Expressions in a CoCo to ANTLR translator

Question

I'm parsing CoCo/R grammars in a utility to automate CoCo -> ANTLR translation. The core ANTLR grammar is:

rule '=' expression '.' ;

expression
     : term ('|' term)*
         -> ^( OR_EXPR term term* )
     ;
term
     : (factor (factor)*)? ;

factor
     : symbol
     | '(' expression ')'
         -> ^( GROUPED_EXPR expression )
     | '[' expression']'
         -> ^( OPTIONAL_EXPR expression)
     | '{' expression '}'
         -> ^( SEQUENCE_EXPR expression)
     ;

symbol
     : IF_ACTION
     | ID (ATTRIBUTES)?
     | STRINGLITERAL
     ;

My problem is with constructions such as these:

CS = { ExternAliasDirective }
         { UsingDirective }
         EOF .

CS results in an AST with a OR_EXPR node although no '|' character actually appears. I'm sure this is due to the definition of expression but I cannot see any other way to write the rules.

I did experiment with this to resolve the ambiguity.

// explicitly test for the presence of an '|' character
expression
@init { bool ored = false; }
     : term {ored = (input.LT(1).Type == OR); } (OR term)*
         ->  {ored}? ^(OR_EXPR term term*)
         ->            ^(LIST term term*)

It works but the hack reinforces my conviction that something fundamental is wrong.

Any tips much appreciated.

Answer 1

Your rule:

expression
  : term ('|' term)*
      -> ^( OR_EXPR term term* )
  ;

always causes the rewrite rule to create a tree with a root of type OR_EXPR. You can create "sub rewrite rules" like this:

expression
  :  (term -> REWRITE_RULE_X) ('|' term -> ^(REWRITE_RULE_Y))*
  ;

And to resolve the ambiguity in your grammar, it's easiest to enable global backtracking which can be done in the options { ... } section of your grammar.

A quick demo:

grammar CocoR;

options {
  output=AST;
  backtrack=true;
}

tokens {
  RULE;
  GROUP;
  SEQUENCE;
  OPTIONAL;
  OR;
  ATOMS;
}

parse
  :  rule EOF -> rule
  ;

rule
  :  ID '=' expr* '.' -> ^(RULE ID expr*)
  ;

expr
  :  (a=atoms -> $a) ('|' b=atoms -> ^(OR $expr $b))*
  ;

atoms
  :  atom+ -> ^(ATOMS atom+)
  ;

atom
  :  ID
  |  '(' expr ')' -> ^(GROUP expr)
  |  '{' expr '}' -> ^(SEQUENCE expr)
  |  '[' expr ']' -> ^(OPTIONAL expr)
  ;

ID
  :  ('a'..'z' | 'A'..'Z') ('a'..'z' | 'A'..'Z' | '0'..'9')*
  ;

Space
  :  (' ' | '\t' | '\r' | '\n') {skip();}
  ;

with input:

CS = { ExternAliasDirective }
     { UsingDirective }
     EOF .

produces the AST:

and the input:

foo = a | b ({c} | d [e f]) .

produces:

The class to test this:

import org.antlr.runtime.*;
import org.antlr.runtime.tree.*;
import org.antlr.stringtemplate.*;

public class Main {
    public static void main(String[] args) throws Exception {
        /*
        String source = 
                "CS = { ExternAliasDirective } \n" +
                "{ UsingDirective }            \n" + 
                "EOF .                           ";
        */
        String source = "foo = a | b ({c} | d [e f]) .";
        ANTLRStringStream in = new ANTLRStringStream(source);
        CocoRLexer lexer = new CocoRLexer(in);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        CocoRParser parser = new CocoRParser(tokens);
        CocoRParser.parse_return returnValue = parser.parse();
        CommonTree tree = (CommonTree)returnValue.getTree();
        DOTTreeGenerator gen = new DOTTreeGenerator();
        StringTemplate st = gen.toDOT(tree);
        System.out.println(st);
    }
}

and with the output this class produces, I used the following website to create the AST-images: http://graph.gafol.net/

HTH

---

EDIT

To account for epsilon (empty string) in your OR expressions, you might try something (quickly tested!) like this:

expr
  :  (a=atoms -> $a) ( ( '|' b=atoms -> ^(OR $expr $b)
                       | '|'         -> ^(OR $expr NOTHING)
                       )
                     )*
  ;

which parses the source:

foo = a | b | .

into the following AST: