A comparison with TLA+

In this notebook we'll go through the example from Hillel Wayne's Learn TLA+ to find out how concepts in TLA+ correspond to concepts in Imandra.

This notebook is intended to be read side-by-side with Hillel's example. We'll quote certain snippets from Learn TLA+ and explain how we could achieve the same in Imandra.

The Problem

You’re writing software for a bank. You have Alice and Bob as clients, each with a certain amount of money in their accounts. Alice wants to send some money to Bob. How do you model this? Assume all you care about is their bank accounts.

Step One

For this example, we could model the transfer algorithm very simply as a single function in Imandra. However, for comparison we'll try to model in a similar way to the PlusCal style. To do this we'll write our algorithm as a state machine.

The example defines the PlusCal variables alice_account, bob_account and money. For this we'll define a state record:

In [1]:
type state =
  { alice_account : int
  ; bob_account : int
  ; money : int
  }

let init =
  { alice_account = 10
  ; bob_account = 10
  ; money = 5
  }
Out[1]:
type state = { alice_account : Z.t; bob_account : Z.t; money : Z.t; }
val init : state = {alice_account = 10; bob_account = 10; money = 5}

A: and B: are labels. They define the steps the algorithm takes. Understanding how labels work is critical to writing more complex algorithms, as they define the places where your concurrency can go horribly awry.

We choose to model the steps A and B as a variant type action. The function step, given an action and a state, computes one step of the algorithm.

In [2]:
type action =
  | A
  | B

let step action state =
  match action with
  | A -> { state with alice_account = state.alice_account - state.money }
  | B -> { state with bob_account = state.bob_account + state.money }
Out[2]:
type action = A | B
val step : action -> state -> state = <fun>

Now we can put everything together to define the transfer algorithm:

In [3]:
let transfer state =
  state |> step A |> step B
Out[3]:
val transfer : state -> state = <fun>

So how do we run this? Well, we can’t.

In Imandra, this is real code! We can execute it, leveraging the OCaml runtime. Let's take a look at the results of executing each step:

In [4]:
init;;
init |> step A;;
init |> step A |> step B;;
Out[4]:
- : state = {alice_account = 10; bob_account = 10; money = 5}
- : state = {alice_account = 5; bob_account = 10; money = 5}
- : state = {alice_account = 5; bob_account = 15; money = 5}

Assertions and Sets

Can Alice’s account go negative? Right now our spec allows that, which we don’t want. We can start by adding a basic assert check after the transaction.

In Imandra, we encode contracts and properties using verify statements.

In [5]:
verify (fun state ->
  state.alice_account = 10 &&
  state.bob_account = 10 &&
  state.money = 5
  ==>
  let state' = transfer state in
  state'.alice_account >= 0
)
Out[5]:
- : state -> bool = <fun>
Proved
proof
ground_instances:0
definitions:0
inductions:0
search_time:
0.016s
details:
Expand
smt_stats:
rlimit count:232
num allocs:5935542
time:0.007000
memory:15.960000
max memory:15.960000
Expand
  • start[0.016s]
      let (_x_0 : int) = ….alice_account in
      let (_x_1 : int) = ….bob_account in
      let (_x_2 : int) = ….money in
      ( :var_0: ).alice_account = 10
      && ( :var_0: ).bob_account = 10 && ( :var_0: ).money = 5
      ==> (if B = A
           then {alice_account = _x_0 - _x_2; bob_account = _x_1; money = _x_2}
           else {alice_account = _x_0; bob_account = _x_1 + _x_2; money = _x_2}).alice_account
          >= 0
  • simplify

    into:
    let (_x_0 : int) = ( :var_0: ).alice_account in
    let (_x_1 : int) = ( :var_0: ).money in
    not ((_x_0 = 10 && ( :var_0: ).bob_account = 10) && _x_1 = 5)
    || (_x_0 + (-1) * _x_1) >= 0
    expansions:
    []
    rewrite_steps:
      forward_chaining:
      • Unsat

      Here we've used the ==> operator. a ==> b can be read as "a implies b". We're saying: if both Alice and Bob's accounts are 10, and money is 5, then after the transfer Alice's account will not be negative.

      Imandra has shown that our statement is true!

      At the very least, it works for the one number we tried. That doesn’t mean it works for all cases. When testing, it’s often hard to choose just the right test cases to surface the bugs you want to find. This is because most languages make it easy to test a specific state but not a large set of them. In TLA+, though, testing a wide range is simple.

      The only thing we changed was money = 5 to money \in 1..20

      Let's amend our verify statement to test the same range:

      In [6]:
      verify (fun state ->
        state.alice_account = 10 &&
        state.bob_account = 10 &&
        List.mem state.money List.(1 -- 20)
        ==>
        let state' = transfer state in
        state'.alice_account >= 0
      )
      
      Out[6]:
      - : state -> bool = <fun>
      module CX : sig val state : state end
      
      Counterexample (after 32 steps, 0.022s):
      let state : state = {alice_account = 10; bob_account = 10; money = 11}
      
      Refuted
      proof attempt
      ground_instances:32
      definitions:0
      inductions:0
      search_time:
      0.022s
      details:
      Expand
      smt_stats:
      num checks:65
      arith assert lower:26
      rlimit count:11116
      mk clause:82
      datatype occurs check:144
      mk bool var:341
      arith assert upper:8
      datatype splits:21
      decisions:113
      propagations:94
      conflicts:77
      datatype accessor ax:30
      arith assert diseq:22
      datatype constructor ax:72
      num allocs:11637382
      final checks:54
      added eqs:302
      del clause:70
      arith eq adapter:27
      memory:16.670000
      max memory:16.670000
      Expand
      • start[0.022s]
          let (_x_0 : int) = ….alice_account in
          let (_x_1 : int) = ….bob_account in
          let (_x_2 : int) = ….money in
          ( :var_0: ).alice_account = 10
          && ( :var_0: ).bob_account = 10
             && List.mem ( :var_0: ).money (List.( -- ) 1 20)
          ==> (if B = A
               then {alice_account = _x_0 - _x_2; bob_account = _x_1; money = _x_2}
               else {alice_account = _x_0; bob_account = _x_1 + _x_2; money = _x_2}).alice_account
              >= 0
      • simplify

