N Queens puzzle in OCaml

nqueens.ml

type pos = Position of int * int 

let noAttack (Position (x, y)) (Position (i, j)) = 
    x <> i && y <> j && abs (x-i) <> abs (y-j)

let rec range n m =
    if m < n 
        then [] 
        else n :: range (n+1) m

let gen row =
    range 1 8 
    |> List.map (fun column -> Position (row, column))

let safe n allegedSolution = match allegedSolution with 
    | imposter :: solutions -> 
        List.for_all (fun pos -> noAttack pos imposter) solutions
    | [] -> false

let rec queens = function
    | 0 -> [[]]
    | n -> 
        queens (n-1)
        |> List.map (fun solution -> List.map (fun imposter -> imposter :: solution) (gen n))
        |> List.concat
        |> List.filter (safe n)