Function doesn't execute?

function playstr(msg, text)
	for i = 1, string.len(msg) do
		text = string.sub(msg, 1, i)
		wait()
	end
end

playstr("Hi!", ScreenGui.Frame.TextLabel.Text)

This function is supposed to make a message play one character at a time when called, like so:

H
Hi
Hi!

However, when the function is called, the script seems to skip over the function entirely. For example, the script I have plays a message, waits for 2 seconds, then disappears. The script doesn't change any text, but still waits 2 seconds and removes the message. There are no notes in output. What's wrong with it?

When you pass ScreenGui.Frame.TextLabel.Text, it just gets the text at that time (which is probably blank). It doesn't "link" the text parameter to that property:

When you set the text parameter, you just change the text parameter. It doesn't remember that it came from a TextLabel.

In other words, you didn't tell it to change any property of any text label, so it didn't.

You have to explicitly change properties to modify objects:

function playstr(msg, label)
    for i = 1, string.len(msg) do
        label.Text = string.sub(msg, 1, i)
        wait()
    end
end

playstr("Hi!", ScreenGui.Frame.TextLabel)

There's also a nice syntax, msg:sub(1, i) which I think is often much more readable.