[Closed] Callback doubt calling a function

Ok i have a problem, i have a function called pri() and i want to execute it back when i change the viewport, but i receive an error, here is my script:

rollout pri_v "callback test" width:450 height:400
(
	
	checkbutton start "GO!" checked:false
	button stop "NO GO!"

	
	fn pri = (
		print "c"
	)
	
	fn pro = (
		print "b"
	)
		
	on start changed state do
	(
			if state == on then
			(
			pri()
			callbacks.addScript #viewportChange "pri()" id:#pri_test
			)
			else
			(
			pro()
			callbacks.removeScripts #viewportChange id:#pri_test
			)
		
	)

	on stop pressed do (
		pro()
		callbacks.removeScripts #viewportChange id:#pri_test
	)

)
spdfloater = newRolloutFloater "callback test"  460 90
addRollout pri_v spdfloater

and here is the output when i move the viewport:

-- Error occurred in anonymous codeblock; filename: none; position: 0
--  Frame:
--   pri: undefined
>> MAXScript Callback script Exception: -- Type error: Call needs function or class, got: undefined <<
--   called in anonymous codeblock
--  Frame:
--   pri: undefined
>> MAXScript Callback script Exception: -- Type error: Call needs function or class, got: undefined <<
"b"

Help me please, i don’t understand why is happening that.