rot90.fs

let rot90 m =
    let rec rot90 m acc =
        if List.isEmpty <| List.head m then
            List.rev acc
        else
            rot90 (List.map List.tail m) (List.map List.head m :: acc)
    in
        rot90 m []

let matrix = [[1; 2; 3;];
              [6; 5; 4]]

let matrix' = rot90 matrix

List.iter (printfn "%A") matrix'