On Fri, Oct 29, 2021 at 6:21 PM Jeremy Wright <jeremy@xxxxxxxxxxxx> wrote:
Thank you for the response Hillel, but I still think there is something I don't understand. It seems that if I setup a Spec with no behaviors, I can ask TLC anything and it will tell me it's okay. This bothers me.The spec's I'm using here are "obvious", however if I design a more complex system, and I make a mistake such that my system system has no valid behaviors, TLC won't tell me there is a problem. Not only will TLC not compain, it tells me there are no problems for ANY property I ask it to check.
For example:I'll restate the video, for those that prefer to read.I define a spec with 1 and only 1 state.
------------------------------ MODULE nonsense ------------------------------
EXTENDS TLC, Integers
VARIABLES A
vars == A
Init == A = 1
Next == A' = 1
EventuallyRubbish == <>(A = 999)
Spec == Init /\ [][Next]_vars /\ <>[](A = 0)
=============================================================================The Spec includes a temporal property which is not in the state space. There is no state where A=0, but TLC never warns me that my system does nothing. Here it is obviously not in the state space, but a more complex system that might not be so obvious. I recall the phrase, "beware of success", but this feels fragile to me. Am I missing something fundamental about how I express the Spec line?Thank you for your time.Sincerely,Jeremy--On Thu, Oct 28, 2021 at 4:05 PM Hillel Wayne <hwayne@xxxxxxxxx> wrote:--The problem is the line
Spec == Init /\ [][Next]_vars /\ EventuallySortedAsc /\ EventuallySortedDes
Since there are no models where EventuallySortedAsc /\ EventuallySortedDes, Spec = FALSE. If you change it to
Spec == Init /\ [][Next]_vars
Then it should work as expected.
H
On 10/28/2021 1:17 PM, Jeremy Wright wrote:
--Hello,
I am trying to write a simple spec for sorting. However, I've expressed a temporal property that I feel shouldn't be valid, yet TLC permits it.
Here is a video walkthrough of my question: https://www.loom.com/share/25f4eaaecd914d158551cbc061104a06
This spec is intended to be a simple abstraction of what sorting is. A sequence is (potentially) unsorted, then in one magical step it's sorted. I'm applying a refinement to it in another spec to show that, for example, bubble sort implements "sort by magic".
However, I have two temporal properties, which I feel conflict.
IsSortedAsc(seq) == \A a,b \in 1..N: a < b => seq[a] <= seq[b]
IsSortedDes(seq) == \A a,b \in 1..N: a < b => seq[a] >= seq[b]
EventuallySortedDes == <>[]IsSortedDes(A)
EventuallySortedAsc == <>[]IsSortedAsc(A)
I'm trying to express that eventually, the sequence is sorted In Ascending order or Descending Order. When I apply a model that checks if both are simultaneously true, TLC says this is successful.
Model Screenshot:
Thank you for any potential help. I fully expect I'm doing something incorrect, but I'm not quite sure where.
Sincerely,Jeremy Wright
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