Augmented sequence deletion

[Keith]

This sounds like 'unconsAt', if 'uncons' isn't already too obtuse.

On December 28, 2019 8:06:21 PM UTC, David Feuer <david.feuer at gmail.com> 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 <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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