Evaluation View
This tutorial describes the use of ProB's evaluation view to explore single states of a model. The view shows the details of a particular state during the animation. It can be used to
- learn about the values of a some variables. This feature is overlapping with the State Properties View in the bottom left section of ProB's main window, but the evaluation view allows to inspect a value in greater detail.
- understand the truth value of the invariant and its sub-formulas and the values of the sub-expressions.
- export the content of variables in a state to use them outside of ProB.
Inspecting State Variables and Constants
As an example specification we use the Sieve.mch delivered with the ProB Distribution in the "Less Simple" folder. After opening the model do a couple of animation steps and then open the Evaluation view in the Analyse menu. You should get window that looks similar to the following screenshot:
You can now expand each of the three sections to investigate the current state of the machine. For instance if we expand the Variables section we will get the following:
The tree shows values for each variable in the same way as the State Properties view. The value of the variable numbers in our example tell us that card(numbers) = 2288 and it shows the first and last entries. In contrast to the State Properties View you can see all values by doubleclicking an entry as shown in the next screenshot.
The save button can be used to save the value of the variable (as a B-Expression) to a file. You can also save the values of multiple variables (we will cover this later).
Understanding the Invariant and Properties
The second objective of the evaluation view is to help understanding why the invariant (or the properties) is true or false. Therefore the view allows us to expand a predicate into its sub-predicates and expressions. This can be demonstrated using Priorityqueue.mch from the Folder SchneiderBook. After initializing the machine and a few animation-steps opening the evaluation view yields
The invariant consists of two predicates
- queue : seq(NAT)
- !xx . (xx : 1..size(queue)-1 => (queue(xx) <= queue(xx+1)))
The first invariant is very simple. It is a predicate that has two sub-expressions queue and "seq(NAT)". The view shows the values for both subexpressions. Note that seq(NAT) is kept symbolic. The second invariant is more complex. It consistis of an implication which has two sub-predicates "xx : 1..size(queue)-1" and