Re: [tlaplus] how to updating state and return in a definition

Thanks for the reply. Is there a way to emulate "returning" a value? My module A is being used as an instance in another module B, but I want A to be as isolated as possible.

I came up with something like below. Is this acceptable?

CONSTANTS Participants
VARIABLE counters

Init == counters = [i \in Participants |-> 0]

UpdateCounter(participant, newCounter) ==
    IF /\ counters[participant] < newCounter THEN
       /\ counters' = [counters EXCEPT ![participant] = newCounter]
       /\ TRUE
    ELSE
       /\ FALSE
       /\ UNCHANGED << counters >>

UpdateAndGet(participant, newCounter) ==
    IF UpdateCounter(participant, newCounter) THEN
       [p |-> participant, c |-> newCounter]
    ELSE
       [p |-> participant, c |-> counters[participant]]

On Wednesday, May 15, 2019 at 4:26:35 AM UTC-4, Stephan Merz wrote:

Hello,

if you remove the empty pair of parentheses, your definition is correct TLA+. It defines an operator whose effect it is to increment the state variable val and in can be used inside an action like so:

MyAction ==
/\ ...
/\ Calculate \* update variable val
/\ ...

There is no notion of "returning" an update of a variable.

If you'd rather compute a value that can be used within an _expression_ you can define

ValPlusOne == val + 1

and have something like

f' = [f EXCEPT ![p] = ValPlusOne]

for updating an array at a certain position.

Regards,
Stephan

On 15 May 2019, at 02:54, Balaji Arun <ba2...@xxxxxx> wrote:

I am trying to have a specification with a definition for Calculate()

module A:

VARIABLE val Calculate() == val' = val + 1 (*return [v: val] here*)

How do I return that val to whoever is calling it (even another module that instantiates module A)? Assuming multiple processes call Calculate concurrently.

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