Re: [tlaplus] Converting TLA+ spec to PlusCal

On Wednesday, December 21, 2022 at 2:34:54 PM UTC+2 Stephan Merz wrote:

Within a form [f EXCEPT ![x] = g(@)] (to which a PlusCal assignment f[x] := ... translates), `@' designates f[x]. `@' has no meaning in the right-hand side of an _expression_ [x \in S |-> ...], sorry for the syntax error.

Stephan

On 21 Dec 2022, at 12:58, Deividas Bražėnas <deividas...@xxxxxxxxx> wrote:

Sorry, that was my typo, PlusCal translation seems to be alright.

Talking about your proposed solution, what does "@" means here? I get the following error during TLC "@ used where its meaning is not defined."

Deividas

On Wednesday, December 21, 2022 at 10:46:15 AM UTC+2 Stephan Merz wrote:
The two commas look wrong: this could be an error in the PlusCal translator. If you can confirm this, could you open a GitHub issue? Could you share the entire TLA+ module?

What happens if you write something like

with m = [t |-> "PROPOSE", v |-> input] do
msgs := [p \in peers |-> @ \cup {m}]
end with

Stephan

On 21 Dec 2022, at 09:40, Deividas Bražėnas <deividas...@xxxxxxxxx> wrote:

I'm also wondering, maybe it would be possible to do that in a more simpler way? e.g. something like this:

msgs := [peer_id \in peers |-> msgs[peer_id] \cup {[t |-> "PROPOSE", v |-> input]}];

But this one gives me a syntax error at first "|->" operator when generated to following TLA+:
msgs' = [peer_id[self] \in peers[self] |-> msgs[peer_id[self]] \cup {[t |-> "PROPOSE", , v |-> input[self]]}]

On Wednesday, December 21, 2022 at 10:37:01 AM UTC+2 Deividas Bražėnas wrote:
Thanks for the answer, Stephan!

On Wed, Dec 21, 2022 at 10:32 AM Stephan Merz <stepha...@xxxxxxxxx> wrote:
You can rewrite your loop as follows:

while peers # {} do
with p \in peers do
msgs[p] := @ \union {...};
peers := peers \ {p};
end do;
end while

Note that unlike your version, this loop iterates through peers in a non-deterministic order, and that this will generate more states when you use TLC for model checking.

Stephan

On 21 Dec 2022, at 08:59, Deividas Bražėnas <deividas...@xxxxxxxxx> wrote:

Hey Stephan, thanks for the clarification and answer.

Sorry for not explaining correctly, but I want to simulate the messages sent for peers. So basically, in the end the variable msgs should look like this
msgs = [n_1 |-> {[t |-> "PROPOSE", v |-> "v_1], [t |-> "ECHO", v |-> "v_1]},
n_2 |-> {[t |-> "PROPOSE", v |-> "v_1], [t |-> "ECHO", v |-> "v_1]},
n_2 |-> {[t |-> "PROPOSE", v |-> "v_1], [t |-> "ECHO", v |-> "v_1]}]

And there will be a several processes like SendPropose or SendEcho that would append to set of messages. So sadly your proposed solution will not work :/

I've made loop like this work, but I'm wondering whether it would be possible to make it look more pretty

while index <= Len(SetToSeq(peers)) do
peer_id := peers[index];
msgs[peer_id] := msgs[peer_id] \union {[t |-> "PROPOSE", v |-> input]};
index := index + 1;
end while;

Deividas

On Wednesday, December 21, 2022 at 9:38:07 AM UTC+2 Stephan Merz wrote:
In PlusCal, "with x \in S do ..." expresses non-deterministic choice, not a loop. The body of the "with" instruction is executed for some peer, then TLC backtracks and explores the remaining choices. Remember that TLC is a model checker that explores all runs of an algorithm, not an interpreter that generates one run. Therefore, "print" instructions are often hard to interpret.

I am not sure what exactly you are trying to achieve. The end result can of course be obtained by writing

msgs := [pid \in peers |-> [t |-> "PROPOSE"]]

Stephan

On 20 Dec 2022, at 22:13, Deividas Bražėnas <deividas...@xxxxxxxxx> wrote:

Thank you, Stephan!!

I'm also wondering how I could write the following in PlusCal.

I have these global variables:

peers = {n_1, n_2, n_3, n_4}
msgs = [node_id \in peers |-> {}]

I want to add [t |-> "PROPOSE"] message for every peer in a process.

I've tried to do it like this, but it does not work as expected. What may I be doing wrong?

print <<"with">>;
with peer_id \in peers do
print <<"peers", peers, "peer", peer_id>>;
msgs[peer_id] := msgs[peer_id] \union {[t |-> "PROPOSE"]};
end with;

print <<"msgs", msgs>>;

I get the following output:
<<"with">>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_1>>
<<"msgs", (n_1 :> {[t |-> "PROPOSE"]} @@ n_2 :> {} @@ n_3 :> {} @@ n_4 :> {})>>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_2>>
<<"msgs", (n_1 :> {} @@ n_2 :> {[t |-> "PROPOSE"]} @@ n_3 :> {} @@ n_4 :> {})>>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_3>>
<<"msgs", (n_1 :> {} @@ n_2 :> {} @@ n_3 :> {[t |-> "PROPOSE"]} @@ n_4 :> {})>>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_4>>
<<"msgs", (n_1 :> {} @@ n_2 :> {} @@ n_3 :> {} @@ n_4 :> {[t |-> "PROPOSE"]})>>

Where I would expect:
<<"with">>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_1>>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_2>>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_3>>
<<"peers", {n_1, n_2, n_3, n_4}, "peer", n_4>>
<<"msgs", (n_1 :> {[t |-> "PROPOSE"]} @@ n_2 :> {[t |-> "PROPOSE"]}   @@ n_3 :> {[t |-> "PROPOSE"]}   @@ n_4 :> {[t |-> "PROPOSE"]} )>>
On Monday, December 19, 2022 at 8:30:20 AM UTC+2 Stephan Merz wrote:
Hello,

TLA+ is more general than PlusCal, and a systematic conversion to PlusCal is unlikely to yield idiomatic results, if it is possible at all: you will probably end up with a single while loop containing all possible actions. It is a bit like the normal form theorem in computability theory.

On 18 Dec 2022, at 13:24, Deividas Bražėnas <deividas...@xxxxxxxxx> wrote:

Hello all,
I have a TLA+ spec that I'd like to convert to PlusCal, but I'm facing some issues as I don't know how to convert a few parts of the specification.

1. I want to have this predicate as a process, but I'm not sure how to convert this part "\E n \in CN, p \in BOOLEAN :"

UpdatePredicate ==
\E n \in CN, p \in BOOLEAN :
/\ predicate[n] => p \* Only monotonic updates are make the algorithm to terminate.
/\ predicate' = [predicate EXCEPT ![n] = p]
/\ UNCHANGED <<bcNode, bcValue, output, msgs>>

In PlusCal you could represent this as

with (n \in CN, p \in BOOLEAN) {
  await (predicate[n] => p);
  predicate[n] := p
}

2. I want to specify global variable bcValue in Init based on the set of bcNode

/\ \/ bcNode \in CN /\ bcValue \in Value
  \/ bcNode \in FN /\ bcValue = NotValue

You could perhaps write

variables
  bcNode \in CN \union FN,
  bcValue \in (CASE bcNode \in CN -> Value [] bcNode \in FN -> {NotValue});

Stephan

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