        into:
        let (_x_0 : int) = ( :var_0: ).alice_account in
        let (_x_1 : int) = ( :var_0: ).money in
        not
        ((_x_0 = 10 && ( :var_0: ).bob_account = 10)
         && List.mem _x_1 (List.( -- ) 1 20))
        || (_x_0 + (-1) * _x_1) >= 0
        expansions:
        []
        rewrite_steps:
          forward_chaining:
          • unroll
            expr:
            (|List.mem_395/server|
              (money_1241/client state_1258/client)
              (|List.--_631/boot| 1 20))
            expansions:
            • unroll
              expr:
              (|List.--_631/boot| 1 20)
              expansions:
              • unroll
                expr:
                (|List.--_631/boot| 2 20)
                expansions:
                • unroll
                  expr:
                  (|List.mem_395/server|
                    (money_1241/client state_1258/client)
                    (|get.::.1_394/server| (|List.--_63…
                  expansions:
                  • unroll
                    expr:
                    (|List.--_631/boot| 3 20)
                    expansions:
                    • unroll
                      expr:
                      (|List.--_631/boot| 4 20)
                      expansions:
                      • unroll
                        expr:
                        (|List.mem_395/server|
                          (money_1241/client state_1258/client)
                          (|get.::.1_394/server| (|get.::.1_3…
                        expansions:
                        • unroll
                          expr:
                          (|List.--_631/boot| 5 20)
                          expansions:
                          • unroll
                            expr:
                            (|List.--_631/boot| 6 20)
                            expansions:
                            • unroll
                              expr:
                              (let ((a!1 (|get.::.1_394/server|
                                           (|get.::.1_394/server|
                                             (|get.::.1_394/…
                              expansions:
                              • unroll
                                expr:
                                (|List.--_631/boot| 7 20)
                                expansions:
                                • unroll
                                  expr:
                                  (|List.--_631/boot| 8 20)
                                  expansions:
                                  • unroll
                                    expr:
                                    (let ((a!1 (|get.::.1_394/server|
                                                 (|get.::.1_394/server|
                                                   (|get.::.1_394/…
                                    expansions:
                                    • unroll
                                      expr:
                                      (|List.--_631/boot| 9 20)
                                      expansions:
                                      • unroll
                                        expr:
                                        (let ((a!1 (|get.::.1_394/server|
                                                     (|get.::.1_394/server|
                                                       (|get.::.1_394/…
                                        expansions:
                                        • unroll
                                          expr:
                                          (|List.--_631/boot| 10 20)
                                          expansions:
                                          • unroll
                                            expr:
                                            (|List.--_631/boot| 11 20)
                                            expansions:
                                            • unroll
                                              expr:
                                              (let ((a!1 (|get.::.1_394/server|
                                                           (|get.::.1_394/server|
                                                             (|get.::.1_394/…
                                              expansions:
                                              • unroll
                                                expr:
                                                (|List.--_631/boot| 12 20)
                                                expansions:
                                                • unroll
                                                  expr:
                                                  (let ((a!1 (|get.::.1_394/server|
                                                               (|get.::.1_394/server|
                                                                 (|get.::.1_394/…
                                                  expansions:
                                                  • unroll
                                                    expr:
                                                    (|List.--_631/boot| 13 20)
                                                    expansions:
                                                    • unroll
                                                      expr:
                                                      (|List.--_631/boot| 14 20)
                                                      expansions:
                                                      • unroll
                                                        expr:
                                                        (let ((a!1 (|get.::.1_394/server|
                                                                     (|get.::.1_394/server|
                                                                       (|get.::.1_394/…
                                                        expansions:
                                                        • unroll
                                                          expr:
                                                          (|List.--_631/boot| 15 20)
                                                          expansions:
                                                          • unroll
                                                            expr:
                                                            (let ((a!1 (|get.::.1_394/server|
                                                                         (|get.::.1_394/server|
                                                                           (|get.::.1_394/…
                                                            expansions:
                                                            • unroll
                                                              expr:
                                                              (|List.--_631/boot| 16 20)
                                                              expansions:
                                                              • unroll
                                                                expr:
                                                                (|List.--_631/boot| 17 20)
                                                                expansions:
                                                                • unroll
                                                                  expr:
                                                                  (let ((a!1 (|get.::.1_394/server|
                                                                               (|get.::.1_394/server|
                                                                                 (|get.::.1_394/…
                                                                  expansions:
                                                                  • unroll
                                                                    expr:
                                                                    (|List.--_631/boot| 18 20)
                                                                    expansions:
                                                                    • unroll
                                                                      expr:
                                                                      (let ((a!1 (|get.::.1_394/server|
                                                                                   (|get.::.1_394/server|
                                                                                     (|get.::.1_394/…
                                                                      expansions:
                                                                      • unroll
                                                                        expr:
                                                                        (|List.--_631/boot| 19 20)
                                                                        expansions:
                                                                        • unroll
                                                                          expr:
                                                                          (|List.--_631/boot| 20 20)
                                                                          expansions:
                                                                          • Sat (Some let state : state = {alice_account = (Z.of_nativeint (10n)); bob_account = (Z.of_nativeint (10n)); money = (Z.of_nativeint (11n))} )

                                                                          Now Imandra has found a counterexample to our verify statement. Like TLA+, Imandra gives us concrete values that make our statement false. We can also compute with these values to get more insight:

                                                                          In [7]:
                                                                          CX.state;;
                                                                          CX.state |> step A;;
                                                                          CX.state |> step A |> step B;;
                                                                          
                                                                          Out[7]:
                                                                          - : state = {alice_account = 10; bob_account = 10; money = 11}
                                                                          - : state = {alice_account = -1; bob_account = 10; money = 11}
                                                                          - : state = {alice_account = -1; bob_account = 21; money = 11}
                                                                          

                                                                          We can fix this by wrapping the check in an if-block:

                                                                          In [8]:
                                                                          let transfer state =
                                                                            if state.alice_account >= state.money then
                                                                              state |> step A |> step B
                                                                            else
                                                                              state
                                                                          
                                                                          Out[8]:
                                                                          val transfer : state -> state = <fun>
                                                                          

                                                                          Which now runs properly.

