Proposal: Add First and Last wrappers around Maybe to Data.Monoid

[Jeffrey Yasskin]

On 3/5/07, Andriy Palamarchuk <apa3a at yahoo.com> wrote:
> Jeffrey, would it be difficult to add an example or
> two for the intended usage?

Not at all. Two reasonable examples are:

> findLast :: Foldable t => (a -> Bool) -> t a -> Maybe a
> findLast pred = getLast . foldMap (\x -> if pred x then Last (Just x) else Last Nothing)

And, assuming Data.Map were extended with
  alterA :: (Applicative f, Ord k) => (Maybe a -> f (Maybe a)) -> k ->
Map k a -> f (Map k a)

> instance Monoid a => Applicative ((,) a)  -- from Control.Applicative
>
> insertLookupWithKey :: Ord k => (k -> v -> v -> v) -> k -> v -> Map k v -> (Maybe v, Map k v)
> insertLookupWithKey combine key value =
>   Arrow.first getFirst . alterA doChange key
>   where
>   doChange Nothing = (First Nothing, Just value)
>   doChange (Just oldValue) =
>     (First (Just oldValue),
>      Just (combine key value oldValue))

I haven't run these yet, so they may need some tweaking.

Since the rest of Data.Monoid doesn't have example uses, where do you
want me to put these?


Jeffrey