Using local variables inside WhenEvent

Question

Using WhenEvent inside Module is quite strange. The local variables are not evaluated in WhenEvent. For example

eq = Module[{time1 = 200, time2 = 400},
  {WhenEvent[t > time1, a[t] -> 1],
   WhenEvent[t > time2, a[t] -> 0]}]

returns

{WhenEvent[t > time1$1064, a[t] -> 1], 
 WhenEvent[t > time2$1064, a[t] -> 0]}

I guess it has something to do with the attribute of the WhenEvent?

Attributes[WhenEvent]
(* {HoldAll, Protected} *)

Is there anyway to force the local variables in WhenEvent to be evaluated?

Answer 1

Yes, it's related to HoldAll, so, just Evaluate:

Module[{time = 200}, WhenEvent[t > time // Evaluate, a[t] -> 1]]

Alternatively, use With instead:

With[{time = 200}, WhenEvent[t > time, a[t] -> 1]]

To understand the behavior of With you may want to read:

What are the use cases for different scoping constructs?