# Re: [tlaplus] State sequences

hi Stephan, thanks for the suggestion. I was doing something similar using variables checking1, checking2, etc, although I prefer your approach as its more compact. Nevertheless even this approach still seems long-winded compared to what I would like to have written. I know its not possible to write it that way in TLA+, but would my solution work in principle - I thought I'd taken  care of the stuttering issue.

thanks

On Friday, November 15, 2019 at 11:55:28 PM UTC-8, Stephan Merz wrote:
You cannot nest primes in TLA+ formulas, for good reasons related to stuttering invariance. You want to specify a state machine that monitors the progress of unlocking. Perhaps something along the following lines:

Digit == 0 .. 9
CONSTANT Code  \* can be instantiated to <<3,2,1>>, for example
ASSUME Code \in Seq(Digit)

VARIABLES
locked,   \* status of the lock
idx       \* monitor progression in input of the code

Init ==
/\ locked = TRUE
/\ idx = 1

Input(x) ==
\/ \* user input correct digit
/\ idx <= Len(Code) /\ x = Code[idx]
/\ idx' = idx+1
/\ locked' = (idx < Len(Code))  \* unlock when last digit has been input
\/ \* user input incorrect digit
/\ idx <= Len(Code) /\ x # Code[idx]
/\ idx' = Len(Code)+1  \* make further inputs useless
/\ locked' = locked
\/ \* ignore input when beyond the length of the code
/\ idx > Len(Code)
/\ UNCHANGED <<locked, idx>>

Spec == Init /\ [][\E x \in Digit : Input(x)]_<<locked,idx>>

The last disjunct of the Input action is in fact useless here, since stuttering is always allowed, but it may be useful when interaction with the user is modeled through an explicit interface. Also, you may choose to have a different strategy of handling input errors such as tolerating a certain number of wrong inputs before blocking.

Stephan

On 16 Nov 2019, at 03:29, ns <nedsr...@xxxxxxxxx> wrote:

hello, I was wondering if there was a convenient way to specify a required sequence of states in TLA+. As an example, suppose the code 3 2 1 unlocks something, we would like to say

Next ==
...
\/    num = 3 /\ num'=2 /\ num''=1 /\ unlock'''
...

and the usual [] [Next]_{vars}. I would like for this to mean
[] (Next \/ (num'=num /\ unlock'=unlock) \/ (num''=num' /\ unlock''=unlock') \/ (num'''=num'' /\ unlock'''=unlock'')
but I don't know if there's a way to write this easily in TLA"

thanks!

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