Action Rules for Programming Constraint Propagators and

Interactive User Interfaces

Neng-Fa ZhouCUNY Brooklyn College & GC

zhou@sci.brooklyn.cuny.eduwww.sci.brooklyn.cuny.edu/~zhou

Outline of Talk

Motivation Action Rules Programming Constraint Propagators

(www.sci.brooklyn.cuny.edu/~zhou/papers/arule.ps)

CGLIB: Constraint-based Graphics Library(www.sci.brooklyn.cuny.edu/~zhou/papers/acmdoc01.pdf)

Demo of B-Prolog v.6 (www.probp.com)

Motivation

The need for a language for programming reactive systems– Constraint propagators– Event handling– Concurrency– Interactive user interfaces– Agents

Operational semantics– An agent (sub-goal) C will be suspended if:

• C matches Agent,

• ConditionSeq is met, and • EventSet is not empty

– C is activated whenever an event in EventSet is posted

– ActionSeq is executed when C is activated and ConditionSeq is met

– C is suspended again after ActionSeq is executed

– The next rule is tried if ConditionSeq fails

– C fails if ActionSeq fails

Agent, ConditionSeq, {EventSet} => ActionSeqAgent, ConditionSeq, {EventSet} => ActionSeq

Syntax

Action Rule

Posting Eventspost(E) event(X,O)

– X – suspension variable – O – event object

Events for programming propagators– ins(X) – X is instantiated– …

Events for graphics programming– actionPerformed(O) – action performed on O– …

Example 1An Echoing Agent

echo_agent(X), {event(X,Message)} => write(Message),nl.

Example 2freeze(X,G)

freeze(X,G), var(X), {ins(X)} => true.freeze(X,G) => call(G).

Programming Constraint Propagators Action rules are expressive

– Propagators for maintaining node, interval, arc consistency

– Propagators for global constraints

Action rules are efficient– B-Prolog is competitive with Ilog solver and

GNU-Prolog

Constraint Propagation

X = Y+1 (X,Y in 1..5)X = Y+1 (X,Y in 1..5)

Algorithm Change Algorithm Change PropagationPropagation

Node consistencyNode consistency X=3X=3 X = 3, Y = 2X = 3, Y = 2

Interval consistencyInterval consistency generatedgenerated X in 2..5, Y in 1..4X in 2..5, Y in 1..45 notin X 5 notin X X in 2..4, Y in 1..3X in 2..4, Y in 1..3

Arc consistencyArc consistency 4 notin X4 notin X X in [2,3,5], Y in [1,2,4]X in [2,3,5], Y in [1,2,4]

Events for Programming Propagators ins(X)

– X is instantiated

minmax(X)– The bound of X is updated

dom(X,E)– An inner element E has been excluded from X

Propagators for aX=bY+c

Forward checking

'aX=bY+c_forward'(A,X,B,Y,C),var(X),var(Y),{ins(X),ins(Y)} => true.

'aX=bY+c_forward'(A,X,B,Y,C),var(X) => T is B*Y+C,Ex is T//A,

(A*Ex=:=T->X = Ex; true). 'aX=bY+c_forward'(A,X,B,Y,C) => T is A*X-C, Ey is T//B,

(B*Ey=:=T->Y is Ey;true).

When either X or Y is instantiated, instantiate the other variable.

Propagators for aX=bY+c

Interval consistency

'aX in bY+c_interval'(A,X,B,Y,C),var(X),var(Y),{minmax(Y)} => 'aX in bY+c_reduce_domain'(A,X,B,Y,C).

'aX in bY+c_interval'(A,X,B,Y,C) => true.

Whenever the bound of Y’s domain is updated,reduce X’s domain to achieve interval consistency.

Propagators for aX=bY+c

Arc consistency

'aX in bY+c_arc'(A,X,B,Y,C),var(X),var(Y),{dom(Y,Ey)} => T is B*Ey+C, Ex is T//A, (A*Ex=:=T -> exclude(X,Ex);true).

'aX in bY+c_arc'(A,X,B,Y,C) => true.

When an element Ey is excluded from Y’s domain,exclude Ey’s counterpart Ex from X’s domain.

