Question

I have a simple f# quick sort function defined as:

let rec qsort(xs:List<int>) =

let smaller = xs |> List.filter(fun e -> e < xs.Head)
let larger = xs |> List.filter(fun e -> e > xs.Head)
match xs with
| [] -> []
| _ -> qsort(smaller)@[xs.Head]@qsort(larger)

Is there a way in f# to write it more like Haskell:

qsort       :: [Int] -> [Int]
qsort []     = []
qsort (x:xs) =
qsort smaller ++ [x] ++ qsort larger
where
  smaller = [a | a <- xs, a <= x]
  larger  = [b | b <- xs, b >= x]

I know the f# algorithm is missing a <= and >=. The question is more about syntax/readibility.

Thanks.

Answer 1

You want your second match clause to be x :: xs, and to use the @ (append) operator where your Haskell example uses ++:

let rec qsort xs =
  match xs with
  | [] -> []
  | x :: xs ->
      let smaller = qsort (xs |> List.filter(fun e -> e <= x))
      let larger  = qsort (xs |> List.filter(fun e -> e >  x))
      smaller @ [x] @ larger

It’s not quite the same as the Haskell definition by cases syntax, but hopefully similar enough for you!