Re: [tlaplus] Initial state doesn't satisfy type invariant

On Tuesday, December 8, 2020 at 12:01:26 AM UTC+5:30 Stephan Merz wrote:

Your type invariant states that edges_stack is a sequence of functions that associate nodes to subsets of labels. So, given any subset of labels ({}, {1}, ...), the function should return a node. This is not what << <<"A">> >> does: the domain of <<"A">> is just {1}, as you say.

Perhaps you mean

node_stack \in Seq(UNION {X -> Nodes : X \in SUBSET Labels})

which would allow each function to have a subset of Labels as domain.

Stephan

On 7 Dec 2020, at 19:14, Tim Leonard <t...@xxxxxxxxxxxxxx> wrote:

I'm starting to use the TLA+ toolkit after a decades-long hiatus in my use of TLA+ and TLC, and am struggling with the basics. My spec's type invariant is failing on the initial state, and I don't see why. Here are the spec, the model configuration, and the error:

--------------------------------------------------------------------------------
(* The relevant part of the spec. *)
--------------------------------------------------------------------------------

CONSTANT Nodes,
Labels,
StartNodes

VARIABLE node_stack,
edges_stack

Init == /\ node_stack = EMPTY
/\ edges_stack = Push( EMPTY, StartNodes )

TypeOK == /\ node_stack \in Seq(Nodes)
/\ edges_stack \in Seq([SUBSET(Labels) -> Nodes])

--------------------------------------------------------------------------------
(* The relevant part of the model configuration. *)
--------------------------------------------------------------------------------

Nodes <- { "A", "B", "C", "D", "E", "F" }
Labels <- 1..Cardinality(Nodes)
StartNodes <- << "A" >>

--------------------------------------------------------------------------------
(* The error. *)
--------------------------------------------------------------------------------

Invariant TypeOK is violated by the initial state:
/\ edges_stack = <<<<"A">>>>
/\ node_stack = <<>>

I assume that node_stack is satisfying the type invariant, so the problem is
with edges_stack. As I read it, its initial value is a sequence of length one,
with the only element being a function with a domain of { 1 } that maps 1 to "A".
I thought that would also satisfy the type invariant. What am I misunderstanding?

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