Propagator for A1*X1+...+An*Xn+C = 0

'A1*X1+...+An*Xn+C=0'(C,A1,A2,...,An,X1,X2,..,Xn),n_vars_gt(n,2),{ins(X1),minmax(X1),...,ins(Xn),minmax(Xn)}

=> reduce domains of X1,..,Xn to achieve ic. 'A1*X1+...+An*Xn+C=0'(C,A1,A2,...,An,X1,X2,..,Xn)

=> nary_to_binary(NewC,B1,B2,Y1,Y2), call_binary_propagator(NewC,B1,Y1,B2,Y2).

When the constraint has more than 2 variables,achieve interval consistency.

When the constraint is binaryarchive arc consistency.

Performance Evaluation

B-PrologIlogGNU-Prolog

CHIP

EclipseSicstusMozart Oz

Fast

Slow

1

2

4

CGLIB --A Constraint-based Graphics Library Features

– Use constraints to specify the layouts of objects

– Use action rules to specify interactions

Implementation– Implemented in B-Prolog, Java, and C

Applications– Drawing editors, interactive user interfaces, document

authoring, animation, information visualization, intelligent agents, and games.

An Illustrative Example

go:-

cgButton(B), (1)

B^text #= “Hello World!”, (2)

handleButtonClick(B), (3)

cgShow(B). (4)

 

handleButtonClick(B), (5)

{actionPerformed(B)} (6)

=> (7)

halt. (8)

Objects

cgButton(X)– Create a button X

cgButton([X1,…,Xn])– Create a list of buttons

Attributes of Objects

Base Derivative

x

y

width

height

B is a button

B^windowB^xB^y B^widthB^heightB^text B^colorB^font

B^rightXB^bottomYB^centerXB^centerYB^leftTopPointB^leftBottomPointB^rightTopPointB^rightBottomPointB^center

Constraints

Arithmetic constraints•O1^x +O1^width//2 #= O2^x •O1^centerX #= O2^x

Positioning constraints•cgLeft(O1,O2)

Same property constraints•cgSame([O1,O2],size)•cgSame([O1,O2],width,100)

Constraints (Cont.)

Grid constraints

cgGrid([[_, S1,_], [S2,S3,S4], [_,S5,_]])

cgGrid([[Bc,Bdiv,Bmul,Bsub], [B7,B8, B9, Badd], [B4,B5, B6, Badd], [B1,B2, B3, Beq], [B0,B0, Bdot,Beq]],1,1),

Constraints (Cont.)

Table constraints

cgTable([[Larc,Arc,Button,Lbutton],

[Lcheckbox, Checkbox, Choice, Lchoice],

[Lcircle,Circle,Image,Limage],

[Llabel,Label,Line,Lline],

[Llist,List,Oval,Loval],

[Lrrectangle,Rrectangle,Rectangle,Lrectangle],

[Lpolygon,Polygon,Square,Lsquare],

[Lstar,Star,TextArea,Ltextarea],

[Ltextfield,TextField,TextBox,Ltextbox],

[Ltriangle,Triangle,_,_]],20,20),

Constraints (Cont.)

Tree constraintsT=node(C0,[node(C1,[node(C3,[]

node(C4,[])]), node(C2,[node(C5,[]),

node(C6,[])])]),cgTree(T,top_down,10,10,centered).

Event Handling

Action rule

Examples

Agent ConditionSeq {Event} => ActionSeq

handleButtonClick(B),{actionPerformed(B)} => …

handleMousePress(O),{mousePressed(O,E)} =>E^x #= X, E^y #= Y,

write(mouse_pressed_at(X,Y)),nl.

handleKeyType(O),{keyTyped(O,E)} =>E^char #= Char, write(Char).

Other Primitives in CGLIB

Packing and showing objects• cgPack(O)& cgShow(O)

Altering and moving objects• O^attr #:= Value• cgScale(O,F)& cgResize(O,W,H)• cgMove(O,X,Y)

Animation• cgSleep(Time)

Generating Java applets• cgStartRecord(FileName) & cgStopRecord

Demo of B-Prolog

B-Prolog (Version 6.0)(www.probp.com)

– Graphical user interfaces

– Animation

– Information visualization

– Constraint satisfaction problems

– Games

Related Work

Programming Constraint Propagators• Indexicals (GNU-Prolog & Sicstus)• Delay constructs• Attribute variables (Eclipse)• Constraint handling rules

Graphics programming– Postscript & Latex

• Every detail must be specified

– Java & Tcl/Tk• Layout managers are helpful for certain layouts

– Constraints & Constraint programming• Sketchpad, ThinkLab, Amulet, SubArctic