1Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 511Centralized vs. DecentralizedInterpreter PatternVisitor PatternObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 512Defining a pictureTextFig.DrawingCompFig.CompFig.RectangleFig.TextFig.RectangleFig.2Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 513Defining a Language for PicturesFigure - displayOn: is abstract methodCompositeFigureDrawingRectangleFigureTextFigureObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 514Interpreter PatternTree of objects is a kind of programInterpret program by sending message toroot, which is recursively sent to entiretree.Not about parsing!!!3Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 515Object-Oriented InterpretersTo write a little object-oriented interpreterfor a language L:• 1) make a subclass of LParseNode foreach rule in the grammar of L• 2) for each subclass, define aninterpreter method that takes thecurrent context as an argument.Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 516Object-Oriented Interpreters• 3) define protocol for making a tree ofLParseNodes.• 4) define a program for L by buildingan abstract syntax tree.4Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 517Example: ValueModelsValueModelvalueValueHoldervaluevalueBlockValueblockargumentsvalueObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 518ValueModels| x y |x := 3 asValue.y := 7 asValue.x + yValueModel>> + aValue^ BlockValueblock: [:a :b | a value * b value]arguments: (Array with: self with:aValue asValue)ValueHolderBlockValueValueHolder5Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 519Example: Regular Expression CheckerGrammar: exp ::= string | exp ‘+’ exp | exp ‘&’ exp | exp‘repeat’Step 1: define RegExpNode, MatchRENode,AlternationRENode, SequenceRENode,RepeatRENodeStep 2: define a match: method for each classthat takes an input state as an argumentRegular Expression ClassesRegExpNodeSequenceRENodeMatchMatchMatchRENodecontentsMatchRepeatRENodeMatchAlternationRENodeMatch6Regular Expression Objects'this is a ' ('very ' *) ('big ’ + 'small ') ('dog' + 'cat' + 'mouse')SequenceRENodeMatchRENode'this is a 'RepeatRENodeMatchRENode'big 'AlternationRENodeAlternationRENodeMatchRENode'small 'MatchRENode'very 'MatchRENode'dog'MatchRENode'mouseMatchRENode'cat'this is a very very very big mouseObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 522MatchingSequenceRENode>>match: inputState^componentsinject: inputStateinto: [:nextState :exp |exp match: nextState]7Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 523MatchingRepeatRENode>>match: inputState| aState finalState |aState := inputState.finalState := inputState copy....Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 524(continued)[aState notEmpty]whileTrue:[aState := component match: aState.finalState addAll: aState].^finalState8Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 525MatchingAlternationRENode>>match: inputState| finalState |finalState := REState new.components do: [ :exp | finalStateaddAll: (exp match: inputState)]^finalStateObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 526MatchingMatchRENode>>match: inputState| finalState tStream |finalState := REState new....REState is a collection of streams.9Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 527(continued)inputStatedo: [:stream |tStream := stream copy.(tStream nextAvailable:components size) = components ifTrue: [finalState add: tStream]].^finalStateObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 528Protocol for Building TreeDefine ”+" and "repeat" and ”&" asmethods on RegExpNodes and anyliterals that are to be treated as patterns,such as strings.Then ((’dog ’ + ’cat ’) repeat & ’weather’)matches: ’dog dog cat weather’ is true.10Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 529Interface methodRegExpNode>>matches: anArg| inputState |inputState := (streamOrCollection isKindOf:Collection)ifTrue: [REState with: (ReadStream on:streamOrCollection)]ifFalse: [REState with:streamOrCollection].(self match: inputState) do: [:stream | streamEdifT[^ ]]Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 530Interface methodsDefine +, &, repeat, and asRENode inRegExpNode and String+ anArg^AlternationRENode newcomponents: (Array with: self with:anArg asRENode)11Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 531Other examples of Interpreter pattern:• • producing Postscript for a document• • figuring out the value of an insurancepolicy• • spreadsheet• • compiling a programC program would use case statementObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 532Replacing Cases with SubclassesAdvantages• • instead of modifying casestatements, add a newsubclass• • easier to parameterize• • can use inheritance tomake new options12Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 533Replacing Cases with SubclassesDisadvantages• • program is spread out,• + harder to understand• + harder to replace algorithm• • state of object can change, but classcan notObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 534How To Centralize AlgorithmsDefine isAlternative, isRepeat, isSequence,isMatch, childrenDo:fix: anRENodeanRENode isMatchifTrue: [anRENode contents:(anRENode contents capitalize)]ifFalse: [anRENode childrenDo: [:child| self fix: child]]13Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 535When to Centralize AlgorithmUse centralized algorithm when youneed to• • change entire algorithm at once• • look at entire algorithm at once• • change algorithm, but not add newclasses of componentsObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 536Visitor patternVisitor lets you centralize algorithm, letsyou create a family of algorithms byinheritance, and makes it easy to createnew algorithms.Major problem is that adding a new kind ofparse node requires adding a newmethod to each visitor.14Object-oriented