N Queens in F#

NQueens.fs

type position = int * int
type board = position list

let queens n = 
    let isNotAttack (p1 : position) (p2 : position) = 
        fst p1 <> fst p2 && snd p1 <> snd p2 && abs (fst p1 - fst p2) <> abs (snd p1 - snd p2)

    let isSafe (b : board) = 
        match b with 
        | [] -> true
        | h :: tail -> List.forall (isNotAttack h) tail 

    let addQueen n (bs : board list) : board list = 
        [for b in bs do yield! [for col in 0 .. (n-1) do yield (b.Length, col) :: b]]
        |> List.filter isSafe
 
    List.fold (fun acc _ -> addQueen n acc) [[]] [0 .. (n-1)]