Contextual Analysis. Overview of Lecture 3. Ch 4 Syntactic Analysis. Mededelingen

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1 Overview of Lecture 3 otextual Aalysis Mededelige h 4 Sytactic Aalysis Abstract Sytax Trees VB H3 Vertalerbouw H3 h 5 otextual Aalysis 5.1 Idetificatio 5.2 Type heckig 5.3 otextual Aalysis algorithm 5.4 ase study: otextual Aalysis for Triagle Theo Ruys Uiversity of Twete epartmet of omputer Sciece Formal Methods & Tools Michael Weber!! kamer: Zi 5037" telefoo: 3716" michaelw@utwete.l! 2 Mededelige h 4 Sytactic Aalysis Opgaveserie 1 komt z.s.m. beschikbaar op de Vertalerbouwwebsite! deadlie: woesdag 18 mei 2011 om uur! wees precies: slordighede zij meestal fout 4.1 Subphrases of sytactic aalysis 4.2 Grammars revisited 4.3 Parsig 4.4 Abstract Sytax Trees 4.5 Scaig 4.6 ase study: Triagle compiler 3 4

2 Recursive-escet Parsig H2 Recursive-escet Parsig H2 Systematic developmet of a recursive-descet parser: 1. Express the grammar i EBNF. 2. Grammar trasformatios:! elimiate left recursio! left-factorizatio 3. reate a Java Parser class with! protected variable currettoke! methods to call the scaer: accept ad acceptit! public method parse which gets the first toke from the scaer, ad calls the parse method of the root o-termial of the grammar 4. Implemet protected parsig methods! add protected methods parsen for each o-termial N public class MicroEglishParser { protected Toke currettoke; public void parse() { currettoke = first toke; parsesetece(); check that o toke follows the setece protected void accept(toke expected) { protected void parsesetece() { protected void parsesubject() { protected void parseobject() { protected void parsenou() { protected void parseverb() { iterface with the scaer which provides the tokes I Watt & Brow, the parse methods are declared private. This is ufortuate as it does ot allow customizatio of the parser through iheritace. 5 6 Abstract Sytax Trees (1) Abstract Sytax Trees (2) A recursive-descet parser builds the sytax tree implicitly by the call graph of the parse methods.! I a oe-pass compiler this is OK.! I a multi-pass compiler we eed a explicit represetatio of the (abstract) sytax tree. Remember that each otermial XYZ is coverted to a parse method parsexyz: protected void parsexyz( ) { Istead of returig othig, the method could retur somethig iterestig. What about a AST ode? Furthermore, other parse methods that call this method could pass useful iformatio to this method usig parameters. 7 Program ::= ommad Program ommad ::= ommad ; ommad Sequetialmd V-ame := Expressio Assigmd Idetifier ( Expressio ) allmd Grammar for Mii-Triagle s abstract sytax. if Expressio Ifmd the ommad else ommad while Expressio do ommad Whilemd let eclaratio i ommad Letmd Expressio ::= Iteger-Literal ItegerExpr V-ame VameExpr Operator Expressio UaryExpr Expressio Operator Expressio BiaryExpr V-ame ::= Idetifier SimpleVame eclaratio ::= eclaratio ; eclaratio Seqecl cost Idetifier ~ Expressio ostecl var Idetifier : Type-deoter Type-deoter ::= Idetifier AST odes of Mii-Triagle ypee 8

