Lecture 12: Parser-Generating Tools

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1 Lecture 12: Parser-Generating Tools Dr Kieran T. Herley Department of Computer Science University College Cork KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

2 Summary Overview here KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

3 Parser-Generating Tools Parser-Generating Tools Yacc Tool to automate production of parsers Input: CFG (suitably formatted and annotated) Output: Parser code Yet Another Compiler Compiler Default parser generator tool in earlier years Produces LALR parser C-based KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

4 Javacc Java-based parser-generating tool Input:.jj file containing suitably formatted grammer etc. Output:.java file containing LL(1) parser Note: javacc (and related tools) installed in WGB labs. KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

5 Using Javacc Usage javacc Simple.jj generates Simple.java (plus possibly others) KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

6 Using Javacc Usage javacc Simple.jj javac Simple.java generates Simple.java (plus possibly others) generates Simple.class (plus others) KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

7 Using Javacc Usage javacc Simple.jj generates Simple.java (plus possibly others) javac Simple.java generates Simple.class (plus others) java Simple runs parser in the usual way Notes (Syntax) errors can occur at javacc or javac stage KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

8 Typical Javacc Code PARSER BEGIN(ExprParser) / Java main method that calls grammars start symbol / PARSER END(ExprParser) / Token descriptions here / / Grammar here /... Built around context-free grammar expressed in eccentric javacc syntax (details later) KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

9 Typical Javacc Code [ExprParser0.jj] PARSER BEGIN(ExprParser) / Java main method that calls Expression () / PARSER END(ExprParser) / Token descriptions here / / <Expression> > <SimpleExpression> / void Expression () : { { SimpleExpression()... / <SeqOfTerms> > <Term> {<Addop> <Term> / void SeqOfTerms() : { { Term() ( Addop() Term() )... KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

10 Java Code Generated Generated code forms recursive descent parser with one method per grammar production.... static final public void SeqOfTerms() throws ParseException { Term(); label 1 : while (true) { switch (( jj ntk == 1)?jj ntk():jj ntk) { case PLUS: case MINUS: ; break; default : jj la1 [0] = jj gen ; break label 1 ; Addop(); Term(); Note: Term()... Addop()... Term()... calling pattern Hideous code not intended for human consumption KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

11 Capabilities of Javacc Javacc very flexible Can augment basic parser with actions to accomplish tasks in syntax-aware way Can be used to build trees, generate code etc KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

12 Simple Balanced Parentheses Example Javacc s Grammar Syntax I MB EOF MB { [ MB ] Javacc s grammar notation void Input() :{ { MatchedBraces() ( \n \r ) EOF void MatchedBraces() :{ { { [ MatchedBraces() ] Terminals Java-function notation, e.g. MatchedBraces() Nonterminals quoted strings, e.g. { NB [...] denotes... optional (also (...)? NB < EOF > special end-offile symbol KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

13 Simple Balanced Parentheses Example Javacc s Grammar Syntax cont d MB { [ MB ] Each grammar production modelled by pseudo-java method: void MatchedBraces() :{ { / RHS goes here / KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

14 Simple Balanced Parentheses Example Javacc s Grammar Syntax cont d MB { [ MB ] Each grammar production modelled by pseudo-java method: void MatchedBraces() :{ { / RHS goes here / RHS symbols listed inside {: <MB> -> { [ <MB> ] v v v v void MatchedBraces():{{ "{" [ MatchedBraces() ] "" method call syntax for nonterminals (MatchedBraces()) terminals quoted ("{") KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

15 Simple Balanced Parentheses Example KHSimple1.jj PARSER BEGIN(KHSimple1) public class KHSimple1 { public static void main(string args []) throws ParseException { KHSimple1 parser = new KHSimple1(System.in); parser.input (); PARSER END(KHSimple1) / Grammar as before / Method main drives parse: stipulates input taken from System.in; calls method Input() to get ball rolling KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

16 Simple Balanced Parentheses Example Tidying Up SKIP : { \t \n \r... TOKEN : { <LBRACE: { > <RBRACE: > Include SKIP clause (before grammar) to specify whitespace characters to be ignored Can name token specifications (regular expressions) and use in grammar e.g. int MatchedBraces() : { { <LBRACE> [MatchedBraces() ] <RBRACE> NB confusing use of angle brackets for terminals here KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

17 Simple Balanced Parentheses Example Actions Can annotate CFG with actions (Java snippets) akin to jflex actions KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

18 Simple Balanced Parentheses Example Actions Can annotate CFG with actions (Java snippets) akin to jflex actions void Input() : { int count; { count=matchedbraces() <EOF> { System.out. println ( The levels of nesting is + count); / Action / int MatchedBraces() : { int nested count=0; { <LBRACE> [ nested count=matchedbraces() ] <RBRACE> { return ++nested count; / Action / Collectively actions compute number of nested { and outputs result KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

19 Simple Balanced Parentheses Example Actions cont d / 1 / int MatchedBraces() : / 2 / { int nested count=0; / 3 / { <LBRACE> [ nested count=matchedbraces() ] <RBRACE> / 4 / { return ++nested count; 1 Changing return type to int, generates int-returning method MatchedBraces KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

