# TLA

TLA is an actively used programming language created in 1999. TLA+ (pronounced as tee ell a plus, ) is a formal specification language developed by Leslie Lamport. It is used to design, model, document, and verify concurrent systems. TLA+ has been described as exhaustively-testable pseudocode, and its use likened to drawing blueprints for software systems; TLA is an acronym for Temporal Logic of Actions. Read more on Wikipedia...

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- TLA ranks in the top 25% of languages
- the TLA wikipedia page
- TLA first appeared in 1999
- file extensions for TLA include tla
- See also: java, latex, ascii, eclipse-editor, isabelle, aws, azure, alloy, z-notation
- Have a question about TLA not answered here? Email me and let me know how I can help.

### Example code from Linguist:

--------------------------- MODULE AsyncInterface --------------------------- EXTENDS Naturals CONSTANT Data VARIABLE chan Values == <<"foo", "bar", "baz">> TypeInvariant == chan \in [val: Data, rdy: {0,1}, ack: {0,1}] Init == /\ TypeInvariant /\ chan.ack = chan.rdy Send(d) == /\ chan.rdy = chan.ack /\ chan' = [chan EXCEPT !.val = d, !.rdy = 1 - @] Rcv == /\ chan.rdy # chan.ack /\ chan' = [chan EXCEPT !.ack = 1 - @] Next == (\E d \in Data : Send(d)) \/ Rcv Spec == Init /\ [][Next]_chan THEOREM Spec => []TypeInvariant =============================================================================

