Short cut fusion
Short cut fusion is an optimizer method that merges some function calls into one.
map f . map g can be substituted by
map (f . g) and
filter p . filter q can be substituted by
filter (\x -> q x && p x).
It can also help to remove intermediate data structures.
sum [1..n] does not require an explicit list structure, and the expression is actually translated into a simple loop.