Knowledge and Systems Research Group, University of Huddersfield

B vs OCL: Comparing Specification Languages for

Planning DomainsDiane Kitchin, Lee McCluskey, Margaret West

TALK to be given by: Ron Simpson

Knowledge and Software Systems

Research Group,

The University of Huddersfield

Knowledge and Systems Research Group, University of Huddersfield

Formal Software Specification Languages

Artificial Intelligence Planning Domain Model Languages

Process/ Systems/ Requirements Modelling Languages

CSP

Petri Nets

Z

BVDMUML

SSADM

PDDL OCLh

Declarative/ Descriptive Languages

SE

AI

Knowledge and Systems Research Group, University of Huddersfield

Formal Specification Languages (eg B) are ‘close’ to Planning Domain Model Languages (eg OCLh)

They share: the concept of a `state' the technique of using pre and post conditions in

state transformation, via operations to specify state dynamics

the assumptions of closed world, default persistence and instantaneous operator execution

the presence of state invariants for validity and documentation purposes.

Knowledge and Systems Research Group, University of Huddersfield

Capturing domain knowledge very accurately.. the need:

Real Planning Applications now exist in safety-related areas:

Space mission control Military resource control Robot control

- among many other implications (!) there is a need to establish and maintain highly accurate domain knowledge

Knowledge and Systems Research Group, University of Huddersfield

Accurate domain knowledge.. How? Our current method using GIPO:

Knowledge Sources

Operational Domain Model

Knowledge Formulation

Validation Processes

GIPOOCLh

Knowledge and Systems Research Group, University of Huddersfield

Accurate domain knowledge.. How? Method using a FSL:

Knowledge Sources

Formal Specification

Operational Domain Model

Knowledge Formulation

Validation and Translation

Knowledge and Systems Research Group, University of Huddersfield

Accurate domain knowledge.. How? FSL = ‘B’, Tool = B-Toolkit

Knowledge Sources

Formal Specification

Operational Domain Model

Knowledge Formulation

Validation and Translation

‘BToolkit’B

Knowledge and Systems Research Group, University of Huddersfield

Paper content: Compare GIPO to B-Toolkit (OCLh vs B)

Both GIPO and B-Toolkit are tools environments that provide tools for the capture and debugging of formal models

The B method is of industrial-strength and while the B-Toolkit is commercially available, GIPO is a research tool

Knowledge and Systems Research Group, University of Huddersfield

Simple Comparison Method

Take some typical worlds which have been represented in OCLh using GIPO (Blocks World, Tyre World).

Re-formulate them using the B-Toolkit. Compare the Process and Result of the

formulation.

Knowledge and Systems Research Group, University of Huddersfield

Domain Model Formulation in OCLh : GIPO

. GIPO contains domain model editors basic static analysis checking tools.

is object class hierarchy consistent?do object state descriptions satisfy invariants?are predicate structures and operator schema mutually consistent?are task specifications consistent with the domain model?

a plan stepper a reachability analyser solution animator a random task generator hierarchy consistency check operator learning mechanism

Acquisition of Objects/Classes /Invariants

Operator Acquisition

Static Analysis Tools

More Static Analysis

Solving simple tasks using The Plan Stepper

Solving more complexTasks with Planners

Knowledge and Systems Research Group, University of Huddersfield

GIPO’s GUI

Knowledge and Systems Research Group, University of Huddersfield

Domain Model Formulation in B: B-Toolkit

Define a set of abstract machines To define each machine, define its variables, its

invariant, its initialisation and operations

e.g.

MACHINE: Blocks_World

SETS: Block, Gripper

VARIABLES: On_Block, On_Table, Gripped, Free

Knowledge and Systems Research Group, University of Huddersfield

B-Toolkit Animator

Knowledge and Systems Research Group, University of Huddersfield

Domain Model Formulation in B: Language Comparison

OCL B

Primitive sorts Sets

Predicate Names Variable Names

Operator Schema Operations

Properties Boolean-valuesFunctions

Predicates(X,Y) Relationsbetween X and Y

Knowledge and Systems Research Group, University of Huddersfield

Domain Model Formulation in B: B-Toolkit

Generate and Discharge ‘Proof Obligations’

A set of proof obligations involving consistency properties of a system can be automatically generated by the B-Tool. e.g.:

(1) Consistency of initialisation: the initialisation must establish the invariant.

(2) Consistency of operation: each operation must preserve the invariant.

Execute (Animate) Operations

Knowledge and Systems Research Group, University of Huddersfield

Knowledge and Systems Research Group, University of Huddersfield

Knowledge and Systems Research Group, University of Huddersfield

Knowledge and Systems Research Group, University of Huddersfield

Discharged Proof Obligation

Knowledge and Systems Research Group, University of Huddersfield

General Results - 1 B allows the user to encode more precise details about

the relations in the domain than GIPO - they can be relations, 1-1 functions etc, and can be given accurate restrictions. This level of precision is certainly not available in most planning languages, and is attractive in safety-related applications.

As OCLh is aimed specifically at planning, it has in-built structures and mechanisms that anticipate the entities that are to be represented. This makes the encoding rather more compact than the B specification.

Encoding in B, as one would expect from a general language, one is left with more choices and decisions in the encoding process.

Knowledge and Systems Research Group, University of Huddersfield

General Results -2

Both languages assume default persistence and a closed world. The differences in this respect are subtle, in that in B a variable involved in a precondition remains unaltered by default. However in OCLh there is a local open-world assumption in operator descriptions - a `prevail' is required if precondition variables are unchanged.

Regarding validation and debugging, both GIPO and the B-Toolkit provide effective, automated tool support which performs validation/consistency checks and identifies the presence of bugs. Not surprisingly, the B-Toolkit was more reliable at finding inconsistencies in some cases, as it demands a more detailed specification, and the tool is more developed.

Knowledge and Systems Research Group, University of Huddersfield

ConclusionsThe comparison shows a remarkable similarity between the two languages

and tool functions. The advantages in using a method such as B are that

- it is mathematically based so that formal reasoning can be used to deduce desirable (and potentially undesirable) properties.

- support for the method is available via tools - such as the Toolkit.

However, the disadvantages are that

- there are no special planning - oriented features,

- the B specification, once validated, would have to be translated into a more planner-friendly language in order to be used with current planning engines.

FUTURE WORK:

- build a B -> OCL translator

- try out B on a complex planning world