Top-down hierarchical planning

A standard Artificial Intelligence mechanism used for simulating many

aspects of ‘intelligent’ behaviour

Main reference for planning:

Earl Sacerdoti (1977) A Structure for Plans and Behavior, Elsevier North-Holland.

For text planning:

Johanna Moore & Cécile Paris (1993) “Planning text for advisory dialogues: capturing intentional and rhetorical information”. Computational Linguistics 19(4):651-694

Requirements

• A representation of the ‘state of the world’: also called domain knowledge– Be-at (Ed, Washington)

• A library of plan operators appropriate for our planning tasks

• A planning mechanism, or algorithm, for constructing a plan to change the world from its current state to some goal state.

Plan operators

• The job of a plan operator is to say how some effect can be achieved – i.e., typically changing the world or someone’s

beliefs about the world

• to do this it must also:– say how the world must be in order for the plan

operator to work (the preconditions)– say what must be done in order for the effect to

come into operation

Abstraction hierarchies

• If all possible actions were equal, then planning is made even more complex because there are very many more planning operators to be considered at each step.

• Solution:– hierarchical planning

• actions are divided up into more specific parts• sequences of actions can be grouped together and given a

more abstract label.

– e.g.: dinner > cooking dinner, eating dinner, washing up cooking dinner > preparing the ingredients, cooking, serving

Action hierarchies

have dinner

prepare dinner wash up

eat dinner wash dishes

dry dishes

stack dishes

serve dinner

mix together

cook ingredients

cut ingredients

clean ingredients

Two kinds of operators

• Primitive operators– actions that are directly ‘executable’– no body

• Abstract operators– actions that need to be decomposed into less

abstract actions– decomposition specified in the body

A plan operator: definition

• Operator: names the operator and says what the main objects involved are

• Effect: states what effect (i.e., what change in the world) the successful application of the planning operator would bring

• Preconditions: states what has to be the case in the world for the plan to be used

• Body: says what actions a more abstract action must be broken down to in order to apply the operator

Only when the actions in the body have been applied is the effect achieved!!

A plan operator’s role during the planning process

PLAN OPERATOR

effect

preconditions

body

A complete plan…

• is a hierarchically organized sequence of actions

• carrying out the actions in sequence will guarantee that the preconditions for achieving all subsequent effects are met in time

• when all the sub-actions of a more abstract action have been performed, the abstract action has also been achieved and the corresponding effect also