[pklausler]

mean = liftA2 (/) sum length

[Kranar]

I mean that could satisfy the interviewer, but for me if I asked that question, I'd want an answer that protected against overflow and unfortunately the naive solution is prone to overflow.

[whatshisface]

That could turn in to a very complex question in that case because dividing too many separate sums before adding would introduce rounding error.

[anyfoo]

Does that not have to be

    mean = liftA2 (/) sum (fromIntegral . length)

?