TLA is a programming language created in 1999 by Leslie Lamport.
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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 exhaustivelytestable pseudocode, and its use likened to drawing blueprints for software systems; TLA is an acronym for Temporal Logic of Actions. Read more on Wikipedia...
 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
=============================================================================
 MODULE Elevator 
(***************************************************************************)
(* This spec describes a simple multicar 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 nextstate 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)
=============================================================================
Feature  Supported  Token  Example 

Booleans  âœ“  TRUE FALSE  
MultiLine Comments  âœ“  (* *)  (* A comment *) 
Comments  âœ“  \* A comment 

Line Comments  âœ“  \*  \* A comment 
Semantic Indentation  X 
title  author  year  reviews  ratings  rating 

Specifying Systems: The TLA+ Language and Tools for Hardware and Software Engineers  Leslie Lamport  2002  2  21  4.10 
Practical Tla+: DesignDriven Programming  Hillel Wayne  0  2  4.50 
title  authors  year  publisher 

Practical TLA  Hillel Wayne  20181011  Springer Nature 