A paper relating to Data.Set and Data.Map

Kazu Yamamoto ( 山本和彦 ) kazu at iij.ad.jp
Tue Apr 19 03:20:40 CEST 2011

Hello libraries and cafe,

We (Hirai and I) would like to tell you our paper relating to
Data.Set and Data.Map. "Balancing Weight-Balanced Trees" is now
accepted and will appear in Journal of Functional Programming.
The camera-ready version of the paper is available from:


Please recall that Taylor Campbell reported a bug of Data.Map in last
Summer. In some cases, the balance of Data.Map is broken after delete
operations. This triggered our research.


Though Data.Set/Data.Map are based of a variant Weight-Balanced tree
by Adams, we target the original Weight-Balanced tree by Nievergelt
and Reingold. They are parameterized algorithms and the difference of
two algorithms is quite small. But the original has smaller conditions
which are gentle for proof.

We identified the valid area of two parameters of the original
Weight-Balanced tree and showed <3,2> is only one integer solution.
Soundness is proved in Coq and completeness is verified with four
algorithms to generate counterexamples for the outside of the valid

"wttree.scm" of MIT/GNU Scheme and slib have already incorporated our
fixes. When I offered our fixes to MIT/GNU Scheme, Taylor Campbell
appeared again. I understand that he is an MIT/GNU Scheme guy and
found the bug of Data.Map when re-implementing "wttree.scm" consulting
Data.Map. :-)

We think it's Haskell community turn now. Data.Set/Data.Map are still
based on the variant whose soundness is not proved yet. If interested,
let's discuss whether or not we should replace the variant with the
original in Data.Set/Data.Map.

We guess that Milan Straka, a big contributer of the container
package, has his opinion. We welcome opinions from other people, two.



More information about the Libraries mailing list