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Re: [tlaplus] Re: action never enabled

Hi, Mark

I'm glad to help!

I think I have an answer. You didn't mention invariants, but when I checked your model with TypeOK as an invariant, I saw it had been violated because of coin0 < upperZero in the second step. Since it's violated in the second step, you'll see "<other action> is not enabled." Now, why is the first action in the disjunction always "chosen"? I believe it's because, when TLC is generating the state graph, it's the first action it parses (instead of deciding between "A" and "B" in "A \/ B", it always picks "A" to evaluate first). When  evaluated, TLC immediately verifies all invariants. Since TypeOK is false during this verification step, TLC stops, nothing else happens.

This is my speculation, because I'm fairly new to TLA+. Maybe a few experts in this group will be able to respond to your question and resolve our doubts.


On Friday, 24 December 2021 at 21:00:18 UTC-3 jetti...@xxxxxxxxx wrote:

Hi Jones,

Here are the constant values.  By the way, thank you very much for helping me out!

liquidityBound <- 10
coin0Init <- 100
coin1Init <- 1000
lowerZero <- 80
upperZero <- 120

On Friday, December 24, 2021 at 3:50:21 PM UTC-8 jone...@xxxxxxxxx wrote:

Yeah, nothing caught my eye… Could you give me the constant values for me to test locally?


On Fri, 24 Dec 2021 at 19:50, Mark Ettinger <jetti...@xxxxxxxxx> wrote:
Here is the actual spec:

----------------------------- MODULE uniswapV1 -----------------------------
EXTENDS Naturals, Integers
CONSTANTS liquidityBound, coin0Init, coin1Init, upperZero, lowerZero
VARIABLES coin0, coin1, liquidityTokens

max(a,b) == IF a > b THEN a ELSE b

outputPrice(delta_y, coin_x, coin_y) == ((delta_y * coin_x) \div (coin_y - delta_y)) + 1
inputPrice(delta_x, coin_x, coin_y) ==  (delta_x * coin_y) \div (coin_x + delta_x)

TypeOK == /\ coin0 \in Nat
          /\ coin1 \in Nat
          /\ (coin0 = 0 <=> coin1 = 0)
          /\ coin0 < upperZero
          /\ coin0 > lowerZero

tradeOneForZero == \exists delta_1 \in 1..outputPrice(coin0-1, coin1, coin0):
        /\ coin1' = coin1 + delta_1
        /\ coin0' = coin0 - inputPrice(delta_1, coin1, coin0)
        /\ liquidityTokens' = liquidityTokens
tradeZeroForOne == \exists delta_0 \in 1..outputPrice(coin1-1, coin0, coin1):
        /\ coin0' = coin0 + delta_0
        /\ coin1' = coin1 - inputPrice(delta_0, coin0, coin1)
        /\ liquidityTokens' = liquidityTokens
Init == coin0 = coin0Init /\ coin1 = coin1Init /\ liquidityTokens = max(coin0,coin1)
\*Next == tradeZeroForOne \/ tradeOneForZero
Next == tradeOneForZero \/ tradeZeroForOne

Spec == Init /\ [][Next]_<<coin0, coin1, liquidityTokens>>

On Friday, December 24, 2021 at 2:13:46 PM UTC-8 jone...@xxxxxxxxx wrote:

No problem. I can't think of a solution unless you show a reduced (if possible) version of your spec?


On Friday, 24 December 2021 at 19:03:36 UTC-3 jetti...@xxxxxxxxx wrote:
 Sorry, I mistyped.  Indeed my spec is already:

Spec == Init /\ [][Next]_vars

On Friday, December 24, 2021 at 1:53:59 PM UTC-8 jone...@xxxxxxxxx wrote:

From your example, it seems Spec should be: Init /\ [][Next]_vars, assuming vars is already a tuple/sequence. I am not sure if that solves your problem, though.


On Friday, 24 December 2021 at 13:59:20 UTC-3 jetti...@xxxxxxxxx wrote:

TLA+ newbie here with what is probably a newbie-ish question.  I'll describe it abstractly with actions A and B.   My spec has the form:

Next == A \/ B
Spec == Init /\ []Next_<<vars>>

When I run TLC I get a warning "B is never enabled".  If I change the ordering in Next to:

Next == B \/ A

I get the warning "A is never enabled."  Am I missing something obvious about disjunction?

Thanks for any insights in advance!

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