### Example code from Wikipedia:

------------------------------ MODULE Elevator ------------------------------ (***************************************************************************) (* This spec describes a simple multi-car elevator system. The actions in *) (* this spec are unsurprising and common to all such systems except for *) (* DispatchElevator, which contains the logic to determine which elevator *) (* ought to service which call. The algorithm used is very simple and does *) (* not optimize for global throughput or average wait time. The *) (* TemporalInvariant definition ensures this specification provides *) (* capabilities expected of any elevator system, such as people eventually *) (* reaching their destination floor. *) (***************************************************************************) EXTENDS Integers CONSTANTS Person, \* The set of all people using the elevator system Elevator, \* The set of all elevators FloorCount \* The number of floors serviced by the elevator system VARIABLES PersonState, \* The state of each person ActiveElevatorCalls, \* The set of all active elevator calls ElevatorState \* The state of each elevator Vars == \* Tuple of all specification variables <<PersonState, ActiveElevatorCalls, ElevatorState>> Floor == \* The set of all floors 1 .. FloorCount Direction == \* Directions available to this elevator system {"Up", "Down"} ElevatorCall == \* The set of all elevator calls [floor : Floor, direction : Direction] ElevatorDirectionState == \* Elevator movement state; it is either moving in a direction or stationary Direction \cup {"Stationary"} GetDistance[f1, f2 \in Floor] == \* The distance between two floors IF f1 > f2 THEN f1 - f2 ELSE f2 - f1 GetDirection[current, destination \in Floor] == \* Direction of travel required to move between current and destination floors IF destination > current THEN "Up" ELSE "Down" CanServiceCall[e \in Elevator, c \in ElevatorCall] == \* Whether elevator is in position to immediately service call LET eState == ElevatorState[e] IN /\ c.floor = eState.floor /\ c.direction = eState.direction PeopleWaiting[f \in Floor, d \in Direction] == \* The set of all people waiting on an elevator call {p \in Person : /\ PersonState[p].location = f /\ PersonState[p].waiting /\ GetDirection[PersonState[p].location, PersonState[p].destination] = d} TypeInvariant == \* Statements about the variables which we expect to hold in every system state /\ PersonState \in [Person -> [location : Floor \cup Elevator, destination : Floor, waiting : BOOLEAN]] /\ ActiveElevatorCalls \subseteq ElevatorCall /\ ElevatorState \in [Elevator -> [floor : Floor, direction : ElevatorDirectionState, doorsOpen : BOOLEAN, buttonsPressed : SUBSET Floor]] SafetyInvariant == \* Some more comprehensive checks beyond the type invariant /\ \A e \in Elevator : \* An elevator has a floor button pressed only if a person in that elevator is going to that floor /\ \A f \in ElevatorState[e].buttonsPressed : /\ \E p \in Person : /\ PersonState[p].location = e /\ PersonState[p].destination = f /\ \A p \in Person : \* A person is in an elevator only if the elevator is moving toward their destination floor /\ \A e \in Elevator : /\ (PersonState[p].location = e /\ ElevatorState[e].floor /= PersonState[p].destination) => /\ ElevatorState[e].direction = GetDirection[ElevatorState[e].floor, PersonState[p].destination] /\ \A c \in ActiveElevatorCalls : PeopleWaiting[c.floor, c.direction] /= {} \* No ghost calls TemporalInvariant == \* Expectations about elevator system capabilities /\ \A c \in ElevatorCall : \* Every call is eventually serviced by an elevator /\ c \in ActiveElevatorCalls ~> \E e \in Elevator : CanServiceCall[e, c] /\ \A p \in Person : \* If a person waits for their elevator, they'll eventually arrive at their floor /\ PersonState[p].waiting ~> PersonState[p].location = PersonState[p].destination PickNewDestination(p) == \* Person decides they need to go to a different floor LET pState == PersonState[p] IN /\ ~pState.waiting /\ pState.location \in Floor /\ \E f \in Floor : /\ f /= pState.location /\ PersonState' = [PersonState EXCEPT ![p] = [@ EXCEPT !.destination = f]] /\ UNCHANGED <<ActiveElevatorCalls, ElevatorState>> CallElevator(p) == \* Person calls the elevator to go in a certain direction from their floor LET pState == PersonState[p] IN LET call == [floor |-> pState.location, direction |-> GetDirection[pState.location, pState.destination]] IN /\ ~pState.waiting /\ pState.location /= pState.destination /\ ActiveElevatorCalls' = IF \E e \in Elevator : /\ CanServiceCall[e, call] /\ ElevatorState[e].doorsOpen THEN ActiveElevatorCalls ELSE ActiveElevatorCalls \cup {call} /\ PersonState' = [PersonState EXCEPT ![p] = [@ EXCEPT !.waiting = TRUE]] /\ UNCHANGED <<ElevatorState>> OpenElevatorDoors(e) == \* Open the elevator doors if there is a call on this floor or the button for this floor was pressed. LET eState == ElevatorState[e] IN /\ ~eState.doorsOpen /\ \/ \E call \in ActiveElevatorCalls : CanServiceCall[e, call] \/ eState.floor \in eState.buttonsPressed /\ ElevatorState' = [ElevatorState EXCEPT ![e] = [@ EXCEPT !.doorsOpen = TRUE, !.buttonsPressed = @ \ {eState.floor}]] /\ ActiveElevatorCalls' = ActiveElevatorCalls \ {[floor |-> eState.floor, direction |-> eState.direction]} /\ UNCHANGED <<PersonState>> EnterElevator(e) == \* All people on this floor who are waiting for the