Also note that this optimisation depends on O(1) access from right to left ( or just plain O(1) random access ).

The Haskell version uses cons-lists, so no cheap right-to-left access. Other non-esoteric languages probably all have it though.

LC requires exporting snoc as cons for this to work.

There is the eternal tension between performance and simplicity, readability and maintainability, in the light of the ranking of the kata.

I tried once, for a too short while, to achieve what you did, but didn't think it through enough and gave up too quickly; I never got there. Kudos for your solution.

But being disappointed in less performant but correct and readable solutions to a 7kyu kata seems setting yourself up for a lot of disappointment. :P