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 [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries |
deleteLookup :: Int -> Seq a -> Maybe (a, Seq a) If it were written natively, it'd probably use some of the machinery from splitAt. On 13:25, Sat, Dec 28, 2019 David Feuer <[hidden email] wrote: Data.Sequence offers _______________________________________________ Libraries mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries |
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 <[hidden email]> wrote:
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This sounds like 'unconsAt', if 'uncons' isn't already too obtuse.
On December 28, 2019 8:06:21 PM UTC, David Feuer <[hidden email]> wrote:
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In reply to this post by David Feuer
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 > [hidden email] > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries > Libraries mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries |
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 <[hidden email]> wrote: I'd advocate a Swiss army knife like _______________________________________________ Libraries mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries |
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