How to make expression for grammar rule in Bison with Jflex

Question

I've been trying to make a parser which takes tokens from a lexer (jflex), and ive used Java with bison for the parser. Here is my code so far for the parser:

%define api.prefix {EXAMPLE}
%define api.parser.class {EXAMPLE}
%define api.parser.public
%define parse.error verbose

%code imports{
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.io.Reader;
  import java.io.IOException;
}

%code {
  public static void main(String args[]) throws IOException {
    EXAMPLELexer lexer = new EXAMPLELexer(System.in);
    EXAMPLE parser = new EXAMPLE(lexer);
    if(parser.parse()){
      System.out.println("VALID FROM PARSER");
    }
    else{
      System.out.println("ERROR FROM PARSER");
    }
      
    return;
  }
}

/* Bison Declarations */
%token <Integer> NUM
%type exp
%%

input: line | input line;
line: '\n'
| exp '\n' { System.out.println($exp); }
| error '\n'
;
exp:
 NUM { $$ = $1; }
| exp '=' exp { if ($1.intValue() != $3.intValue()) yyerror("calc: error: " + $1 + " != " + $3); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = (int) Math.pow($1, $3); }
| '(' exp ')' { $$ = $2; }
| '(' error ')' { $$ = 1111; }
| '!' { $$ = 0; return YYERROR; }
| '-' error { $$ = 0; return YYERROR; }

%%

class EXAMPLELexer implements EXAMPLE.Lexer {
  InputStreamReader it;
  Yylex yylex;

  public EXAMPLELexer(InputStream is){
    it = new InputStreamReader(is);
    yylex = new Yylex(it);
  }

  @Override
  public void yyerror (String s){
   
  }

  @Override
  public Object getLVal() {
    return null;
  }

  @Override
  public int yylex () throws IOException{
    return yylex.yylex();
  }
}

When I try to use the jflex and bison commands I get this: $$ of ‘exp’ has no declared type, same with $1, $3

Please help, i've been stuck on this for a while! (p.s i've already tried %union{} and it just gave rise to more errors unfortunately, if you have any sample code that actually works with it, i'd love to learn as there seems to be a lack of documentation with Java and bison working with Jflex.)

Answer 1

There was no declared type for exp. Additionally, token NEG was not declared.

%token <String> NEG
%type<Object> exp

The lexer interface needs to be declared in a %code lexer { }; block, before the %% parser end marker.

Following your file example, here is example.y

%language "Java"
%define api.prefix {EXAMPLE}
%define api.parser.class {EXAMPLE}
%define api.parser.public
%define parse.error verbose

%code imports{
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.io.Reader;
  import java.io.IOException;
}

%code {
  public static void main(String args[]) throws IOException {
    EXAMPLELexer lexer = new EXAMPLELexer(System.in);
    EXAMPLE parser = new EXAMPLE(lexer);
    if(parser.parse()){
      System.out.println("VALID FROM PARSER");
    }
    else{
      System.out.println("ERROR FROM PARSER");
    }
      
    return;
  }
}

/* Bison Declarations */
%token <Integer> NUM
%token <String> NEG
%type<Object> exp
%%

input: line | input line;
line: '\n'
| exp '\n' { System.out.println($exp); }
| error '\n'
;
exp:
 NUM { $$ = Integer.parseInt($1); }
| exp '=' exp { if ($1.intValue() != $3.intValue()) yyerror("calc: error: " + $1 + " != " + $3); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = (int) Math.pow($1, $3); }
| '(' exp ')' { $$ = $2; }
| '(' error ')' { $$ = 1111; }
| '!' { $$ = 0; return YYERROR; }
| '-' error { $$ = 0; return YYERROR; }

%code lexer {

private EXAMPLELexer lexer;

private Object yylval;

public YYLexer (EXAMPLELexer elexer)
{
    lexer = elexer;
}

@Override
public void yyerror (String s) {

}

@Override
public Object getLVal() {
    return yylval;
}

@Override
public int yylex () throws IOException {
    int v = lexer.scan();
    yylval = lexer.getValue();
    return v;
}

};

%%

And here is the start of an example.jf

import java.util.*;
import java.io.InputStreamReaader;

import static EXAMPLE.*;

%%

%class EXAMPLELexer 
%integer
%unicode

%{

EXAMPLELexer(InputStream ios) {
    this (new InputStreamReader(ios));
}
    String lexeme;

    private void setValue(String val)
    {
        lexeme = val;
    }

    public String getValue()
    {
        return lexeme;
    }

%}

INTNUM = ([0-9]*)

%%

{INTNUM} { setValue( yytext() ); return EXAMPLE.Lexer.NUM; }