Trouble passing panel.data through an inline function for mouseCallback

[VirtualVirgin]

I am trying to make my panel data drag and drop more modular by making an inline function for the mouseCallback:

	p.setMouseCallback(function(event)
	{	
		altDragAndDrop(this.data.test);
	});

inline function altDragAndDrop(dataCopy)
{
	// show "+" cursor when alt/option is pressed
	local cursor;
	event.altDown ? cursor = "CopyingCursor" : cursor = "NormalCursor";
	this.setMouseCursor(cursor, Colours.white, 1);
	// alt/option drag copies to clipboard
	if (event.drag && event.altDown)
	{
		Clipboard = dataCopy;
	}
	// releasing the mouse on a panel copies the clipboard to the panel, then clears the clipboard
	if (!event.clicked && event.altDown && dataCopy != Clipboard && Clipboard != undefined)
	{
		dataCopy = Clipboard;
		this.repaint();
		Clipboard = undefined;
	}
};

From the mouseCallback, "this" and "event" and my "Clipboard" are all passing into the inline function properly and can be seen in Console printouts:

But my problem is that trying to use the this.data.test passed into the inline function from my panels is only half-working.
It will take that reference and copy the dataCopy (this.data.test) contents (an object) to the Clipboard,
But then it does not allow me to copy Clipboard contents to the dataCopy (this.data.test).

Interface:! Line 65, column 12: Cannot assign to this expression!

So a strange thing-
If I just replace the argument name in (dataCopy) with a direct reference this.data.test in that particular spot, the code now works without any errors,
but that defeats the basic convenience of having an inline function.

[d.healey]

@VirtualVirgin Parameters are passed by value, you can't change them within the function.

Minimal example:

reg data = 10;

test(data);

inline function test(myParam)
{
    myParam = 50;
}

It's the same as writing 10 = 50.

[VirtualVirgin]

@d-healey I thought in this case the parameter is the address for the data "this.data.test" being a path, not the actual data.
So I was thinking the path variable is static, but it is just rewriting the contents at that path.
Anyway, I fixed it by using .getValue() and .setValue():

	p.set("allowCallbacks", "All Callbacks");
	p.setMouseCallback(function(event)
	{	
		altDragAndDrop();
	});

inline function altDragAndDrop()
{
	// show "+" cursor when alt/option is pressed
	local cursor;
	event.altDown ? cursor = "CopyingCursor" : cursor = "NormalCursor";
	this.setMouseCursor(cursor, Colours.white, 1);
	// alt/option drag copies to clipboard
	if (event.drag && event.altDown)
	{
		Clipboard = this.getValue();
	}
	// releasing the mouse on a panel copies the clipboard to the panel, then clears the clipboard
	if (!event.clicked && event.altDown && this.getValue() != Clipboard && Clipboard != undefined)
	{
		this.setValue(Clipboard);
		this.repaint();
		Clipboard = undefined;
	}
};