3 Abstract Sytax Trees (3) Abstract Sytax Trees (4) ommad ::= ommad ; ommad Sequetialmd V-ame := Expressio Assigmd Idetifier ( Expressio ) allmd if Expressio the sigle-ommad Ifmd else sigle-ommad while Expressio do sigle-ommad Whilemd let eclaratio i sigle-ommad Letmd We eed to defie Java classes to capture the structure of Mii- Triagle ASTs. We itroduce the abstract class AST. public abstract class AST { Sequetialmd Assigmd allmd Every ode i the AST will be a object of a subclass of AST. Each subclass has istace variables for the childre odes. 1 2 Ifmd V E Whilemd Idet E Letmd public class Program exts AST { public ommad ; ommad is the abstract base class for all ommad AST odes. E 1 2 E public abstract class ommad exts AST { 9 10 ASTs (5) abstract class ommad exts AST { ommad ::= ommad ; ommad Sequetialmd V-ame := Expressio Assigmd Idetifier ( Expressio ) allmd if Expressio the sigle-ommad Ifmd else sigle-ommad while Expressio do sigle-ommad Whilemd let eclaratio i sigle-ommad Letmd public class Sequetialmd exts ommad { public ommad 1, 2; Sequetialmd public class Assigmd exts ommad { 1 2 public Vame V; public Expressio E; Assigmd V E public class allmd exts ommad { allmd public Idetifier I; public Expressio E; Idet E Ifmd public class Ifmd exts ommad { public Expressio E; public ommad 1, 2; E 1 2 etc. The AST subclasses should have costructors to build a object of these classes. 11 Abstract Sytax Trees (6) It is straightforward to make a recursive-descet parser costruct a AST to represet the phrase structure.! We make each method parsen (as well as parsig a N-phrase), retur the N-phrase s AST.! We let the body of a method parsen costruct the N-phrase AST by combiig the ASTs of ay subphrases. Thus, for productio rule N::=X protected AST N parsen() { AST N itsast; parse X, at the same time costructig itsast retur itsast; 12

4 Abstract Sytax Trees (7) h 5 otextual Aalysis EBNF ommad ::= sigle-ommad (; sigle-ommad)* AST ommad ::= ommad ; ommad Sequetialmd protected ommad parseommad() { ommad c1ast = parsesigleommad(); while (currettoke.kid == Toke.SEMIOLON) { acceptit(); ommad c2ast = parsesigleommad(); c1ast = ew Sequetialmd(c1AST, c2ast); retur c1ast; Seqmd 5.1 Idetificatio 5.2 Type checkig 5.3 otextual Aalysis algorithm 5.4 ase study: Triagle compiler Seqmd md 3 md 1 md ompiler Phases otext ostraits scope rules H1 Source Program Scope rule - examples sometimes referred to as sematic aalysis. h. 8: Iterpreter Sytax Aalysis AST otextual Aalysis ecorated AST AST ode Geeratio Object ode (Abstract) Machie h. 4 h. 5 h. 7 heckig the laguage s cotextual costraits. h. 6: Ru-Time Orgaizatio?? let cost m ~ 2; var : Iteger := m*2; declaratio of : bidig occurrece use of : applied occurrece let If there is o eclosig scope var : Iteger; (i.e. letmd) where m is declared, the bidig occurece of m is missig: scope error! := m*2; applied occurrece 15 16

5 otext ostraits type rules H1 Sytax Aalysis Source Program Sytax Aalysis H2 Errors Type rule - example let var : Iteger while > 0 do := -1; Type rule for E 1 > E 2 (GreaterOp): If E 1 ad E 2 are both of type it, the the result is of type bool. Type rule for while E do (Whilemd): E must be a boolea. let var : Iteger; var c: har c := & ; := +1 Sequetialecl Program Abstract Sytax Tree Letmd Sequetialmd Assigmd Assigmd BiaryExpr harexpr VameExpr Type rule for V:=E (Assigmd): The types of V ad E must be equivalet. Type rule for E 1 -E 2 (SubOp): If E 1 ad E 2 are both of type it, the the result is of type it. Idet har-lit Idet Idet Op ItExpr It-Lit Iteger c har c & otextual Aalysis Abstract Sytax Tree otext Aalysis H2 Errors Thigs to check Two sub phases: scope rules are checked i the idetificatio phase type rules are checked i the type checkig phase Sequetialecl Program Letmd Assigmd Iteger c har c & ecorated Abstract Sytax Tree Sequetialmd BiaryExpr harexpr :char VameExpr ItExpr :char Idet har-lit Idet Idet Op It-Lit Assigmd + 1 A applied occurrece of a idetifier must have a matchig defiig occurrece. The relatio betwee applied ad defiig occurrece might be added to the AST. Fuctio calls must refer to defied fuctios. I a assigmet, the idetifier o the left-had side should refer to a variable. The expressio of a if or while should be boolea. Whe callig procedures, the umber ad types of the actual parameters (argumets) should match the umber ad types of the formal parameters. etc. All follow from the cotext costraits defied for the (programmig) laguage

