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
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
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