tap action only possible for items on active screen

I'd like to add on-tap-actions to different items of my face, but it should only react, if the the item is visible on the active screen.

I tried with a script in tap action, for example:
var_screen==1 and var_1 = var_1 + 1 or var_1=var_1
but this will not work :-(

If I define a global Function, it works, but how is the syntax for open apps on the watch?
For example: (in this example I'd need the right expressin for line 4: "watch: alarm"

function set_alarm()
if var_screen == 1
then
watch: alarm
else
0
end
end

Greetings Radagast