# Re: [tlaplus] help understanding TLC error

• From: Stephan Merz <stephan.merz@xxxxxxxxx>
• Date: Fri, 27 Sep 2019 18:03:16 +0200

For TLC to be able to evaluate an action, the first occurrence of a primed variable should be of the form

x' = ....     or     x' \in ....

Later occurrences refer to the value assigned to that variable by this initial "assignment".

In your spec, the first occurrence of carts' appears in the expression carts'[c], and TLC attempts to retrieve the value of carts from the (partly constructed) successor state, but doesn't find a variable of that name. Hence the error message.

Hope this helps,
Stephan

> On 27 Sep 2019, at 17:52, 'Marko Schuetz-Schmuck' via tlaplus <tlaplus@xxxxxxxxxxxxxxxx> wrote:
>
> Dear All,
>
> I'm puzzled as to why TLC gives me an error when checking the
> following. I'm aware that the two lines before the commented-out line
> and the commented-out line are not equivalent. What I don't understand
> is why TLC give the error message:
>
> In evaluation, the identifier carts is either undefined or not an operator.
> line 12, col 45 to line 12, col 49 of module Dummy1
>
> The location is the first occurrence of carts' in the Next action.
>
> Thanks in advance for help with this.
>
> Best regards,
>
> Marko
>
> ------------------------------- MODULE Dummy1 -------------------------------
> EXTENDS Integers, Sequences
> VARIABLES carts, count, item
> Customer == 1..3
> MaxBag == 10
> Items == 1..10
> Init ==
>  /\ carts = [c \in Customer |-> <<>>]
>  /\ count = 1
>  /\ item = 1
> Next ==
>  /\ \A c \in Customer \ {(count % 3) + 1}: carts'[c] = carts[c]
>  /\ carts'[(count % 3) + 1] = carts[(count % 3) + 1]  \o <<item, count>>
>  \*  /\ carts' = [carts EXCEPT ![(count % 3) + 1] = @ \o <<item, count>>] \* this does not cause an error
>  /\ count' = count + 1
>  /\ item' = item + 1
> =============================================================================
>
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