baylor

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baylors Planning Talk

• STRIPS
• November 25, 2008

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Announcements

• No class on Thursday
• Homework 5 is out
• 2 ontology questions, 4 planning
• Homework due next class
• Tuesday December 2
• Almost end of the semester

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Question What Is This?
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Why Is Knowing This Important?
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(No Transcript)
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How Do You Make Cupcakes?

• Buy ingredients
• Mix ingredients
• Bake
• Put on frosting
• Put on even more frosting
• Give to TA

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How Do You Write That In Logic?
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How Do You Write That In Logic?

• um, i dont know
• We havent seen any way to write actions
• Our logic has no sense of time
• If action giveCupcake turns hasCupcake from False
  to True, what is the value of hasCupcake?
• No way to say False now, True in future
• Could do if we used a new type of logic such as
  temporal logic, event calculus, situation
  calculus, etc.
• Could pass time as a parameter to every
  predicate
• This is all kinda hard and messy

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What About Search?
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What About Search?
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Some Problems With Search

• Its a graph
• Means it can loop
• And in this case, it should (frosting x2)
• No obvious heuristic
• No way to specify ordering constraints
• Doesnt make sense to bake ingredients if you
  havent mixed them together
• Vague
• What exactly does each action do?
• How do we know if weve reached the goal?

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So How Do We Represent The Bribe-TA-With-Cupcake
Process?

• Both logic and search have problems
• Logic can represent facts but not actions
• Search cant facts but not actions
• Well, not facts and actions at the same time
• In CSP, we searched assignments, not actions
• Obvious solution use both
• States represented in logic
• Actions modify logic
• Solve the time problem each node on the path
  has its own state (set of logic facts)
• Heuristic Pick action that results in the most
  goal conditions being satisfied

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STRIPS

• A representation of how action affects state
• Has preconditions to make sure you dont execute
  an illegal action
• One of the first (and oldest) planning languages
• Modern languages such as PDDL are based on STRIPS
• Cant do everything you want
• Some things are very hard to do
• Missing features on purpose
• Done for computational efficiency
• Uses closed-world assumption

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Action Schema Structure
Action(actionName(parm1, parm2,
parmEtc), PRECOND Predicate1(parm2)
? SomethingElse(parm1), ADD
SomePredicate(parm2) ? YetAnotherPredicate(parm2
), DELETE Predicate1(parm1))
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Action Schema Structure (version 2)
Action(actionName(parm1, parm2,
parmEtc), PRECOND Predicate1(parm2)
? SomethingElse(parm1), EFFECT
SomePredicate(parm2) ? YetAnotherPredicate(parm2
) ? Predicate1(parm1))
This is a slightly newer syntax that merges the
ADD and DELETE lists. You can use whichever you
prefer.
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Example STRIPS Action Schema
Action(giveCupcake(from, to, cupcake), PRECOND
Has(from, object) ? IsCupcake(cupcake), ADD
Has(to, cupcake), DELETE Has(from, cupcake))
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STRIPS Action Schema Rules

• You cannot use any variable not passed in as a
  parameter
• Global constants OK
• No disjunctions (?)
• No existential quantifiers
• No functions
• No negations in PRECOND
• But OK in the EFFECT clause

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Things You Cant Do In STRIPS

• Action(give(from, to, object),PRECOND from?to
• Action(give(from, to, object),PRECOND Have(to,
  object)
• Action(give(from, to, object),PRECOND ?place
  At(from, place) ? At(to, place)
• Action(stealsPurse(victim),EFFECT Have(victim,
  all her stuff)
• Action(stealsPurse(victim),EFFECT if
  HadCash(victim) ? Crying(victim)
• Goal HasResearchUniversityJob ? HasIndustryJob

You can do all this stuff in ADL
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Lets Play Spot The Problem!
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Good or Bad?

• Action( Buy(person, ticket, from, to),
• PRECOND At(person, from) ? Connected(from, to)
  ?
• (Cost(ticket) lt Savings(person))
• ADD At(person, to)
• DELETE At(person, from)

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You Cannot Have Functions. Period.

• Action( Buy(person, ticket, from, to),
• PRECOND At(person, from) ? Connected(from, to)
  ?
• (Cost(ticket) lt Savings(person))
• ADD At(person, to)
• DELETE At(person, from)

Functions recursion possibly infinite search
space STRIPS no likey functions
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Good or Bad?

• Action( Buy(person, ticket, from, to),
• PRECOND At(person, from) ? Connected(from, to)
• ADD At(person, to)
• DELETE At(person, from) ? Savings(person,
  Price(ticket))

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Seriously, No Functions

• Action( Buy(person, ticket, from, to),
• PRECOND At(person, from) ?
• Connected(from, to)
• ADD At(person, to)
• DELETE At(person, from) ? Savings(person,
  Price(ticket))

Just making sure you were paying attention
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Good or Bad?

• Action( Buy(person, ticket, from, to),
• PRECOND At(person, from) ? Connected(from, to)
  ?
• (CanAfford(person, ticket) ? HasFreebie(person))
  
• ADD At(person, to)
• DELETE At(person, from)

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AND not OR

• Action( Buy(person, ticket, from, to),
• PRECOND At(person, from) ? Connected(from, to)
  ?
• (CanAfford(person, ticket) ? HasFreebie(person))
  
• ADD At(person, to)
• DELETE At(person, from)

No disjunctions in STRIPS
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OK, Yeah, Its Bad. But Why?

• Action( Fly(plane, from, to),
• PRECOND At(plane, from) ?
• Connected(from, to) ? Broken(plane)
• EFFECT At(plane, to) ? At(plane, from)

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Be Positive

• Action( Fly(plane, from, to),
• PRECOND At(plane, from) ?
• Connected(from, to) ? Broken(plane)
• EFFECT At(plane, to) ? At(plane, from)

You cant negate things in preconditions (but
effects are OK)
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Whats Wrong With This

• Action( Go(person, from, to),
• PRECOND At(person, from) ?
• ?door Connects(door, from, to)
• EFFECT At(person, from) ? At(person, to)

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No Existentials

• Action( Go(person, from, to),
• PRECOND At(person, from) ?
• ?door Connects(door, from, to)
• EFFECT At(person, from) ? At(person, to)

You cannot use variables in the precondition that
arent in the argument list.
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In-Class Exercise
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Give the TA Cupcakes!!!

• Actions
• buyIngredients
• mixIngredients
• bake
• addFrosting
• giveToTA

• Predicates
• HaveIngredients(person)
• HaveBatter(person)
• HaveCupcake(person)
• IsFrosted(cupcake)
• TA is a constant