6 Symbol Table (1) See exercise 1.3 of the laboratory sessio of week 1. Block Symbol table (called idetificatio table i W&B)! ictioary-style data structure i which idetifiers are stored together with their attributes.! Attributes type: it, char, boolea, record, array, poiter, etc. kid: costat, variable, procedure, fuctio, value-parameter, referece-parameter, etc. visibility: public, private, protected other importat characteristics! Typical operatios eter a idetifier ad its attributes ito symbol table retrieve the attributes for a idetifier other operatios dep o the block structure of the laguage. scope of a declaratio: area of the program over which the declaratio takes effect. block: area of program text that delimits the scope of declaratios withi it.! Triagle s block commads let eclaratios i ommads proc P(formal-parameters) ~ ommads! I Java: A block is a sequece of statemets, local class declaratios ad local variable declaratio statemets withi braces. for (it i=0; i<a.legth; i++) { Strig s = a[i]; System.out.pritl(s); Both i ad s caot be used outside the for-loop Moolithic block structure Flat block structure program eclaratios sequece of declaratios sequece of commads begi ommads haracteristics! Oly oe block: etire program! All declaratios are global i scope. Scope rules! No idetifier may be declared more tha oce.! No idetifier may be used uless declared. Symbol table! For every idetifier there is a sigle etry i the symbol table.! Retrieval should be fast (e.g. biary search tree or hash table). BASI, OBOL,! 23 program procedure P begi procedure Q begi begi haracteristics! program has several disjoit blocks! two scope levels: global ad local Scope rules! No globally declared idetifier may be redeclared globally.! No locally declared idetifier may be redeclared i the same block.! No idetifier may be used uless declared (globally or locally) Symbol table! Symbol table should cotai etries for both global ad local declaratios.! After aalysis of a block has completed, its local declaratios ca be discarded. Fortra, 24

7 Nested block structure (1) Nested block structure (2) program procedure P procedure PP proc PPP begi begi procedure Q begi begi haracteristics! blocks ca be ested withi each other! may scope levels Scope rules! No idetifier may be declared more tha oce i the same block.! No idetifier may be used uless declared (i local or eclosed blocks). Symbol table! Several etries for each idetifier.! But, at most oe etry for each (scope level, idetifier) combiatio.! Highest-level etry of a idetifier should be retrieved (fast). Pascal, Modula, Ada, Java, etc. 25 let!level 1 var a, b, c ; let!level 2 var a, b ; let!level 3 var a, c ; a := b + c ; ; a := b + c ; ; a := b + c ; Scope ad visibility a ad b of level 1 get redefied ad are ot visible o level 2 a of level 2 ad c of level 1 get redefied ad are ot visible o level 3 26 Scope structure Symbol Table (2) Example For a statically scoped laguage with ested block structure, the structure of the scopes ca be see as a tree. Global P Q P1 P2 P3 * P Global P1 P2 P3 Q Lookup path for a applied occurrece withi P3 At ay time (whe aalysig the program), oly a sigle path i the tree is visible. let!level 1 (1) var a: Iteger; var b: Boolea (2) let!level 2 (3) var b: Iteger; var c: Boolea (4) let!level 3 (5) cost x ~ 3 i let!level 2 (6) var d: Boolea; (7) var e: Iteger level id Attr. 1 a (1) 1 level b (2) id Attr. 1 a (1) 1 b (2) 2 b (3) 2 level c (4) id Attr. 1 a (1) 1 b (2) 2 b (3) 2 c (4) 3 x (5) level id Attr. 1 a (1) 1 b (2) 2 d (6) 2 e (7) 27 28