                                                                          In [9]:
                                                                          verify (fun state ->
                                                                            state.alice_account = 10 &&
                                                                            state.bob_account = 10 &&
                                                                            List.mem state.money List.(1 -- 20)
                                                                            ==>
                                                                            let state' = transfer state in
                                                                            state'.alice_account >= 0
                                                                          )
                                                                          
                                                                          Out[9]:
                                                                          - : state -> bool = <fun>
                                                                          
                                                                          Proved
                                                                          proof
                                                                          ground_instances:0
                                                                          definitions:0
                                                                          inductions:0
                                                                          search_time:
                                                                          0.008s
                                                                          details:
                                                                          Expand
                                                                          smt_stats:
                                                                          num checks:2
                                                                          arith assert lower:1
                                                                          arith tableau max rows:3
                                                                          arith tableau max columns:9
                                                                          arith pivots:2
                                                                          rlimit count:787
                                                                          mk clause:3
                                                                          mk bool var:29
                                                                          arith assert upper:5
                                                                          decisions:2
                                                                          arith row summations:5
                                                                          propagations:5
                                                                          conflicts:4
                                                                          datatype accessor ax:3
                                                                          arith conflicts:1
                                                                          arith num rows:3
                                                                          arith assert diseq:1
                                                                          datatype constructor ax:2
                                                                          num allocs:20524647
                                                                          added eqs:25
                                                                          del clause:1
                                                                          arith eq adapter:3
                                                                          memory:16.920000
                                                                          max memory:16.920000
                                                                          Expand
                                                                          • start[0.008s]
                                                                              let (_x_0 : int) = ( :var_0: ).alice_account in
                                                                              let (_x_1 : int) = ( :var_0: ).money in
                                                                              let (_x_2 : state) = {alice_account = …; bob_account = …; money = …}
                                                                              in
                                                                              _x_0 = 10
                                                                              && ( :var_0: ).bob_account = 10 && List.mem _x_1 (List.( -- ) 1 20)
                                                                              ==> (if _x_0 >= _x_1 then if B = A then _x_2 else _x_2 else ( :var_0: )).alice_account
                                                                                  >= 0
                                                                          • simplify

                                                                            into:
                                                                            let (_x_0 : int) = ( :var_0: ).alice_account in
                                                                            let (_x_1 : int) = ( :var_0: ).bob_account in
                                                                            let (_x_2 : int) = ( :var_0: ).money in
                                                                            not ((_x_0 = 10 && _x_1 = 10) && List.mem _x_2 (List.( -- ) 1 20))
                                                                            || (if _x_0 >= _x_2
                                                                                then
                                                                                  {alice_account = _x_0 + (-1) * _x_2; bob_account = _x_1 + _x_2;
                                                                                   money = _x_2}
                                                                                else ( :var_0: )).alice_account
                                                                               >= 0
                                                                            expansions:
                                                                            []
                                                                            rewrite_steps:
                                                                              forward_chaining:
                                                                              • Unsat

                                                                              Quick aside: this is closer to testing all possible cases, but isn’t testing all possible cases. Would the algorithm break if money was, say, 4997? If we actually wanted to test all possible cases, we could replace money \in 1..20 with money \in Nat, where Nat is the set of natural numbers. This is perfectly valid TLA+. Unfortunately, it’s also something the model checker can’t handle. TLC can only check a subset of TLA+, and infinite sets aren’t part of that.

                                                                              Imandra is capable of handling infinite sets. We can do this by simply removing the constraint that state.money is in the range 1 to 20. Instead, we'll require that it is non-negative:

                                                                              In [10]:
                                                                              verify (fun state ->
                                                                                state.alice_account = 10 &&
                                                                                state.bob_account = 10 &&
                                                                                state.money >= 0
                                                                                ==>
                                                                                let state' = transfer state in
                                                                                state'.alice_account >= 0
                                                                              )
                                                                              
                                                                              Out[10]:
                                                                              - : state -> bool = <fun>
                                                                              
                                                                              Proved
                                                                              proof
                                                                              ground_instances:0
                                                                              definitions:0
                                                                              inductions:0
                                                                              search_time:
                                                                              0.016s
                                                                              details:
                                                                              Expand
                                                                              smt_stats:
                                                                              num checks:1
                                                                              arith assert lower:2
                                                                              arith tableau max rows:3
                                                                              arith tableau max columns:7
                                                                              arith pivots:2
                                                                              rlimit count:796
                                                                              mk clause:3
                                                                              mk bool var:24
                                                                              arith assert upper:3
                                                                              decisions:1
                                                                              arith row summations:5
                                                                              propagations:3
                                                                              conflicts:2
                                                                              datatype accessor ax:3
                                                                              arith conflicts:1
                                                                              arith num rows:3
                                                                              datatype constructor ax:2
                                                                              num allocs:31112549
                                                                              added eqs:23
                                                                              del clause:1
                                                                              arith eq adapter:2
                                                                              time:0.008000
                                                                              memory:17.040000
                                                                              max memory:17.450000
                                                                              Expand
                                                                              • start[0.016s]
                                                                                  let (_x_0 : int) = ( :var_0: ).alice_account in
                                                                                  let (_x_1 : int) = ( :var_0: ).money in
                                                                                  let (_x_2 : state) = {alice_account = …; bob_account = …; money = …}
                                                                                  in
                                                                                  _x_0 = 10 && ( :var_0: ).bob_account = 10 && _x_1 >= 0
                                                                                  ==> (if _x_0 >= _x_1 then if B = A then _x_2 else _x_2 else ( :var_0: )).alice_account
                                                                                      >= 0
                                                                              • simplify

                                                                                into:
                                                                                let (_x_0 : int) = ( :var_0: ).alice_account in
                                                                                let (_x_1 : int) = ( :var_0: ).bob_account in
                                                                                let (_x_2 : int) = ( :var_0: ).money in
                                                                                not ((_x_0 = 10 && _x_1 = 10) && _x_2 >= 0)
                                                                                || (if _x_0 >= _x_2
                                                                                    then
                                                                                      {alice_account = _x_0 + (-1) * _x_2; bob_account = _x_1 + _x_2;
                                                                                       money = _x_2}
                                                                                    else ( :var_0: )).alice_account
                                                                                   >= 0
                                                                                expansions:
                                                                                []
                                                                                rewrite_steps:
                                                                                  forward_chaining:
                                                                                  • Unsat

                                                                                  Imandra has shown that this property holds for all possible non-negative integer values of money.

                                                                                  TLA+ and Invariants

                                                                                  Can you transfer a negative amount of money? We could add an assert money > 0 to the beginning of the algorithm. This time, though, we’re going to introduce a new method in preparation for the next section.

