Begin Expression Variable State Rule

If Python is in the "begin_expr" state at time t, and the current expression is variable x, and there are no ancestor expressions, then at time t + 1, the state is "end_expr." More formally:

if the following are true:

  • expression state at time t = "begin_expr"
  • the expression at time t = v
  • v is a variable
  • parent stack at time t = [ ]

then expression state at time (t + 1) = "end_expr"

If the expression is a variable, then the evaluation is complete in 1 step (look up the variables map). If there is no parent expression to return to, the evaluation is complete.

For example,

if the following are true:

  • the expression at time 5 = x
  • expression state at time 5 = "begin_expr"
  • parent stack at time 5 = [ ]

then expression state at time (5 + 1) = "end_expr"

This simplifies to:

expression state at time 6 = "end_expr"

Try stepping through the simulator to see the "Expression State" change to "end_expr."

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