Help regarding Hierarchical planning

[MannavaVivek]

I am designing HTN planning problem for a domestic robot scenario and this here is a mockup of the same.

Code

  import unified_planning as up
  import sys
  from unified_planning.shortcuts import *
  from unified_planning.model.htn import *
  
  pb = HierarchicalProblem()
  
  Location = UserType('Location')
  Robot = UserType('Robot', father=Location)
  Robot_at = UserType('Robot_at', father=Location)
  
  Objects = UserType('Objects')
  
  # adding the fluents
  
  on = pb.add_fluent('on', Location, objects=Objects)
  at = pb.add_fluent('at', Location, robot=Robot)
  
  # adding the objects and the initial state
  
  cup = pb.add_object('cup', Objects)
  box = pb.add_object('box', Objects)
  phone = pb.add_object('phone', Objects)
  
  kitchen = pb.add_object('kitchen', Location)
  living_room = pb.add_object('living_room', Location)
  bedroom = pb.add_object('bedroom', Location)
  hallway = pb.add_object('hallway', Location)
  
  lucy = pb.add_object('lucy', Robot)
  
  pb.set_initial_value(on(cup), kitchen)
  pb.set_initial_value(on(box), kitchen)
  pb.set_initial_value(on(phone), bedroom)
  pb.set_initial_value(at(lucy), hallway)
  
  pick = InstantaneousAction('pick', objects=Objects, robot=Robot, location=Location)
  pick.add_precondition(Equals(at(pick.robot), pick.location))
  pick.add_precondition(Equals(on(pick.objects), pick.location))
  pick.add_effect(on(pick.objects), pick.robot)
  pb.add_action(pick)
  
  place = InstantaneousAction('place', objects=Objects, robot=Robot, location=Location)
  place.add_precondition(Equals(at(place.robot), place.location))
  place.add_precondition(Equals(on(place.objects), place.robot))
  place.add_effect(on(place.objects), place.location)
  pb.add_action(place)
  
  move = InstantaneousAction('move', robot=Robot, source=Location, destination=Location)
  move.add_precondition(Equals(at(move.robot), move.source))
  move.add_effect(at(move.robot), move.destination)
  pb.add_action(move)
  
  move_object = pb.add_task('move-object', objects=Objects, robot=Robot, destination=Location)
  
  # no-op method for case when object is already at the destination
  m = Method("move-object-np-op", objects=Objects, robot=Robot, destination=Location)
  m.set_task(move_object, m.objects, m.robot, m.destination)
  m.add_precondition(Equals(on(m.objects), m.destination))
  pb.add_method(m)
  
  # method for moving the object
  m = Method("move-object-actual", objects=Objects, robot=Robot, source=Location, destination=Location)
  m.set_task(move_object, m.objects, m.robot, m.destination)
  m.add_precondition(Equals(at(m.robot), m.source))
  t1 = m.add_subtask(move, m.robot, m.source, on(m.objects))
  t2 = m.add_subtask(pick, m.objects, m.robot, m.destination)
  t3 = m.add_subtask(move, m.objects, at(m.robot), m.destination)
  t4 = m.add_subtask(place, m.objects, m.robot, m.destination)
  m.set_ordered(t1, t2, t3, t4)
  pb.add_method(m)
  
  # adding the tasks
  def solve(pb):
      result = OneshotPlanner(problem_kind=pb.kind).solve(pb, output_stream=sys.stdout)    
      if result.plan is not None:
          print("Plan found:")
          print(result.plan)
      else:
          print("No plan found")
          print(result.status)
  
  pb_clone = pb.clone()
  pb_clone.task_network.add_subtask(move_object, cup, lucy, bedroom)
  
  # print(pb_clone)
  solve(pb_clone)

and when I run it, I get the following error:

Serving: 127.0.0.1:57634
ERROR: No temporal qualifier on state variable access.
    Context: In problem _domain/

PlanGenerationResultStatus.INTERNAL_ERROR

My initial assumption is that it is not finding the source in

m = Method("move-object-actual", objects=Objects, robot=Robot, source=Location, destination=Location)
m.set_task(move_object, m.objects, m.robot, m.destination)
m.add_precondition(Equals(at(m.robot), m.source))
t1 = m.add_subtask(move, m.robot, m.source, on(m.objects))
t2 = m.add_subtask(pick, m.objects, m.robot, m.destination)
t3 = m.add_subtask(move, m.objects, at(m.robot), m.destination)
t4 = m.add_subtask(place, m.objects, m.robot, m.destination)
m.set_ordered(t1, t2, t3, t4)
pb.add_method(m)

but I cannot understand how it has to be passed. There is a similar use in the example from the docs, but I cannot understand how the orig is passed there as well.

example from documentation

# Option 2: truck not at target location, move it
m = Method("bring-truck-move", truck=Truck, orig=Loc, dest=Loc)
# declares that m achieves the `bring-truck(truck, to)` task`
m.set_task(bring_truck, m.truck, m.dest) 
m.add_precondition(Equals(at(m.truck), m.orig))      # restrict applicability to cases where the truck is
m.add_precondition(Not(Equals(m.orig, m.dest)))        # in a different location
m.add_precondition(Equals(city(m.orig), city(m.dest))) # of the same city
# accomplishing this method requires executing a `move` action
m.add_subtask(move, m.truck, m.orig, m.dest, ident="move-subtask")  

pb.add_method(m)

[arbimo]

The problem in the model is in the two lines:

 t1 = m.add_subtask(move, m.robot, m.source, on(m.objects))
 t3 = m.add_subtask(move, m.objects, at(m.robot), m.destination)

There you use the value of non-static fluents (at, on). This is ambiguous as it is unclear at which time the fluent should be evaluated and this would impact the value passed to the subtasks.
Instead you should use a parameter (that is, invariant in time), whose value may be restricted in preconditions.

Here is a corrected model (with other minor fixes). Note that ltmp is only implicitly constrained by its usage but you may as well impose its value in preconditions.

m = Method("move-object-actual", objects=Objects, robot=Robot, source=Location, destination=Location, ltmp=Location, ltmp2=Location)
m.set_task(move_object, m.objects, m.robot, m.destination)
m.add_precondition(Equals(at(m.robot), m.source))
t1 = m.add_subtask(move, m.robot, m.source, m.ltmp)
t2 = m.add_subtask(pick, m.objects, m.robot, m.ltmp)
t3 = m.add_subtask(move, m.robot, m.ltmp, m.destination)
t4 = m.add_subtask(place, m.objects, m.robot, m.destination)
m.set_ordered(t1, t2, t3, t4)