                                                                                  TLA+ allows you write down invariants that will be checked for each state of the system.

                                                                                  We can achieve the same with Imandra:

                                                                                  In [11]:
                                                                                  verify (fun action state ->
                                                                                    state.alice_account = 10 &&
                                                                                    state.bob_account = 10 &&
                                                                                    state.money >= 0
                                                                                    ==>
                                                                                    let state' = step action state in
                                                                                    state'.money >= 0
                                                                                  )
                                                                                  
                                                                                  Out[11]:
                                                                                  - : action -> state -> bool = <fun>
                                                                                  
                                                                                  Proved
                                                                                  proof
                                                                                  ground_instances:0
                                                                                  definitions:0
                                                                                  inductions:0
                                                                                  search_time:
                                                                                  0.016s
                                                                                  details:
                                                                                  Expand
                                                                                  smt_stats:
                                                                                  num checks:1
                                                                                  arith assert lower:4
                                                                                  arith tableau max rows:3
                                                                                  arith tableau max columns:7
                                                                                  rlimit count:795
                                                                                  mk clause:3
                                                                                  mk bool var:30
                                                                                  arith assert upper:2
                                                                                  datatype splits:1
                                                                                  decisions:3
                                                                                  propagations:8
                                                                                  conflicts:4
                                                                                  datatype accessor ax:5
                                                                                  arith conflicts:1
                                                                                  arith num rows:3
                                                                                  arith assert diseq:1
                                                                                  datatype constructor ax:2
                                                                                  num allocs:43621981
                                                                                  added eqs:36
                                                                                  del clause:1
                                                                                  arith eq adapter:3
                                                                                  time:0.008000
                                                                                  memory:17.270000
                                                                                  max memory:17.670000
                                                                                  Expand
                                                                                  • start[0.016s]
                                                                                      let (_x_0 : int) = ( :var_0: ).alice_account in
                                                                                      let (_x_1 : int) = ( :var_0: ).bob_account in
                                                                                      let (_x_2 : int) = ( :var_0: ).money in
                                                                                      _x_0 = 10 && _x_1 = 10 && _x_2 >= 0
                                                                                      ==> (if ( :var_1: ) = A
                                                                                           then {alice_account = _x_0 - _x_2; bob_account = _x_1; money = _x_2}
                                                                                           else {alice_account = _x_0; bob_account = _x_1 + _x_2; money = _x_2}).money
                                                                                          >= 0
                                                                                  • simplify

                                                                                    into:
                                                                                    let (_x_0 : int) = ( :var_0: ).alice_account in
                                                                                    let (_x_1 : int) = ( :var_0: ).bob_account in
                                                                                    let (_x_2 : int) = ( :var_0: ).money in
                                                                                    not ((_x_0 = 10 && _x_1 = 10) && _x_2 >= 0)
                                                                                    || (if ( :var_1: ) = A
                                                                                        then
                                                                                          {alice_account = _x_0 + (-1) * _x_2; bob_account = _x_1; money = _x_2}
                                                                                        else {alice_account = _x_0; bob_account = _x_1 + _x_2; money = _x_2}).money
                                                                                       >= 0
                                                                                    expansions:
                                                                                    []
                                                                                    rewrite_steps:
                                                                                      forward_chaining:
                                                                                      • Unsat

                                                                                      Imandra has proven that for any state and after processing any action, money is always non-negative: the invariant holds for all states of the system.

                                                                                      One step further: checking Atomicity

                                                                                      So far we haven’t done anything too out of the ordinary. Everything so far is easily coverable in a real system by unit and property tests. There’s still a lot more ground to cover, but I want to show that we can already use what we’ve learned to find more complicated bugs. Alice wants to give Bob 1,000 dollars. If we’re unlucky, it could play out like this:

                                                                                      1. System checks that Alice has enough money
                                                                                      2. \$1,000 is deducted from her account
                                                                                      3. Eve smashes in the server with a baseball bat
                                                                                      4. Bob never receives the money
                                                                                      5. \$1,000 has completely disappeared from the system
                                                                                      6. The SEC shuts you down for fraud.

                                                                                      We already have all of the tools to check this. First, we need to figure out how to represent the broken invariant. We could do that by requiring the total money in the system is always the same:

                                                                                      In our Imandra model we could express this as follows: given any state and any action, the total money in the system at the start should be equal to the total money in the system afterwards:

                                                                                      In [12]:
                                                                                      let account_total state =
                                                                                        state.alice_account + state.bob_account
                                                                                      
                                                                                      verify (fun action state ->
                                                                                        state.money >= 0
                                                                                        ==>
                                                                                        let state' = step action state in
                                                                                        account_total state = account_total state'
                                                                                      )
                                                                                      
                                                                                      Out[12]:
                                                                                      val account_total : state -> Z.t = <fun>
                                                                                      - : action -> state -> bool = <fun>
                                                                                      module CX : sig val action : action val state : state end
                                                                                      
                                                                                      Counterexample (after 0 steps, 0.016s):
                                                                                      let action : action = B
                                                                                      let state : state = {alice_account = 0; bob_account = 1796; money = 1}
                                                                                      
                                                                                      Refuted
                                                                                      proof attempt
                                                                                      ground_instances:0
                                                                                      definitions:0
                                                                                      inductions:0
                                                                                      search_time:
                                                                                      0.016s
                                                                                      details:
                                                                                      Expand
                                                                                      smt_stats:
                                                                                      num checks:1
                                                                                      arith assert lower:3
                                                                                      arith tableau max rows:5
                                                                                      arith tableau max columns:12
                                                                                      arith pivots:3
                                                                                      rlimit count:988
                                                                                      mk clause:10
                                                                                      datatype occurs check:5
                                                                                      mk bool var:32
                                                                                      arith assert upper:8
                                                                                      datatype splits:1
                                                                                      decisions:6
                                                                                      arith row summations:8
                                                                                      propagations:5
                                                                                      conflicts:2
                                                                                      arith fixed eqs:2
                                                                                      datatype accessor ax:6
                                                                                      arith num rows:5
                                                                                      arith assert diseq:1
                                                                                      datatype constructor ax:5
                                                                                      num allocs:58915037
                                                                                      final checks:1
                                                                                      added eqs:31
                                                                                      del clause:2
                                                                                      arith eq adapter:3
                                                                                      time:0.008000
                                                                                      memory:17.670000
                                                                                      max memory:18.060000
                                                                                      Expand
                                                                                      • start[0.016s]
                                                                                          let (_x_0 : int) = ( :var_0: ).money in
                                                                                          let (_x_1 : int) = ( :var_0: ).alice_account in
                                                                                          let (_x_2 : int) = ( :var_0: ).bob_account in
                                                                                          let (_x_3 : state)
                                                                                              = if ( :var_1: ) = A
                                                                                                then {alice_account = _x_1 - _x_0; bob_account = _x_2; money = _x_0}
                                                                                                else {alice_account = _x_1; bob_account = _x_2 + _x_0; money = _x_0}
                                                                                          in _x_0 >= 0 ==> _x_1 + _x_2 = _x_3.alice_account + _x_3.bob_account
                                                                                      • simplify