elevator and travelling the same direction enter the elevator. LET eState == ElevatorState[e] IN LET gettingOn == PeopleWaiting[eState.floor, eState.direction] IN LET destinations == {PersonState[p].destination : p \in gettingOn} IN /\ eState.doorsOpen /\ eState.direction /= "Stationary" /\ gettingOn /= {} /\ PersonState' = [p \in Person |-> IF p \in gettingOn THEN [PersonState[p] EXCEPT !.location = e] ELSE PersonState[p]] /\ ElevatorState' = [ElevatorState EXCEPT ![e] = [@ EXCEPT !.buttonsPressed = @ \cup destinations]] /\ UNCHANGED <<ActiveElevatorCalls>> ExitElevator(e) == \* All people whose destination is this floor exit the elevator. LET eState == ElevatorState[e] IN LET gettingOff == {p \in Person : PersonState[p].location = e /\ PersonState[p].destination = eState.floor} IN /\ eState.doorsOpen /\ gettingOff /= {} /\ PersonState' = [p \in Person |-> IF p \in gettingOff THEN [PersonState[p] EXCEPT !.location = eState.floor, !.waiting = FALSE] ELSE PersonState[p]] /\ UNCHANGED <<ActiveElevatorCalls, ElevatorState>> CloseElevatorDoors(e) == \* Close the elevator doors once all people have entered and exited the elevator on this floor. LET eState == ElevatorState[e] IN /\ ~ENABLED EnterElevator(e) /\ ~ENABLED ExitElevator(e) /\ eState.doorsOpen /\ ElevatorState' = [ElevatorState EXCEPT ![e] = [@ EXCEPT !.doorsOpen = FALSE]] /\ UNCHANGED <<PersonState, ActiveElevatorCalls>> MoveElevator(e) == \* Move the elevator to the next floor unless we have to open the doors here. LET eState == ElevatorState[e] IN LET nextFloor == IF eState.direction = "Up" THEN eState.floor + 1 ELSE eState.floor - 1 IN /\ eState.direction /= "Stationary" /\ ~eState.doorsOpen /\ eState.floor \notin eState.buttonsPressed /\ \A call \in ActiveElevatorCalls : \* Can move only if other elevator servicing call /\ CanServiceCall[e, call] => /\ \E e2 \in Elevator : /\ e /= e2 /\ CanServiceCall[e2, call] /\ nextFloor \in Floor /\ ElevatorState' = [ElevatorState EXCEPT ![e] = [@ EXCEPT !.floor = nextFloor]] /\ UNCHANGED <<PersonState, ActiveElevatorCalls>> StopElevator(e) == \* Stops the elevator if it's moved as far as it can in one direction LET eState == ElevatorState[e] IN LET nextFloor == IF eState.direction = "Up" THEN eState.floor + 1 ELSE eState.floor - 1 IN /\ ~ENABLED OpenElevatorDoors(e) /\ ~eState.doorsOpen /\ nextFloor \notin Floor /\ ElevatorState' = [ElevatorState EXCEPT ![e] = [@ EXCEPT !.direction = "Stationary"]] /\ UNCHANGED <<PersonState, ActiveElevatorCalls>> (***************************************************************************) (* This action chooses an elevator to service the call. The simple *) (* algorithm picks the closest elevator which is either stationary or *) (* already moving toward the call floor in the same direction as the call. *) (* The system keeps no record of assigning an elevator to service a call. *) (* It is possible no elevator is able to service a call, but we are *) (* guaranteed an elevator will eventually become available. *) (***************************************************************************) DispatchElevator(c) == LET stationary == {e \in Elevator : ElevatorState[e].direction = "Stationary"} IN LET approaching == {e \in Elevator : /\ ElevatorState[e].direction = c.direction /\ \/ ElevatorState[e].floor = c.floor \/ GetDirection[ElevatorState[e].floor, c.floor] = c.direction } IN /\ c \in ActiveElevatorCalls /\ stationary \cup approaching /= {} /\ ElevatorState' = LET closest == CHOOSE e \in stationary \cup approaching : /\ \A e2 \in stationary \cup approaching : /\ GetDistance[ElevatorState[e].floor, c.floor] <= GetDistance[ElevatorState[e2].floor, c.floor] IN IF closest \in stationary THEN [ElevatorState EXCEPT ![closest] = [@ EXCEPT !.floor = c.floor, !.direction = c.direction]] ELSE ElevatorState /\ UNCHANGED <<PersonState, ActiveElevatorCalls>> Init == \* Initializes people and elevators to arbitrary floors /\ PersonState \in [Person -> [location : Floor, destination : Floor, waiting : {FALSE}]] /\ ActiveElevatorCalls = {} /\ ElevatorState \in [Elevator -> [floor : Floor, direction : {"Stationary"}, doorsOpen : {FALSE}, buttonsPressed : {{}}]] Next == \* The next-state relation \/ \E p \in Person : PickNewDestination(p) \/ \E p \in Person : CallElevator(p) \/ \E e \in Elevator : OpenElevatorDoors(e) \/ \E e \in Elevator : EnterElevator(e) \/ \E e \in Elevator : ExitElevator(e) \/ \E e \in Elevator : CloseElevatorDoors(e) \/ \E e \in Elevator : MoveElevator(e) \/ \E e \in Elevator : StopElevator(e) \/ \E c \in ElevatorCall : DispatchElevator(c) TemporalAssumptions == \* Assumptions about how elevators and people will behave /\ \A p \in Person : WF_Vars(CallElevator(p)) /\ \A e \in Elevator : WF_Vars(OpenElevatorDoors(e)) /\ \A e \in Elevator : WF_Vars(EnterElevator(e)) /\ \A e \in Elevator : WF_Vars(ExitElevator(e)) /\ \A e \in Elevator : SF_Vars(CloseElevatorDoors(e)) /\ \A e \in Elevator : SF_Vars(MoveElevator(e)) /\ \A e \in Elevator : WF_Vars(StopElevator(e)) /\ \A c \in ElevatorCall : SF_Vars(DispatchElevator(c)) Spec == \* Initialize state with Init and transition with Next, subject to TemporalAssumptions /\ Init /\ [][Next]_Vars /\ TemporalAssumptions THEOREM Spec => [](TypeInvariant /\ SafetyInvariant /\ TemporalInvariant) =============================================================================

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Last updated January 18th, 2020