How does this ANTLR 3 convert to ANTLR4? [duplicate] - parsing

Question

In ANTLR 3 you could just do the following:
andExpression
: (andnotExpression -> andnotExpression)
(AND? a=andnotExpression -> ^(AndNode $andExpression $a))*
;
Any idea how to do it in the new version?

Answer 1

As mentioned by Sam (280Z28), ANTLR 4 does not have rewrite operators.
When generating the parser, ANTLR 4 creates some listener classes that you can use to listen for "enter" and "exit" events of all parser rules.
Also, ANTLR 4 supports "direct left recursive rules", so your expression rules can be defined in a single rule as demonstrated below:
grammar Expr;
parse
: expression EOF
;
expression
: '(' expression ')'
| IDENTIFIER
| NOT expression
| expression AND? expression
| expression OR expression
;
LPAREN : '(';
RPAREN : ')';
NOT : 'NOT';
AND : 'AND';
OR : 'OR';
IDENTIFIER : [a-zA-Z_] [a-zA-Z_0-9]*;
SPACE : [ \t\r\n]+ -> skip;
When parsing the input "a b OR NOT c AND d", the following parse tree will be created:
(image created using ANTLRWorks2, thank you Sam! Very impressive IDE, I love it!)
Generate the parser and listener classes:
java -cp antlr-4.0-complete.jar org.antlr.v4.Tool Expr.g4
and the following class is generated that will let you help "walk" the tree:
public class ExprBaseListener implements ExprListener {
#Override public void enterExpression(ExprParser.ExpressionContext ctx) { }
#Override public void exitExpression(ExprParser.ExpressionContext ctx) { }
#Override public void enterParse(ExprParser.ParseContext ctx) { }
#Override public void exitParse(ExprParser.ParseContext ctx) { }
#Override public void enterEveryRule(ParserRuleContext<Token> ctx) { }
#Override public void exitEveryRule(ParserRuleContext<Token> ctx) { }
#Override public void visitTerminal(TerminalNode<Token> node) { }
#Override public void visitErrorNode(ErrorNode<Token> node) { }
}
Now you'll need to inspect the ExprParser.ExpressionContext to see which of the alternatives in expression is matched, which is where "tree-labels" come in handy. Change the expression rule as follows:
expression
: '(' expression ')' # EXPR
| IDENTIFIER # ID_EXPR
| 'NOT' expression # NOT_EXPR
| expression 'AND'? expression # AND_EXPR
| expression 'OR' expression # OR_EXPR
;
and regenerate the parser and listeners, and you'll see that ExprBaseListener now looks like this:
public class ExprBaseListener implements ExprListener {
#Override public void enterAND_EXPR(ExprParser.AND_EXPRContext ctx) { }
#Override public void exitAND_EXPR(ExprParser.AND_EXPRContext ctx) { }
#Override public void enterOR_EXPR(ExprParser.OR_EXPRContext ctx) { }
#Override public void exitOR_EXPR(ExprParser.OR_EXPRContext ctx) { }
#Override public void enterEXPR(ExprParser.EXPRContext ctx) { }
#Override public void exitEXPR(ExprParser.EXPRContext ctx) { }
#Override public void enterNOT_EXPR(ExprParser.NOT_EXPRContext ctx) { }
#Override public void exitNOT_EXPR(ExprParser.NOT_EXPRContext ctx) { }
#Override public void enterID_EXPR(ExprParser.ID_EXPRContext ctx) { }
#Override public void exitID_EXPR(ExprParser.ID_EXPRContext ctx) { }
#Override public void enterParse(ExprParser.ParseContext ctx) { }
#Override public void exitParse(ExprParser.ParseContext ctx) { }
#Override public void enterEveryRule(ParserRuleContext ctx) { }
#Override public void exitEveryRule(ParserRuleContext ctx) { }
#Override public void visitTerminal(TerminalNode node) { }
#Override public void visitErrorNode(ErrorNode node) { }
}
I.e., for each label in expression a separate enter- and exit-method is created.
Now, let's say you're only interested in enter-events of the AND expression. You could create a custom class that extends this ExprBaseListener and override enterAND_EXPR:
public class ExprWalker extends ExprBaseListener {
#Override
public void enterAND_EXPR(ExprParser.AND_EXPRContext ctx) {
java.util.List<ExprParser.ExpressionContext> e = ctx.expression();
System.out.println("AND -> " + e.get(0).getText() + ", " + e.get(1).getText());
}
}
To test this all, create a small driver class:
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
public class Main {
public static void main(String[] args) throws Exception {
String input = "a b OR NOT c AND d";
ExprLexer lexer = new ExprLexer(new ANTLRInputStream(input));
ExprParser parser = new ExprParser(new CommonTokenStream(lexer));
ParseTree tree = parser.parse();
ParseTreeWalker walker = new ParseTreeWalker();
walker.walk(new ExprWalker(), tree);
}
}
and run it:
java -cp antlr-4.0-complete.jar org.antlr.v4.Tool Expr.g4
javac -cp antlr-4.0-complete.jar *.java
java -cp .:antlr-4.0-complete.jar Main
after which you'll see the following being printed to your console:
AND -> a, bORNOTcANDd
AND -> NOTc, d

Answer 2

ANTLR 4 does not have a rewrite operator or an output=AST option like ANTLR 3. Trees produced by ANTLR 4 parsers are parse trees, the shape of which is implicitly defined by the grammar rules.