20 Simple Balanced Parentheses Example Actions cont d / 1 / int MatchedBraces() : / 2 / { int nested count=0; / 3 / { <LBRACE> [ nested count=matchedbraces() ] <RBRACE> / 4 / { return ++nested count; 1 Changing return type to int, generates int-returning method MatchedBraces 2 Adding statement inside : { bit smuggles declaration/initialization into generated code KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

21 Simple Balanced Parentheses Example Actions cont d / 1 / int MatchedBraces() : / 2 / { int nested count=0; / 3 / { <LBRACE> [ nested count=matchedbraces() ] <RBRACE> / 4 / { return ++nested count; 1 Changing return type to int, generates int-returning method MatchedBraces 2 Adding statement inside : { bit smuggles declaration/initialization into generated code 3 Assignment nested_count= assignment, captures value returned by recursive call KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

22 Simple Balanced Parentheses Example Actions cont d / 1 / int MatchedBraces() : / 2 / { int nested count=0; / 3 / { <LBRACE> [ nested count=matchedbraces() ] <RBRACE> / 4 / { return ++nested count; 1 Changing return type to int, generates int-returning method MatchedBraces 2 Adding statement inside : { bit smuggles declaration/initialization into generated code 3 Assignment nested_count= assignment, captures value returned by recursive call 4 Action { return... smuggled into generated code; returned brace count when executed at run time KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

23 Simple Balanced Parentheses Example Actions cont d int MatchedBraces() : { int nested count=0; { <LBRACE> [ nested count=matchedbraces() ] <RBRACE> { return ++nested count; / action / Execution of the action triggered after parse of <LBRACE>...<RBRACE> has been completed KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

24 Simple Balanced Parentheses Example Illustration void Input() : { int count; { count=matchedbraces() <EOF> { System.out. println ( The levels of nesting is + count); int MatchedBraces() : { int nested count=0; { <LBRACE> [ nested count=matchedbraces() ] <RBRACE> { return ++nested count; KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

25 Expression Grammar Simple Expression Grammar Here is our grammar expression simple-expression simple-expression seq-of-terms seq-of-terms term seq-of-terms addop term term seq-of-factors seq-of-factors factor seq-of-factors mulop factor factor INTLIT ( simple-expression ) addop + - mulop * / But Javacc dislikes left recursion so must recast to circumvent this... KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

26 Expression Grammar Modified Grammar Using EBNF to re-express repetition we get expression simple-expression simple-expression seq-of-terms seq-of-terms term { addop term term seq-of-factors seq-of-factors factor { mulop factor factor INTLIT ( simple-expression ) addop + - mulop * / Recall {... indicates zero or more reps of... KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

27 Expression Grammar Overall Structure of ExprParser0.jj File PARSER BEGIN(ExprParser)... PARSER END(ExprParser) / Tokens and whitespace definitions / / Grammar productions / Note PARSER BEGIN specifies name of generated Java class; need not match name of.jj file. KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

28 Expression Grammar Token Definitions SKIP : / WHITE SPACE / { \t \n \r \f TOKEN : { < PLUS: + > < MINUS: > < TIMES: > < DIV: / > < LPAREN: ( > < RPAREN: ) > Javacc supports full regular expression specifications for token definition Also provides for non-tokens SKIP: stuff to be ignored COMMENTS: not shown here TOKEN : / LITERALS / { < NUM: [ 1 9 ] ([ 0 9 ]) > KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

29 Expression Grammar Grammar void Expression () : { { SimpleExpression()<EOF> void SimpleExpression() : { { SeqOfTerms() void SeqOfTerms() : { { Term() ( Addop() Term() ) / ETC ETC ETC / (...) denotes zero or more repetitions (...)+ for one or more reps Note use of named tokens void Mulop() : { { <TIMES> <DIV> KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

30 Expression Grammar Main Method PARSER BEGIN(ExprParser) public class ExprParser { / Main entry point. / public static void main(string args []) throws ParseException { ExprParser parser = new ExprParser(System.in); parser. Expression (); PARSER END(ExprParser) Can specify more sophisticated main to take input from file for example. KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

31 Expression Grammar Extending Expression Parser Modify to compute value of expression Modifications: Change method/nonterminal return types to int Add actions to productions to compute value of corresponding subexpression, / <term> > <seq of factors> / int Term() : { int value ; { value = SeqOfFactors() { return value ; Action code gets integrated into Java code generated KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

32 Expression Grammar More Actions / <seq of factors> > <factor> {<mulop> <factor> / int SeqOfFactors() : { int value ; int temp; char op; { value = Factor() ( op = Mulop() temp = Factor() { if (op == *) value = temp; else value /= temp; ) { System.out. println ( <seqoffactors> > <factor> {<mulop> <factor> ); return value ; Note intermingling of RHS of production KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

33 Expression Grammar Illustration Return actions get executed in postorder fashion Node corresponding to each nonterminal returns value or subexpression to its parent KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

34 Expression Grammar Acknowledgements Examples KHSimple[123].jj are adapted from the Simple[123].jj examples bundled with the official javacc download. KH (31/10/17) Lecture 12: Parser-Generating Tools / 27

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