Question

I want to get a list but the last and the first elements. What is the most efficient way?

middle = init . tail

or:

middle = tail . init

And in the case of drop the n-first elements and the n-last elements?

Answer 1

It doesn’t matter (much) either way. Note that tail is O(1) whereas init is O(n). If you take the tail first then init has to examine (n-1) constructors, for a total of n constructors examined.

If you take the init first then init examines n constructors and tail examines 1, for a total of n+1 constructors examined.

So it’s every so slightly more efficient to do init . tail, but not so much more efficient that you should worry about it.

The same applies to dropping the first k elements from the front and end of the list, except that it might make more of a difference. If you drop the initial elements first, then they don’t have to be examined when dropping the final elements, and you save a bit of time, so you should do

dropLast k . drop k

which examines n constructors, rather than

drop k . dropLast k

which examines n+k constructors.