为什么Data.Sequence中缺少takeR，dropR和splitAtR？

Question

Data.Sequence has takeWhileR and dropWhileR for efficient deconstruction of Seq s from the right. However, takeR , dropR and splitAtR are conspicuously absent. take and drop are implemented in terms of splitAt . So, do finger trees not admit an efficient splitAtR or was this functionality not included for some other reason?

(Separate but somewhat related question: Would a naive dropR implementation in terms of viewR perform decently well?)

This question is based on containers-0.5.6.3 .

Answer 1

length is O(1), so splitAt suffices to define everything you need, in an efficient way.

 splitAtR i s = splitAt (length s - i) s
 takeR i s = snd $ splitAtR i s
 dropR i s = fst $ splitAtR i s

According to the docs, splitAt costs O(log(min(i,length si))) , so by symmetry splitAtR costs the same (just an additional +O(1) , which we can neglect).