(Thanks to Isaac for the n00b-friendly instruction link!)
http://hackage.haskell.org/trac/ghc/ticket/3271
Data.Sequence is meant as a general-purpose implementation of finite
sequences. The range of methods it offers, however, is considerably more
constrained than Data.List, even allowing for the constraint that sequences
are finite.
The following methods cannot be efficiently implemented based on currently
exported methods from Data.Sequence.
- zipWith and its variants. Note: it suffices to define zipWith alone.
- replicate. (This can be implemented in O(log n) time with node
sharing.)
The following methods are relatively simple extensions of already-exported
methods. It may be more appropriate to allow library users to reimplement
them themselves.
- scanl, scanr, and variants. This is relatively straightforward using
methods borrowed from the Traversable instance.
- tails and inits. tails is equivalent to scanr (<|) empty; inits is
similar.
- takeWhile, dropWhile, span, break (and perhaps from-the-end
variations). Finding a breakpoint index can be done as efficiently on lists
as on sequences; find the appropriate breakpoint index after converting to a
list and use splitAt.
- unfoldr and iterate, though the latter would require a finite number of
iterations to perform.
- partition. I can conceive of a slightly more efficient implementation
than the trivial one, but the gains may be minimal.
Discussion deadline: two weeks from today (June 16).
Of these, I have implemented the first three bullets.
Louis Wasserman
wasserman.louis@gmail.com
On Mon, Jun 1, 2009 at 9:52 PM, Isaac Dupree wrote: Louis Wasserman wrote: This patch contains several methods for Data.Sequence I've always thought
were long missing: Excellent! Can you make an official "library proposal"? It's not hard.
http://www.haskell.org/haskellwiki/Library_submissions Basically, see section 2: make a Trac ticket; choose a discussion deadline
(perhaps, 2 weeks from now); and then post back on this mailing-list
including a link to the ticket, a copy of the description, and stating the
end-of-discussion date. -Isaac