Augmented sequence deletion

[David Feuer]

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 <zemyla at gmail.com> 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 <david.feuer at gmail.com 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?
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>
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