(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
<ml@isaac.cedarswampstudios.org> wrote:
Louis Wasserman wrote:
This patch contains several methods for Data.Sequence I've always thought were long missing:
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