# Re: TLC choose a new unique element everytime

Thank you for pointing me to the "when" construct. I'll study it in greater details next.


For the TLC error, the PlusCal algorithm is on my previous email and the model values are:

Records <- [ model values ] {r1, r2, r3}
N <- 5
defaultInitValue <- [ model value ]
NoRecord <- [ model value ] (This is actually in the definition override)


Under the Invariants under the "What to check?" section, I'm checking DataOK.

If there's anything else I can provide, I'll be happy to do so.

Thanks again,
Shuhao

On 2017-07-14 03:15 PM, Leslie Lamport wrote:

First of all, when reporting a TLC error, please include the relevant model
values that produce the error.

mention.  Some operators in the TLC module, including RandomElement, are
not mathematics.  TLA+ specs should be mathematics.  Therefore,
RandomElement should not be used in a TLA+/PlusCal spec.  Randomness is
relevant for obtaining statistics.  You are apparently using RandomElement
to introduce nondeterminism.  If you look at any examples of PlusCal
algorithms, you will see that nondeterminism is expressed with the *when*
construct.

Leslie

On Friday, July 14, 2017 at 11:06:52 AM UTC-7, Shuhao Wu wrote:


Hello,

I've been trying to specify a simple problem where one process writes
arbitrary data to a source datastore and a log and a second process
follows and applies that log to a different datastore (similar to MySQL
binlogs). The corresponding Pluscal implementation is shown at the end
of the email. The implementation is supposed to show a violation of the
DataOK invariance as lfread and lfwrite are two steps rather than one
atomic steps.

The idea is that the DataUpdater process will write an entry into
source. This is done via the source[currentI] :=
RandomElement(PossibleRecords). In TLC, I have to specify
PossibleRecords as a set of finite number of model values. This does not
seem like it's "correct" so to speak, as it is possible for
RandomElement(PossibleRecords) = RandomElement(PossibleRecords) if two
of the same values happened to be picked.

Is there a better way to specify this for TLC? I feel this may be
impossible as I can't give TLC an infinite set. *In which case: is there
a better way to model this so TLC can work on it?

Furthermore, it seems like even if I use RandomElement, the operation of
source[currentI] := RandomElement(PossibleRecords) causes TLC to
somehow loses traces. If I run the below algorithm in TLC with
Invariance of DataOK, I will get:

Invariant DataOK is violated.
Failed to recover the initial state from its fingerprint.
This is probably a TLC bug(1).

I see that people have reported this in the past
after reading the thread, I'm not quite sure if there is a workaround
for this. Does anyone know?

Thanks,
Shuhao

----------------------------------------------------------------------------

EXTENDS TLC, Integers, Sequences

CONSTANT N, Records

NoRecord == CHOOSE r : r \notin Records

PossibleRecords == Records \cup {NoRecord}

(***************************************************************************

--algorithm foo {
variables
source = [k \in 1..N |-> NoRecord],
target = [k \in 1..N |-> NoRecord],
log = <<>>
;

process (DataUpdater = "DataUpdater")
variable currentI = 1;
{
duloop: while (currentI <= N) {
source[currentI] := RandomElement(PossibleRecords);
log := Append(log, source[currentI]);
currentI := currentI + 1;
}
}

process (LogFollower = "LogFollower")
variable currentR;
{
lfloop: while (Len(target) < N) {