8 Symbol Table (3) Symbol Table (4) Symbol table additioal operatios! ope a ew scope level! close the highest scope level public class SymbolTable { /** Ope a ew scope. */ public void opescope() Attribute: holds all importat iformatio o a defied occurrece (type, kid, level, visibility, etc.) differs for differet laguages /** losest the highest (curret) scope. */ public void closescope() /** Eters a id together with its Attribute. */ public void eter(strig id, Attribute attr); /** Returs the Attribute of id, defied o the * highest level. Retur ull if ot i table. */ public Attribute retrieve(strig id) /** Returs the curret scope level. */ public it curretlevel() Would have bee better OO-practice if we had declared the SymbolTable to be a iterface or a abstract class. 29 Possible implemetatio: public class SymbolTable { private Map<Strig,Stack<Attribute>> symtab; private Stack<List<Strig>> scopestack; Oly used to optimize the closig of a scope. The symtab is a Map from Strigs to Stack-objects. The keys are the Strig represetatios of the idetifiers. The value of a idetifier s Strig is a Stack of Attributes; the Attributes of the idetifier declared o the highest scope level is always o top. The scopestack is a Stack of List-objects (cotaiig Strigs). Whe a scope is opeed, a empty List is pushed o the scopestack; the Strig-represetatio of each idetifier foud i this curret scope will be added to this list. Whe a scope is closed, the idetifiers of this old scope (which are all i the top List of scopestack) are removed from symtab. The List of the old scope is popped from scopestack. 30 Symbol Table (5) Previous example usig a map of <Strig, Stack<Attribute>>. Attributes (1) let!level 1 (1) var a: Iteger; var b: Boolea (2) let!level 2 (3) var b: Iteger; var c: Boolea; (4) let!level 3 (5) cost x ~ 3 i let!level 2 (6) var d: Boolea (7) var e: Iteger id a b c x level id Attr. 1 a (1) 1 level b (2) id Attr. 1 a (1) 1 b (2) 2 b (3) 2 level c (4) id Attr. 1 (1) 1 a (1) 1 b (2) 2 (3) 2 b (3) 1 (2) 2 c (4) 2 (4) 3 x (5) 3 (5) Attributes should (at least) cotai iformatio for! checkig the scope rules Successful retrieval of a applied occurrece i the symbol table is eough: it meas that there is a bidig occurece.! checkig the type rules The type of a idetifier has to be stored.! (code geeratio) address of the variable Possible approaches! Store the attribute completely ito the symbol table.! As the AST ode of the bidig occurrece should have access to all ecessary iformatio, we could also store refereces (poiters) to these odes i the symbol table

9 Attributes (2) Imperative approach for storig attributes explicitly. public class Attribute { public static fial byte // kid ONST = 0, VAR = 1, PRO = 2, ; public static fial byte // type BOOL = 0, HAR = 1, INT = 2, ARRAY = 3, ; Attributes (3) OO approach for storig attributes explicitly. public class Attribute { public Kid kid; public Type type; Kid ostkid VarKid ProcKid FucKid... Type Works, but for a realistic laguage this ca become quite tedious ad complex. public byte kid; public byte type; suitable for little laguages BoolType hartype ItType ArrayType Attributes (4) Usig refereces to the AST. x Sequetialecl Iteger y Program Letmd har... Letmd z... Boolea let var x: Iteger; var y: har let var z: Boolea i level id Attr. 1 x 1 y 2 z Very coveiet whe we eed to decorate the AST. Types What is a type?! A restrictio o the possible iterpretatios of a segmet of memory or other program costruct.! A set of values. Why use types?! error avoidace: prevet programmer from makig type errors (e.g. roud peg i square hole).! rutime optimizatio: earlier bidig leads to fewer rutime decisios (e.g. ). Are types really eeded?! No, may laguages ca operate (fie) without them assembly laguages, script laguages (e.g. Pytho, Tcl) 35 36

