f# - 在f＃中向列表添加内容

Question

我正在f＃中执行一个名为“ 2D Shape editor”的项目。之前我已经在C＃中完成了这个项目，所以我已经掌握了如何连接两个形状的所有逻辑。因此，我知道我将需要一个列表来保存要添加的所有泰斯形状。但是我根本无法使我的addToList方法正常工作。

我的ShapeList：

let mutable ShapeList:List<RectangleZ> =  [RectangleZ(100,100)] 

我的添加方法：

let addToList (listan:List<RectangleZ>) (element:RectangleZ) = let ShapeList =     ShapeList@[element] in ShapeList
//Method to add into the ShapeList

let addToList (listan:List<RectangleZ>) (element:RectangleZ) = element::ShapeList
//Other try on adding into shapeList

应该在ShapeList中添加矩形的按钮：

btn.Click.Add(fun _ -> new RectangleZ(500, 100) |> addToList ShapeList |>ignore |> saver)
//Button click method that should be adding the RectangleZ(500, 100) to my ShapeList

当然还有我的矩形：

type RectangleZ(x:int, y:int)= 
  let mutable thisx = x
  let mutable thisy = y
  let mutable thiswidth = 50
  let mutable thisheight = 20
  let brush = new SolidBrush(Color.Black)
  member obj.x with get () = thisx and set x = thisx <- x
  member obj.y with get () = thisy and set y = thisy <- y
  member obj.width with get () = thiswidth and set width = thiswidth <- width
  member obj.height with get () = thisheight and set height = thisheight <- height
  member obj.thisColor = Color.FromArgb(167, 198, 253)
  member obj.draw(paper:Graphics) = paper.FillRectangle(brush, thisx, thisy, 50, 20)
  member obj.ShapeType = "Rectangle"

由于某种原因，两个我的addToList函数都没有将该元素添加到列表中。我的问题是为什么？

Answer 1

F＃中的列表是不可变的。这意味着当您将项目添加到列表中时，如下所示：

let newlist = elem :: tail;;

旧列表（尾部）不会更改，而是会创建新列表。因此，您需要从 addToList函数返回新列表，然后更新可变变量：

let addToList (listan:List<RectangleZ>) (element:RectangleZ) = element::listan
ShapeList <- addToList ShapeList newElement

在您的代码中， let ShapeList是局部的，不会影响全局ShapeList变量。