually would benefit from a name

---

\xs -> catMaybes <$> traverse f xs

is generalized to witherM from witherable. (I'd appreaciate a good name for flipped variant, like we have mapM and forM or traverse and for) (FWIW: similar request #87 for foldMap)