Re: [tlaplus] Re: Doing arithmetic with current state value and next state value of a variable in TLA+

On Thursday, May 27, 2021 at 3:48:40 PM UTC-7 joan...@xxxxxxxxx wrote:

The minimal spec follows.
---------------------------- MODULE minimal_ex1 ----------------------------
EXTENDS Integers, Reals
VARIABLES x, y
TypeOK == /\ x \in -128..127
          /\ y \in -128..127
Init == ((x \in -128..127) /\ (y = 0))
VS_Pitch_Wheel_Rotation == 
    y' = ((x' - x + 128) % 256) - 128
                             
Next == VS_Pitch_Wheel_Rotation      
Spec == Init /\ [][Next]_<<x,y>> 
=============================================================================
\* Modification History
\* Last modified Thu May 27 19:45:21 BRT 2021 by jefferson
\* Created Thu May 27 19:45:08 BRT 2021 by jefferson
When the model is run, it produces the following error messages:
TLC threw an unexpected exception.
This was probably caused by an error in the spec or model.
See the User Output or TLC Console for clues to what happened.
The exception was a java.lang.RuntimeException
: 
In evaluation, the identifier x is either undefined or not an operator.
line 11, col 12 to line 11, col 12 of module minimal_ex1
The error occurred when TLC was evaluating the nested
expressions at the following positions:
0. Line 11, column 5 to line 11, column 37 in minimal_ex1
1. Line 11, column 10 to line 11, column 37 in minimal_ex1
2. Line 11, column 11 to line 11, column 30 in minimal_ex1
3. Line 11, column 12 to line 11, column 23 in minimal_ex1
4. Line 11, column 12 to line 11, column 17 in minimal_ex1
5. Line 11, column 12 to line 11, column 13 in minimal_ex1
6. Line 11, column 12 to line 11, column 12 in minimal_ex1
Thanks for your help.
On Thu, 2021-05-27 at 14:35 -0700, Andrew Helwer wrote:
Can you post a minimal functional spec which reproduces this error?

On Thursday, May 27, 2021 at 4:11:15 PM UTC-4 joan...@xxxxxxxxx wrote:
Hello,

I want to write a (piece of a) formula like this:
y' = x' - x
But every time I try to run the model I get the error, «In evaluation, the identifier x is either undefined or not an operator.»

What is going on? Is it really not possible to write such formulas? why? How can I work around this?

Thanks.

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