                                                                                        into:
                                                                                        let (_x_0 : int) = ( :var_0: ).money in
                                                                                        let (_x_1 : int) = ( :var_0: ).alice_account in
                                                                                        let (_x_2 : int) = ( :var_0: ).bob_account in
                                                                                        let (_x_3 : state)
                                                                                            = if ( :var_1: ) = A
                                                                                              then
                                                                                                {alice_account = _x_1 + (-1) * _x_0; bob_account = _x_2; money = _x_0}
                                                                                              else {alice_account = _x_1; bob_account = _x_2 + _x_0; money = _x_0}
                                                                                        in not (_x_0 >= 0) || _x_1 + _x_2 = _x_3.alice_account + _x_3.bob_account
                                                                                        expansions:
                                                                                        []
                                                                                        rewrite_steps:
                                                                                          forward_chaining:
                                                                                          • Sat (Some let action : action = B let state : state = {alice_account = (Z.of_nativeint (0n)); bob_account = (Z.of_nativeint (1796n)); money = (Z.of_nativeint (1n))} )

                                                                                          When we run this, TLC finds a counterexample: between steps A and B the invariant doesn’t hold.

                                                                                          Imandra has found this same counterexample.

                                                                                          How do we solve this? It depends on the level of abstraction we care about. If you were designing a database, you’d want to spec the exact steps required to keep the system consistent. At our level, though, we probably have access to database transactions and can ‘abstract out’ the atomicity checks. The way we do that is to combine A and B into a single “Transaction” step. That tells TLA+ that both actions happen simultaneously, and the system never passes through an invalid state.

                                                                                          There are many different ways we could choose to model this in Imandra. Here is one:

                                                                                          In [13]:
                                                                                          type action =
                                                                                            | Transfer (* In this step we'll check whether Alice's balance is sufficient *)
                                                                                            | A (* In this step we'll transfer the funds *)
                                                                                            | End
                                                                                          
                                                                                          type state =
                                                                                            { alice_account : int
                                                                                            ; bob_account : int
                                                                                            ; money : int
                                                                                            ; next_action : action (* The action that should be executed on the next call to step *)
                                                                                            }
                                                                                          
                                                                                          let is_valid_initial_state state =
                                                                                            state.alice_account >= 0 &&
                                                                                            state.money >= 0 &&
                                                                                            state.next_action = Transfer
                                                                                          
                                                                                          let step state =
                                                                                            match state.next_action with
                                                                                            | Transfer ->
                                                                                              if state.alice_account >= state.money then
                                                                                                { state with next_action = A }
                                                                                              else
                                                                                                { state with next_action = End }
                                                                                            | A ->
                                                                                              { state with
                                                                                                alice_account = state.alice_account - state.money
                                                                                              ; bob_account = state.bob_account + state.money
                                                                                              ; next_action = End
                                                                                              }
                                                                                            | End ->
                                                                                              state
                                                                                          
                                                                                          (* step_n repeatedly calls (step state) n times. *)
                                                                                          let rec step_n n state =
                                                                                            if n > 0 then
                                                                                              step_n (n-1) (step state)
                                                                                            else
                                                                                              state
                                                                                          
                                                                                          let account_total state =
                                                                                            state.alice_account + state.bob_account
                                                                                          
                                                                                          verify (fun n state ->
                                                                                            n < 5 &&
                                                                                            is_valid_initial_state state
                                                                                            ==>
                                                                                            let state' = step_n n state in
                                                                                            account_total state = account_total state' &&
                                                                                            state'.alice_account >= 0
                                                                                          )
                                                                                          
                                                                                          Out[13]:
                                                                                          type action = Transfer | A | End
                                                                                          type state = {
                                                                                            alice_account : Z.t;
                                                                                            bob_account : Z.t;
                                                                                            money : Z.t;
                                                                                            next_action : action;
                                                                                          }
                                                                                          val is_valid_initial_state : state -> bool = <fun>
                                                                                          val step : state -> state = <fun>
                                                                                          val step_n : Z.t -> state -> state = <fun>
                                                                                          val account_total : state -> Z.t = <fun>
                                                                                          - : Z.t -> state -> bool = <fun>
                                                                                          