10 Type heckig (1) I a statically typed laguage every expressio E is either (i) ill-typed, or (ii) has a static type that ca be computed without actually evaluatig E. Whe a expressio E has static type T this meas that whe E is evaluated the the retured value will always have type T. Most moder laguages have a large emphasis o static typecheckig. But object-orieted programmig laguages (e.g. Java) require some rutime type checkig. Type heckig (2) Type checkig ivolves (i) calculatig or iferrig the types of expressios (by usig iformatio about the types of their compoets) ad (ii) checkig that these types are what they should be (e.g. the coditio of ifstatemet must have type Boolea). Bottom-up type checkig algorithm for statically typed programmig laguages:! The types of expressio AST leaves are kow: literals: deotatio (true/false, 2, 3, a ) variables: retrieve from symbol table costats: retrieve from symbol table! Types of iteral odes are iferred from the type of the childre ad the type rule for that kid of expressio Type heckig (3) otextual Aalysis Algorithm x Type rule for biary expr: If op is a operatio of type T 1 xt 2!R the E 1 op E 2 is type correct ad of type R if E 1 ad E 2 are type correct ad have types compatible with T 1 ad T 2 respectively. type ItegerExpr it 2 SimpleVame type x BiaryExpr bool Operator ItegerExpr it x it! bool it < 7 Idetificatio ad type checkig could be doe by two separate passes over the AST.! However, this is ot eeded. Both passes ca be iterleaved, as log as the declaratio of a idetifier is before its use (ad hece its type is available for type checkig to proceed). Possible algorithm! Oe depth-first left-to-right traversal of the AST, doig both idetificatio ad type checkig.! Results of the aalysis are recorded i the AST by decoratig it

11 Abstract Sytax Trees AST lass Hierarchy See W&B.2 Program ::= ommad Program ommad ::= ommad ; ommad Sequetialmd V-ame := Expressio Assigmd Idetifier ( Expressio ) allmd Grammar for Mii-Triagle s abstract sytax. if Expressio Ifmd the ommad else ommad while Expressio do ommad Whilemd let eclaratio i ommad Letmd Expressio ::= Iteger-Literal ItegerExpr V-ame VameExpr Operator Expressio UaryExpr Expressio Operator Expressio BiaryExpr V-ame ::= Idetifier SimpleVame eclaratio ::= eclaratio ; eclaratio Seqecl cost Idetifier ~ Expressio ostecl var Idetifier : Type-deoter Type-deoter ::= Idetifier AST odes of Mii-Triagle ypee 41 AST eclaratio ommad Seqecl ostecl... Expressio ItegerExpr VameExpr UaryExpr BiaryExpr 42 AST Hierarchy i Java otextual Aalysis Abstract Sytax Tree otext Aalysis H2 Errors Expressio ::= Iteger-Literal ItegerExpr V-ame VameExpr Operator Expressio UaryExpr Expressio Operator Expressio BiaryExpr public class BiaryExpr exts Expressio { public Expressio E1, E2; public Operator O; public class UaryExpr exts Expressio { public Expressio E; public Operator O; Two sub phases: scope rules are checked i the idetificatio phase type rules are checked i the type checkig phase Sequetialecl Program Letmd Assigmd Iteger c har c & ecorated Abstract Sytax Tree Sequetialmd BiaryExpr harexpr :char VameExpr ItExpr :char Idet har-lit Idet Idet Op It-Lit Assigmd

