Given the recent faster trie implementations (hooray!), I thought it
would be worth pointing out that a pretty thorough discussion of the
tradeoffs of functional trie design is available at
http://www.nada.kth.se/~snilsson/publications/Functional-trie-compression-experiment/
An experimental study of compression methods for functional tries
Jukka-Pekka Iivonen, Stefan Nilsson, and Matti Tikkanen
WAAAPL'99
FWIW, apparently clojure uses a 32-ary hashtrie:
http://blog.ezyang.com/2010/03/the-case-of-the-hash-array-mapped-trie/
http://blog.higher-order.net/2009/09/08/understanding-clojures-persistenthashmap-deftwice/
Because it is only binary, Data.IntSet is sometimes quite deep. All of
that pointer chasing makes Data.Set actually faster than Data.IntSet
for looking up 0 in the set 0:[2^i | i <-[0..30]].
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