                                                                                          termination proof

                                                                                          Termination proof

                                                                                          call `step_n (n - 1) (step state)` from `step_n n state`
                                                                                          original:step_n n state
                                                                                          sub:step_n (n - 1) (step state)
                                                                                          original ordinal:Ordinal.Int (_cnt n)
                                                                                          sub ordinal:Ordinal.Int (_cnt (n - 1))
                                                                                          path:[n > 0]
                                                                                          proof:
                                                                                          detailed proof
                                                                                          ground_instances:1
                                                                                          definitions:0
                                                                                          inductions:0
                                                                                          search_time:
                                                                                          0.009s
                                                                                          details:
                                                                                          Expand
                                                                                          smt_stats:
                                                                                          num checks:3
                                                                                          arith assert lower:6
                                                                                          arith tableau max rows:4
                                                                                          arith tableau max columns:9
                                                                                          arith pivots:2
                                                                                          rlimit count:1109
                                                                                          mk clause:5
                                                                                          datatype occurs check:2
                                                                                          mk bool var:17
                                                                                          arith assert upper:3
                                                                                          decisions:2
                                                                                          arith row summations:3
                                                                                          propagations:2
                                                                                          conflicts:2
                                                                                          arith fixed eqs:2
                                                                                          datatype accessor ax:2
                                                                                          arith conflicts:1
                                                                                          arith num rows:4
                                                                                          num allocs:75923272
                                                                                          final checks:1
                                                                                          added eqs:4
                                                                                          del clause:5
                                                                                          arith eq adapter:2
                                                                                          memory:18.650000
                                                                                          max memory:18.650000
                                                                                          Expand
                                                                                          • start[0.009s]
                                                                                              let (_x_0 : int) = if n >= 0 then n else 0 in
                                                                                              let (_x_1 : int) = n - 1 in
                                                                                              let (_x_2 : int) = if _x_1 >= 0 then _x_1 else 0 in
                                                                                              n > 0 && _x_0 >= 0 && _x_2 >= 0
                                                                                              ==> not (_x_1 > 0) || Ordinal.( << ) (Ordinal.Int _x_2) (Ordinal.Int _x_0)
                                                                                          • simplify
                                                                                            into:
                                                                                            (n <= 0 || n <= 1)
                                                                                            || Ordinal.( << ) (Ordinal.Int (if n >= 1 then (-1) + n else 0))
                                                                                               (Ordinal.Int (if n >= 0 then n else 0))
                                                                                            expansions:
                                                                                            []
                                                                                            rewrite_steps:
                                                                                              forward_chaining:
                                                                                              • unroll
                                                                                                expr:
                                                                                                (|Ordinal.<<| (|Ordinal.Int_79/boot|
                                                                                                                (ite (>= n_558/server 1) (+ (- 1) n_558/server)…
                                                                                                expansions:
                                                                                                • Unsat
                                                                                                Proved
                                                                                                proof
                                                                                                ground_instances:5
                                                                                                definitions:0
                                                                                                inductions:0
                                                                                                search_time:
                                                                                                0.015s
                                                                                                details:
                                                                                                Expand
                                                                                                smt_stats:
                                                                                                arith offset eqs:29
                                                                                                num checks:11
                                                                                                arith assert lower:150
                                                                                                arith tableau max rows:35
                                                                                                arith tableau max columns:59
                                                                                                arith pivots:91
                                                                                                rlimit count:15346
                                                                                                mk clause:233
                                                                                                datatype occurs check:69
                                                                                                mk bool var:509
                                                                                                arith assert upper:159
                                                                                                datatype splits:26
                                                                                                decisions:159
                                                                                                arith row summations:374
                                                                                                arith bound prop:15
                                                                                                propagations:362
                                                                                                interface eqs:8
                                                                                                conflicts:38
                                                                                                arith fixed eqs:108
                                                                                                arith assume eqs:8
                                                                                                datatype accessor ax:23
                                                                                                minimized lits:12
                                                                                                arith conflicts:4
                                                                                                arith num rows:35
                                                                                                arith assert diseq:39
                                                                                                datatype constructor ax:26
                                                                                                num allocs:94526401
                                                                                                final checks:13
                                                                                                added eqs:997
                                                                                                del clause:130
                                                                                                arith eq adapter:107
                                                                                                memory:19.460000
                                                                                                max memory:19.460000
                                                                                                Expand
                                                                                                • start[0.015s]
                                                                                                    let (_x_0 : int) = ( :var_1: ).alice_account in
                                                                                                    let (_x_1 : state) = step_n ( :var_0: ) ( :var_1: ) in
                                                                                                    let (_x_2 : int) = _x_1.alice_account in
                                                                                                    ( :var_0: ) < 5
                                                                                                    && _x_0 >= 0
                                                                                                       && ( :var_1: ).money >= 0 && ( :var_1: ).next_action = Transfer
                                                                                                    ==> _x_0 + ( :var_1: ).bob_account = _x_2 + _x_1.bob_account && _x_2 >= 0
                                                                                                • simplify

                                                                                                  into:
                                                                                                  let (_x_0 : int) = ( :var_1: ).alice_account in
                                                                                                  let (_x_1 : state) = step_n ( :var_0: ) ( :var_1: ) in
                                                                                                  let (_x_2 : int) = _x_1.alice_account in
                                                                                                  not
                                                                                                  (((not (5 <= ( :var_0: )) && _x_0 >= 0) && ( :var_1: ).money >= 0)
                                                                                                   && ( :var_1: ).next_action = Transfer)
                                                                                                  || _x_0 + ( :var_1: ).bob_account = _x_2 + _x_1.bob_account && _x_2 >= 0
                                                                                                  expansions:
                                                                                                  []
                                                                                                  rewrite_steps:
                                                                                                    forward_chaining:
                                                                                                    • unroll
                                                                                                      expr:
                                                                                                      (step_n_1296/client n_1301/client state_1302/client)
                                                                                                      expansions:
                                                                                                      • unroll
                                                                                                        expr:
                                                                                                        (step_n_1296/client
                                                                                                          (+ (- 1) n_1301/client)
                                                                                                          (step_1294/client state_1302/client))
                                                                                                        expansions:
                                                                                                        • unroll
                                                                                                          expr:
                                                                                                          (step_n_1296/client
                                                                                                            (+ (- 2) n_1301/client)
                                                                                                            (step_1294/client (step_1294/client state_1302/clien…
                                                                                                          expansions:
                                                                                                          • unroll
                                                                                                            expr:
                                                                                                            (step_n_1296/client
                                                                                                              (+ (- 3) n_1301/client)
                                                                                                              (step_1294/client (step_1294/client (step_1294/clien…
                                                                                                            expansions:
                                                                                                            • unroll
                                                                                                              expr:
                                                                                                              (let ((a!1 (step_1294/client (step_1294/client (step_1294/client (step_1294/client state_1302/client…
                                                                                                              expansions:
                                                                                                              • Unsat

                                                                                                              Multiprocess Algorithms

                                                                                                              As a final part of our example, let’s discuss concurrency.

                                                                                                              The accounts are global variables, while money is a local variable to the process.

                                                                                                              In our model, we will treat state (which holds the accounts) as a global variable. The state of our processes will be represented by the process_state type - each has a local money variable. The world type will hold the global state of the accounts and the process_state for each process.

                                                                                                              We'll then define a function run_world, which takes a world state and a scheduled execution order in the form of a list of process contexts, and executes the processes according to the schedule.

