Although this version is very simple, concatenating and creating lists at runtime ruins performance (and it takes extra memory).
Please make it a good example by doing a version of Quicksort with in-place partition.
That's actually a nice nab for functional programmers. They run around all the time telling how nice their language of choice is, showing off quicksort in three lines of code.
But please don't have the audacity of pointing out that original quicksort was supposed to be an efficient in place sort, not a mental exercise in beauty of a language. Then you get a Haskell version that is in place, and supposedly fast. It's about 10 lines longer than the C version, uses Monads, ST, recursion, "unsafeRead/Write", "rscan", "lscan", "swap", "sloop", etc., all together about 20 concepts, built-in functions, etc.
Also, of course, if it really should be fast, you have to use several compiler hints and macros to make it type specialize and all. Obviously.
The C version is shorter, uses nothing but array accesses, do/while, if, int comparisons, and recursion. If you want to explain the C version to a novice programmer, it will maybe take you half a day. The inplace Haskell version? Uhhh, yeah.
I can't help it, but things like these leave me with the impression of Haskell being a theoretically really nice language whose utility in actually getting things done seems somewhat questionable.
