F＃List.iter不打印

Question

我在F＃中尝试OOP并遇到了问题。我正在尝试构建一个无人机和空域类，但由于某种原因我的printDrones功能不起作用，所以我无法检查其余部分是否有效。我没有得到任何错误，但该功能只是没有打印任何东西。我想我的类型或列表函数可能有些问题，但我已经尝试了很多东西，现在我要放弃了。希望有人可以帮助我！

type Drone(startposition: (float * float), destination: (float * float), speed: float) =
    let mutable position = startposition
    let a = fst destination - fst position
    let b = snd destination - snd position
    let c = sqrt(a**2.0 + b**2.0)
    let speeddir = (a/c, b/c)
    member this.fly =
        position <- (fst position + (fst speeddir * speed), snd position + (snd speeddir * speed))
    member this.getPos = position
    member this.isFinished =
        (fst startposition - fst destination)*(fst position - fst destination) < 0.0


type Airspace() =
    let mutable drones: Drone List = []
    member this.droneDist (d1: Drone) (d2: Drone) : float =
        let d1pos = d1.getPos
        let d2pos = d2.getPos
        let a = fst d1pos - fst d2pos
        let b = snd d1pos - snd d2pos
        sqrt(a**2.0+ b**2.0)
    member this.flyDrones() =
        drones |> List.iter (fun x -> x.fly)
    member this.addDrone(dr: Drone) =
        drones <- dr :: drones
    member this.printDrones() =
        drones |> List.iter (fun x -> printfn "%A" (x.getPos))

let aspace = Airspace()

aspace.addDrone(Drone((50.0,100.0),(0.0,0.0), 20.0))
aspace.addDrone(Drone((0.0,100.0),(0.0,0.0), 20.0))
aspace.addDrone(Drone((25.0,100.0),(0.0,0.0), 20.0))
aspace.addDrone(Drone((75.0,100.0),(0.0,0.0), 20.0))
aspace.flyDrones
aspace.printDrones