                                                                                                              We want to verify that, given any initial world, the following invariants hold regardless of the order in which the processes are executed:

                                                                                                              1. The total money in the system does not change.
                                                                                                              2. Alice's account never goes negative.
                                                                                                              In [14]:
                                                                                                              (** Global state of accounts *)
                                                                                                              type state =
                                                                                                                { alice_account : int
                                                                                                                ; bob_account : int
                                                                                                                }
                                                                                                              
                                                                                                              (** Actions for an individual process *)
                                                                                                              type process_action =
                                                                                                                | Transfer
                                                                                                                | A
                                                                                                                | End
                                                                                                              
                                                                                                              (** The state of a process *)
                                                                                                              type process_state =
                                                                                                                { money : int
                                                                                                                ; next_action : process_action
                                                                                                                }
                                                                                                              
                                                                                                              (** State of the world *)
                                                                                                              type world =
                                                                                                                { state : state
                                                                                                                ; p1_state : process_state
                                                                                                                ; p2_state : process_state
                                                                                                                }
                                                                                                              
                                                                                                              (** Step a process's next_action. Returns the updated global accounts
                                                                                                                  state and the new state of this process. *)
                                                                                                              let step_process state process_state =
                                                                                                                match process_state.next_action with
                                                                                                                | Transfer ->
                                                                                                                  if state.alice_account >= process_state.money then
                                                                                                                    (state, { process_state with next_action = A })
                                                                                                                  else
                                                                                                                    (state, { process_state with next_action = End })
                                                                                                                | A ->
                                                                                                                   ( { alice_account = state.alice_account - process_state.money
                                                                                                                     ; bob_account = state.bob_account + process_state.money
                                                                                                                     }
                                                                                                                   , { process_state with next_action = End }
                                                                                                                   )
                                                                                                                | End ->
                                                                                                                  (state, process_state)
                                                                                                              
                                                                                                              (** Current execution context *)
                                                                                                              type context =
                                                                                                                | Process_1
                                                                                                                | Process_2
                                                                                                              
                                                                                                              (** Step the world forward for a given execution context. *)
                                                                                                              let step_world context world =
                                                                                                                match context with
                                                                                                                | Process_1 ->
                                                                                                                  let (state, p1_state) = step_process world.state world.p1_state in
                                                                                                                  { world with state; p1_state }
                                                                                                                | Process_2 ->
                                                                                                                  let (state, p2_state) = step_process world.state world.p2_state in
                                                                                                                  { world with state; p2_state }
                                                                                                              
                                                                                                              (** run_world takes an initial world state and executes the processes
                                                                                                                  according to the schedule specified by contexts *)
                                                                                                              let run_world world contexts =
                                                                                                                contexts |> List.fold_right step_world ~base:world
                                                                                                              
                                                                                                              Out[14]:
                                                                                                              type state = { alice_account : Z.t; bob_account : Z.t; }
                                                                                                              type process_action = Transfer | A | End
                                                                                                              type process_state = { money : Z.t; next_action : process_action; }
                                                                                                              type world = {
                                                                                                                state : state;
                                                                                                                p1_state : process_state;
                                                                                                                p2_state : process_state;
                                                                                                              }
                                                                                                              val step_process : state -> process_state -> state * process_state = <fun>
                                                                                                              type context = Process_1 | Process_2
                                                                                                              val step_world : context -> world -> world = <fun>
                                                                                                              val run_world : world -> context list -> world = <fun>
                                                                                                              

                                                                                                              Now we can verify that, for any initial state of the world and for any possible sequence of contexts, the invariants hold.

                                                                                                              In [15]:
                                                                                                              (** A state is a valid initial state if the accounts are non-negative. *)
                                                                                                              let is_valid_initial_state state =
                                                                                                                state.alice_account >= 0 &&
                                                                                                                state.bob_account >= 0
                                                                                                              
                                                                                                              (** This function checks whether a process is in a valid starting state.
                                                                                                                  We'll use it to constrain the input to our verify statement below. *)
                                                                                                              let is_valid_initial_process_state p =
                                                                                                                p.money >= 0 &&
                                                                                                                p.next_action = Transfer
                                                                                                              
                                                                                                              (** The world is a valid initial state if the accounts and processes are all valid. *)
                                                                                                              let is_valid_initial_world world =
                                                                                                                is_valid_initial_state world.state &&
                                                                                                                is_valid_initial_process_state world.p1_state &&
                                                                                                                is_valid_initial_process_state world.p2_state
                                                                                                              
                                                                                                              let account_total state =
                                                                                                                state.alice_account + state.bob_account
                                                                                                              
                                                                                                              verify (fun contexts world ->
                                                                                                                (* Initial states are valid *)
                                                                                                                is_valid_initial_world world
                                                                                                                ==>
                                                                                                                let world' = run_world world contexts in
                                                                                                                account_total world.state = account_total world'.state &&
                                                                                                                world'.state.alice_account >= 0
                                                                                                              )
                                                                                                              
                                                                                                              Out[15]:
                                                                                                              val is_valid_initial_state : state -> bool = <fun>
                                                                                                              val is_valid_initial_process_state : process_state -> bool = <fun>
                                                                                                              val is_valid_initial_world : world -> bool = <fun>
                                                                                                              val account_total : state -> Z.t = <fun>
                                                                                                              - : context list -> world -> bool = <fun>
                                                                                                              module CX : sig val contexts : context list val world : world end
                                                                                                              
                                                                                                              Counterexample (after 5 steps, 0.040s):
                                                                                                              let contexts : context list = [Process_2; Process_1; Process_1; Process_2]
                                                                                                              let world : world =
                                                                                                                {state = {alice_account = 626; bob_account = 7065};
                                                                                                                 p1_state = {money = 1; next_action = Transfer};
                                                                                                                 p2_state = {money = 626; next_action = Transfer}}
                                                                                                              