12 ecoratio Straightforward OO-approach (1) ecoratio is doe by addig some istace variables to some of the AST classes. public abstract class Expressio exts AST { // Every expressio has a type public Type type; public class Idetifier exts Toke { // Bidig occurrece of this idetifier public eclaratio decl; Add to each AST class methods for type checkig (or code-geeratio, pretty pritig, etc.). I each AST ode class, the methods traverse their childre. public abstract AST() { public abstract Object check(object arg); public abstract Object ecode(object arg); public abstract Object prettyprit(object arg); Program program; program.check(ull); Retur value ca be used to pass iformatio up the AST tree. Extra arg ca be used to pass iformatio dow the AST tree. advatage: OO-idea is easy to uderstad ad implemet disadvatage: checkig (ad ecodig) methods are spread over all AST classes: ot very modular Straightforward OO-approach (2) public abstract class Expressio Program exts AST { public Type type; Letmd public class BiaryExpr exts Expressio Sequetialmd { public Expressio E1, E2; public Operator O; Assigmd Assigmd Example public Object check(object arg) { Sequetialecl BiaryExpr Type t1 = (Type) E1.check(ull); Type t2 = (Type) E2.check(ull); harexpr Op op = (Op) O.check(ull); :char VameExpr Type result = op.compatible(t1,t2); ItExpr if (result == ull) report type error :char retur result; Idet har-lit Idet Idet Op It-Lit or Object[] tmp = ew Object[2]; Iteger tmp[0] c = har t1; tmp[1] c = t2; & + 1 Type result = (Type) O.check(tmp); Visitor patter (1) The Visitor patter from the famous esig Patters book by Gamma et. al. (1994) lets you defie a ew operatio o the elemets of a object (e.g. the odes i a AST) without chagig the classes of the elemets o which it operates. This patter is particular useful if may distict ad urelated operatios eed to be performed o objects i a object structure, ad you wat to avoid pollutig their classes with these operatios. Some characteristics:! Visitors makes addig ew operatios easy.! A visitor gathers related operatios ad separates urelated oes.! Visitor patter breaks ecapsulatio

13 Usig a Visitor (1) Idea: use a extra level of idirectio! defie a special Visitor class to visit the odes i the tree.! add (oly-oe) visit method to the AST classes, which lets the visitor actually visit the AST ode. public abstract class AST { public abstract Object visit(visitor v, Object arg); public class Assigmd exts ommad { public Object visit(visitor v, Object arg) { retur v.visitassigmd(this, arg); public class XYZ exts { public Object visit(visitor v, Object arg) { retur v.visitxyz(this, arg); Geeral template for all AST ode classes. I literature o software patters the method visit is usually amed accept. So istead of several methods like check, ecode, etc, oly a sigle visit method. (a implemetatio of) this method will do the type-checkig (or code geeratio, pritig, etc.). 49 Usig a Visitor (2) public class XYZ exts { Object visit(visitor v, Object arg) { retur v.visitxyz(this, arg); public iterface Visitor { public Object visitprogram (Program prog, Object arg); public Object visitassigmd (Assigmd cmd, Object arg); public Object visitsequetialmd (Sequetialmd cmd, Object arg); public Object visitvameexpressio (VameExpressio e, Object arg); public Object visitbiaryexpressio (BiaryExpressio e, Object arg); The Visitor iterface defies visitxyz methods for all AST classes! public Object visitxyz (XYZ x, Object arg); 50 hecker as a Visitor (1) Ay implemetatio of Visitor ca traverse the AST. public class hecker implemets Visitor { private SymbolTable symtab; public void check(program prog) { symtab = ew SymbolTable(); prog.visit(this, ull); + implemetatios of all methods of Visitor All methods for a specific pass over the AST up i the same class, i.e. the same file! Root ode of the AST. hecker as a Visitor (2) public Object visitassigmd (Assigmd com, Object arg) { Type vtype = (Type) com.v.visit(this, ull); Type etype = (Type) com.e.visit(this, ull); if (! com.v.isvariable()) error: left side is ot a variable if (! etype.equals(vtype)) error: types are ot equivalet retur ull; public class XYZ exts { Object visit(visitor v, Object arg) { retur v.visitxyz(this, arg); Assigmd public Object visitletmd (Letmd com, Object arg) { Letmd symtab.opescope(); com..visit(this, ull); com..visit(this, ull); symtab.closescope(); retur ull; Note that the letmd opes (ad closes) the scope of the Symbol Table. V E 51 52

