Question

I am trying to implement the quicksort algorithm in OCaml, and I thought I had it, but it won’t compile, and I just can’t see what is wrong with it. Here’s my code:

let rec quicksort list =
    match list with
    [] -> []
    |h::t -> append((quicksort (filter (largerthan h)
    t))(quicksort(filter (smallerthan h) t)));;

let largerthan x y =
    x<y;;

let smallerthan x y =
    x>y;;

let rec append x y =
match x with
[] -> y
| h::t -> h:: append t y;;  

let rec filter f list =
   match list with
   [] -> []
   |h::t -> (if f h = true then h:: filter f t else filter f t);; 

Now, when I try to use this in OCaml, it says “Error: This expression has type ‘a -> ‘b
but an expression was expected of type ‘a” while pointing to the last line of my quicksort function.

Does anybody know what’s going wrong??

Thanks a lot!

Linus

Edit: Okay, I’ve gotten rid of the original error (thanks to ADEpt :)). However, now the function just outputs an empty list regardless of the input… Does anybody know what’s going on there??

Answer 1

You have extra parens in the “apply” call. Instead of:

append((quicksort (filter (largerthan h)
t))(quicksort(filter (smallerthan h) t))

Write this:

append (quicksort (filter (largerthan h) t))
(quicksort (filter (smallerthan h) t))