Re: [tlaplus] Recursion without StackOverflowError

[Markus Kuppe <tlaplus-google-group@xxxxxxxxxxx>]

Hi Isaac,

you have identified a bug in TLC's handling of lazy values within the 
scope of ENABLED combined with recursion: 
https://github.com/tlaplus/tlaplus/issues/991.  TLC only throws a 
StackOverflowError if it verifies the  TerminationInvariant  property. 
To work around the bug when checking the property, wrap the recursive 
call to  GoodMove  within  TLCEval  (defined in the TLC module).


EXTENDS Integers, TLC

[...]

RECURSIVE GoodMove(_, _)
GoodMove(B, M) ==
    LET B_0 == [B EXCEPT ![M] = "X"] IN
        IF Win(B) \/ Draw(B)
            THEN TRUE
            ELSE \A counter \in PossibleMoves(B_0):
                LET B_1 == [B_0 EXCEPT ![counter] = "O"] IN
                    \E y \in PossibleMoves(B_1): TLCEval(GoodMove(B_1, y))


Markus


On 7/19/24 10:29 AM, Isaac Dynamo wrote:
> Hi,
>
> Looked around online for a TLA+ community and this looks like the best 
> place for a beginner to ask some questions.
>
> I'm trying to model tic-tac-toe with the end goal to show that the 
> starting player doesn't have to lose.
>
> I started with a specification that has all possible game states, and 
> did some sanity checks on it. This all worked as expected.
>
> Next I wanted to show than by restricting the moves of the first player 
> to "good moves" that the system doesn't deadlock and Lose(board) is 
> unreachable.
>
> I wrote a recursive function GoodMove() that tries to ensure that after 
> every counter move there is still a good move to make. Resulting in a 
> Win or a Draw.
>
> However with this addition TLC crashes with an 
> java.lang.StackOverflowError. I understand that recursion can result in 
> a stack overflow, but the recursion in this case should be bounded. Each 
> iteration sets a tile, and at one point all tiles are set and this must 
> be a Win or a Draw. So I don't understand why this results in a stack 
> overflow.
>
> Can somebody explain what is going on? And what techniques can be used 
> to fix this?
>
> I get the feeling that recursion in TLA+ works differently compared to 
> regular programming languages and that I'm missing something.
>
> Thanks,

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