14 hecker as a Visitor (3) public Object visitifmd (Ifmd com, Object arg) { Type etype = (Type)com.E.visit(this, ull); if (! etype.equals(type.bool)) error: coditio is ot a boolea com.1.visit(this, ull); com.2.visit(this, ull); retur ull; public Object visititegerexpr (ItegerExpr expr, Object arg) { expr.type = Type.it; retur expr.type; decoratig the ItegerExpr ode i the AST public class XYZ exts { Object visit(visitor v, Object arg) { retur v.visitxyz(this, arg); Ifmd E 1 2 ItegerExpr IL o eed to visit the Termial leaf hecker as a Visitor (4) public Object visitbiaryexpr (BiaryExpr expr, Object arg) { BiaryExpr Type e1type = (Type) expr.e1.visit(this, ull); Type e2type = (Type) expr.e2.visit(this, ull); Operatoreclaratio opdecl = E 1 (Operatoreclaratio) expr.o.visit(this, ull); if (opdecl == ull) { error: o such operator expr.type = Type.error; else if (opdecl istaceof BiaryOperatoreclaratio) { BiaryOperatoreclaratio bopdecl = (BiaryOperatoreclaratio) opdecl; if (! e1type.equals(bopdecl.operad1type)) error: left operad has the wrog type if (! e2type.equals(bopdecl.operad2type)) error: right operad has the wrog type expr.type = bopdecl.resulttype; else { error: operator is ot a biary operator expr.type = Type.error; O E 2 retur expr.type; See W&B for the other visitor methods Summary of visitig methods Table 5.1 Visitor patter (2) Program visitprogram retur ull ommad visit..md retur ull Expressio visit..expr decorate it with its type retur that type Vame visitsimplevame decorate it with its type set a flag idicatig if it is variable retur the type. eclaratio visit..ecl eter all declared idetifiers ito symbol table retur ull Typeeoter visit..typeeoter decorate it with its type retur that type All compoud ASTs also check for well-formedess. Idetifier visitidetifier check that the idetifier is declared set a referece to its bidig declaratio retur that declaratio Operator visitoperator check that the operator is declared set a referece to its bidig declaratio retur that declaratio The iterface Visitor declares for each AST class XYZ the method visitxyz(xyz x, Object arg). It is possible to reame all visitxyz methods to plai visit ad rely o Java s overloadig mechaism to select the correct visitor method. Not much is gaied by this reamig, though.! Still all AST classes should have a visit method, callig aother overloaded visit method with the this argumet (otherwise the overloadig will ot work).! As i geeral the visit methods for the AST classes are all differet, you will ot profit from iheritig visit methods of superclasses

15 Visitor patter (3) Nice (1) The Visitor patter has some drawbacks.! Argumets ad retur types of the visitig methods have to be kow i advace. For ew type of visitig methods, these methods have to be added to each AST class.! Visitor patter requires (substatial) preparatio: Visitor iterface with a abstract method for each AST ode; Each AST class should have a visit method; ode itself is tedious to write.! Visitor patter should be there from the start.! Visitor code withi the visit methods i the AST classes look obscure: they are meat for visitig, ot for checkig. Furthermore, the visitor patter should ot be used whe the object structure (i.e. AST hierarchy) o which it works is still chagig. Nice = Object-Orieted Programmig Laguage! research project: adds features from fuctioal laguages to Java! implemeted o top of Java, geerates bytecode! additioal features: Java 5 " parametric/geeric fuctios (like templates i ++) aoymous fuctios (istead of the heavy aoymous classes of Java) multi-methods (see below) tuples (to retur several values from fuctio; type-safe) Java 5 " optioal parameters to methods! Nice compiler is writte i Java ad i Nice Nice (2) With Nice the stadard OO-approach ad the Visitor-patter ca be combied quite icely usig multi-methods. Multi-methods (or multiple dispatch )! Allow to defie methods outside classes.! Istead of usig (oly) the receiver class to dispatch the method (usig overridig), oe ca use the argumets of the method to dispatch the method. Object check(ast ode); Object check(ode@biaryexpr) { Object check(ode@ifmd) { Object check(ode@assigmd) { No preparatio eeded (as for the Visitor patter) ode is shorter ad more atural. Withi this method you ca access ode as beig a Assigmd. The Nice project is ot very active (uderstatemet!). The MultiJava project is aother multiple dispatch attempt for Java. 59