                                                                                                              Refuted
                                                                                                              proof attempt
                                                                                                              ground_instances:5
                                                                                                              definitions:0
                                                                                                              inductions:0
                                                                                                              search_time:
                                                                                                              0.040s
                                                                                                              details:
                                                                                                              Expand
                                                                                                              smt_stats:
                                                                                                              arith offset eqs:371
                                                                                                              num checks:11
                                                                                                              arith assert lower:704
                                                                                                              arith tableau max rows:78
                                                                                                              arith tableau max columns:106
                                                                                                              arith pivots:285
                                                                                                              rlimit count:127658
                                                                                                              mk clause:902
                                                                                                              datatype occurs check:404
                                                                                                              mk bool var:2049
                                                                                                              arith assert upper:687
                                                                                                              datatype splits:406
                                                                                                              decisions:1207
                                                                                                              arith row summations:2962
                                                                                                              arith bound prop:223
                                                                                                              propagations:4082
                                                                                                              interface eqs:13
                                                                                                              conflicts:211
                                                                                                              arith fixed eqs:531
                                                                                                              arith assume eqs:13
                                                                                                              datatype accessor ax:213
                                                                                                              minimized lits:176
                                                                                                              arith conflicts:28
                                                                                                              arith num rows:78
                                                                                                              arith assert diseq:202
                                                                                                              datatype constructor ax:326
                                                                                                              num allocs:129191470
                                                                                                              final checks:24
                                                                                                              added eqs:10932
                                                                                                              del clause:321
                                                                                                              arith eq adapter:442
                                                                                                              time:0.006000
                                                                                                              memory:23.720000
                                                                                                              max memory:23.720000
                                                                                                              Expand
                                                                                                              • start[0.040s]
                                                                                                                  let (_x_0 : state) = ( :var_0: ).state in
                                                                                                                  let (_x_1 : int) = _x_0.alice_account in
                                                                                                                  let (_x_2 : int) = _x_0.bob_account in
                                                                                                                  let (_x_3 : process_state) = ( :var_0: ).p1_state in
                                                                                                                  let (_x_4 : process_state) = ( :var_0: ).p2_state in
                                                                                                                  let (_x_5 : state)
                                                                                                                      = (List.fold_right step_world ( :var_0: ) ( :var_1: )).state
                                                                                                                  in
                                                                                                                  let (_x_6 : int) = _x_5.alice_account in
                                                                                                                  (_x_1 >= 0 && _x_2 >= 0)
                                                                                                                  && (_x_3.money >= 0 && _x_3.next_action = Transfer)
                                                                                                                     && _x_4.money >= 0 && _x_4.next_action = Transfer
                                                                                                                  ==> _x_1 + _x_2 = _x_6 + _x_5.bob_account && _x_6 >= 0
                                                                                                              • simplify

                                                                                                                into:
                                                                                                                let (_x_0 : state) = ( :var_0: ).state in
                                                                                                                let (_x_1 : int) = _x_0.alice_account in
                                                                                                                let (_x_2 : int) = _x_0.bob_account in
                                                                                                                let (_x_3 : process_state) = ( :var_0: ).p1_state in
                                                                                                                let (_x_4 : process_state) = ( :var_0: ).p2_state in
                                                                                                                let (_x_5 : state)
                                                                                                                    = (List.fold_right step_world ( :var_0: ) ( :var_1: )).state
                                                                                                                in
                                                                                                                let (_x_6 : int) = _x_5.alice_account in
                                                                                                                not
                                                                                                                (((((_x_1 >= 0 && _x_2 >= 0) && _x_3.money >= 0)
                                                                                                                   && _x_3.next_action = Transfer)
                                                                                                                  && _x_4.money >= 0)
                                                                                                                 && _x_4.next_action = Transfer)
                                                                                                                || _x_1 + _x_2 = _x_6 + _x_5.bob_account && _x_6 >= 0
                                                                                                                expansions:
                                                                                                                []
                                                                                                                rewrite_steps:
                                                                                                                  forward_chaining:
                                                                                                                  • unroll
                                                                                                                    expr:
                                                                                                                    (|List.fold_right_679/server| world_1354/client contexts_1353/client)
                                                                                                                    expansions:
                                                                                                                    • unroll
                                                                                                                      expr:
                                                                                                                      (|List.fold_right_679/server|
                                                                                                                        world_1354/client
                                                                                                                        (|get.::.1_675/server| contexts_1353/client))
                                                                                                                      expansions:
                                                                                                                      • unroll
                                                                                                                        expr:
                                                                                                                        (|List.fold_right_679/server|
                                                                                                                          world_1354/client
                                                                                                                          (|get.::.1_675/server| (|get.::.1_675/server| co…
                                                                                                                        expansions:
                                                                                                                        • unroll
                                                                                                                          expr:
                                                                                                                          (|List.fold_right_679/server|
                                                                                                                            world_1354/client
                                                                                                                            (|get.::.1_675/server|
                                                                                                                              (|get.::.1_675/server…
                                                                                                                          expansions:
                                                                                                                          • unroll
                                                                                                                            expr:
                                                                                                                            (let ((a!1 (|get.::.1_675/server|
                                                                                                                                         (|get.::.1_675/server|
                                                                                                                                           (|get.::.1_675/…
                                                                                                                            expansions:
                                                                                                                            • Sat (Some let contexts : context list = [Process_2; Process_1; Process_1; Process_2] let world : world = {state = {alice_account = (Z.of_nativeint (626n)); bob_account = (Z.of_nativeint (7065n))}; p1_state = {money = (Z.of_nativeint (1n)); next_action = Transfer}; p2_state = {money = (Z.of_nativeint (626n)); next_action = Transfer}} )

                                                                                                                            There’s a gap between when we check that Alice has enough money and when we actually transfer the money. With one process this wasn’t a problem, but with two, it means her account can go negative. TLC is nice enough to provide the initial state and steps required to reproduce the bug.

                                                                                                                            Imandra has also found a counterexample. We can see from the contexts in the counterexample that one process interrupted the other.

                                                                                                                            We can also execute the counterexample to see the final state of the world in this case:

                                                                                                                            In [16]:
                                                                                                                            run_world CX.world CX.contexts
                                                                                                                            
                                                                                                                            Out[16]:
                                                                                                                            - : world =
                                                                                                                            {state = {alice_account = -1; bob_account = 7692};
                                                                                                                             p1_state = {money = 1; next_action = End};
                                                                                                                             p2_state = {money = 626; next_action = End}}
                                                                                                                            

                                                                                                                            Summary

                                                                                                                            We've shown how these TLA+ problems can be modeled in Imandra, in an apples-to-apples fashion.

                                                                                                                            In most cases we've translated the PlusCal examples into a state machine, which is closer to the underlying TLA+ representation. A PlusCal-like DSL for Imandra would be an interesting future project.

                                                                                                                            Both systems can go much deeper: TLA+ has theorem proving capabilities, and we haven't touched on Imandra's lemmas, theorems, rewrite rules or the induction waterfall. Browse the documentation to find out more!