1. Consider the task get milk, bananas, and a cordless drill

2. Consider the task get milk, bananas, and a cordless drill

3. Search vs. planning Standard search algorithms seem to fail miserably:
Problems with search approach
too many irrelevant actions
finding good heuristics is difficult
cannot take advantage of problem decomposition

5. Planning as search with logic
Planning systems do the following:
1. open up action and goal representation to allow selection
2. divide-and-conquer by subgoaling
3. relax requirement for sequential construction of solutions

6. Classic Planning
Planning: devising a sequence of actions achieving a goal
fully observable
deterministic
static
discrete
specific language to represent planning problems

7. Language for planning problems
STRIPS: STanford Research Institute Problem Solver
world described by logical conditions
state as conjunction of positive literals
propositional; e.g., Happy ^ Hungry to represent the state of the agent
first-order ground and function-free terms;
e.g., At(Plane1, Verona) ^ At(Plane2,Malpensa)
closed-world assumption; i.e., any not mentioned condition is false
goal is a partially specified state
a state satisfies a goal if contains all the literals of the goal
e.g. state At(Plane1, Verona) ^ At(Plane2,Malpensa) satisfies goal At(Plane2,Malpensa)

8. STRIPS actions actions are described by actions contain variables
preconditions: when the action can be applied
effects: state changes by the action
delete-list: propositions that become false
add-list: propositions that become true
actions contain variables
a single action schema represents different actions (instantiation of variables)

9. STRIPS operators Have(X) ^ GoesOn(X,Y) ^ Bare(Y) PutOn(X) HaveOn(X)
In order to PutOn(LeftSock) we have to Have(LeftSock) ^ GoesOn(LeftSock,LeftFoot) ^ Bare(LeftFoot)
This leaves us with HaveOn(LeftSock)

10. STRIPS actions (contd.)
Tidily arranged actions descriptions, restricted language
Action schema:
ACTION: specifies name and parameter list
Buy(x)
PRECONDITION: conjunction of positive literals
At(p) ^ Sells(p, x)
EFFECT: conjunction of literals (positive or negative)
Have(x)
[Note: no information on how to execute the action!]
A complete set of STRIPS operators can be translated into a set of successor-state axioms

11. Semantics Given a state (conjunction of literals)
precondition is satisfied if there is a variable assignment s.t. the literals are included in the state; e.g.,
state At(HW) ^ Sell(HW,Drill)
satisfies precondition At(p) ^ Sells(p, x)
with assignment p/HW and x/Drill
actions with preconditions satisfied can be applied
delete items from the delete-list
add items from the add-list
order does matter!
new state: At(HW) ^ Sell(HW,Drill) ^ Have(Drill)

12. Example: Shopping
Start State?
Goal State?

13. Example: Shopping Start State? Goal State? At(Home)
Have(Milk) ^ Have(Banana) ^ Have(Drill)
At(Home) ??

14. Example: Shopping Start State? Goal State? Actions? At(Home)
Have(Milk) ^ Have(Banana) ^ Have(Drill) ^ At(Home)
Actions?
Buy(x)
Go(x, y)

15. Example: Shopping Start State? Goal State? Actions? At(Home)
Have(Milk) ^ Have(Banana) ^ Have(Drill) ^ At(Home)
Actions?
Buy(x)
PRE: At(store), Sells(store, x)
EFF: Have(x)
Go(x, y)
PRE: At(x)
EFF: At(y),¬At(x)