It seems rather funny to write a treatise, with quicksort being a central example, where the shown code requires O(N) temporary space.

C/C++ programmers might not be good at category theory, but no one worthy of their salary would walk past a quicksort routine with O(N) memory without stopping and asking "Wait, what?"

Seriously, I remember when this used to be the first Haskell code shown on haskell.org homepage, and I had to stop and wonder if these folks were just trolling or if they are actually that oblivious of performance. If you want to promote Haskell, you could have hardly chosen a worse piece of code.