[Haskell] Looking for a random-access sequence data structure

[Ross Paterson]

On Fri, Jan 13, 2006 at 01:18:35PM +0000, Duncan Coutts wrote:
> I've been looking around (unsuccessfully) for an efficient data
> structure to implement a sequence data type with indexed
> insert/delete/lookup.
> 
> lookup :: Sequence a -> Int -> Maybe a
> insert :: Sequence a -> Int -> a -> Sequence a
> delete :: Sequence a -> Int -> Sequence a

Have a look at Data.Sequence (in CVS/darcs version), docs at

http://www.haskell.org/ghc/dist/current/docs/libraries/base/Data-Sequence.html

insert and delete aren't provided, but are easily derived:

	insert :: Seq a -> Int -> a -> Seq a
	insert xs i x = front >< x <| back
	  where (front, back) = splitAt i xs

	delete :: Seq a -> Int -> Seq a
	delete xs i = front >< drop 1 back
	  where (front, back) = splitAt i xs

(where splitAt and drop are the sequence versions).

Each of the three operations takes O(log(min(i,n-i))) time.