Adding stuff to a list in f#

Question

I'm doing a project called "2D Shape editor" in f#. I have done this project in c# before so I've got all the logics for how to connect two shapes. So I know that i will need a list to hold all theese shapes that I will be adding. But I simply can't get my addToList method to work.

My ShapeList:

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

My add methods:

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

the button that should be adding rectangles to the ShapeList:

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

And ofcourse my rectangle:

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"

The element dosn't get added into the list for some reason in neither of my addToList functions. My Question is why?

Answer 1

List in F# are immutable. This means that when you add item to list like this:

let newlist = elem :: tail;;

old list (tail) doesn't changes, instead of that new list created. So, you need to return new list from your addToList function and than update mutable variable:

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

In your code let ShapeList is local and doesn't affect global ShapeList variable.

Answer 2

let newList = oldList @ [newElement]

Answer 3

You can use List.append with mutable lists, the below example worked fine with me:

let mutable season_averages = []
for i in 0 .. n_seasons do
     season_averages  <- [i] |> List.append season_averages
printfn "Seasons Average: %A" season_averages