Re: Augmented sequence deletion

Written natively, it would surely borrow the machinery of deleteAt, which does quite a bit less reshuffling. It's actually a finger-twisted version of a classical 2-3 tree deletion. On Sat, Dec 28, 2019, 2:59 PM Zemyla wrote:

deleteLookup :: Int -> Seq a -> Maybe (a, Seq a) deleteLookup n q = case Seq.splitAt n q of (ql, qr) -> case Seq.viewl qr of Seq.EmptyL -> Nothing (Seq.:<) a qr' -> Just (a, ql <> qr')

If it were written natively, it'd probably use some of the machinery from splitAt.

On 13:25, Sat, Dec 28, 2019 David Feuer

...

Data.Sequence offers

deleteAt :: Int -> Seq a -> Seq a

which deletes the element at the given index. Today, I ran into a situation where I wanted to know what was deleted.

deleteLookup :: Int -> Seq a -> Maybe (a, Seq a)

The closest thing I can find in `containers` is in Data.Map:

updateLookupWithKey :: Ord k => (k -> a -> Maybe a) -> k -> Map k a -> (Maybe a,Map k a)

Unfortunately, that function is ugly and strange. A better one, whose name I can't guess at the moment:

flabbergast :: (a -> (b, Maybe a)) -> Int -> Seq a -> Maybe (b, Seq a)

where a Nothing result means the index was out of bounds. There's also a potential

flabbergastF :: Functor f => (a -> f (Maybe a)) -> Int -> Seq a -> Maybe (f (Seq a))

I'm not sure if flabbergast can be made as fast as deleteLookup, so it's possible we may want both. Any opinions? _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries