`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:

`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 s-i)))`

, so by symmetry `splitAtR`

costs the same (just an additional `+O(1)`

, which we can neglect).

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