This sounds like 'unconsAt', if 'uncons' isn't already too obtuse. On December 28, 2019 8:06:21 PM UTC, David Feuer wrote:

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

-- Sent from my Android device with K-9 Mail. Please excuse my brevity.

That approach works great for maps and sets, but not so well for sequences. If a sequence has ten elements, then you can't really insert a twentieth one. On Sat, Dec 28, 2019, 7:25 PM Andreas Abel wrote:

I'd advocate a Swiss army knife like

atM :: Applicative m => Int -> (Maybe a -> m (Maybe a)) -> Seq a -> m (Seq a)

Then you can get your deleteLookup function by a suitable instance of the effect m.

Nothing stands for "index does not point to any element", "Just a" means the index points to value a.

On 2019-12-28 20:24, David Feuer wrote:

...

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

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries