Handling line feed in ANTLR4 grammar with Python target

Question

I am working on an ANTLR4 grammar for parsing Python DSL scripts (a subset of Python, basically) with the target set as the Python 3. I am having difficulties handling the line feed.

In my grammar, I use lexer::members and NEWLINE embedded code based on Bart Kiers's Python3 grammar for ANTLR4 which are ported to Python so that they can be used with Python 3 runtime for ANTLR instead of Java. My grammar differs from the one provided by Bart (which is almost the same used in the Python 3 spec) since in my DSL I need to target only certain elements of Python. Based on extensive testing of my grammar, I do think that the Python part of the grammar in itself is not the source of the problem and so I won't post it here in full for now.

The input for the grammar is a file, catched by the file_input rule:

file_input: (NEWLINE | statement)* EOF;

The grammar performs rather well on my DSL and produces correct ASTs. The only problem I have is that my lexer rule NEWLINE clutters the AST with \r\n nodes and proves troublesome when trying to extend the generated MyGrammarListener with my own ExtendedListener which inherits from it.

Here is my NEWLINE lexer rule:

NEWLINE
 : ( {self.at_start_of_input()}? SPACES
   | ( '\r'? '\n' | '\r' | '\f' ) SPACES?
   )
   {
    import re
    from MyParser import MyParser
    new_line = re.sub(r"[^\r\n\f]+", "", self._interp.getText(self._input)) 
    spaces = re.sub(r"[\r\n\f]+", "", self._interp.getText(self._input)) 
    next = self._input.LA(1)

    if self.opened > 0 or next == '\r' or next == '\n' or next == '\f' or next == '#':
        self.skip()
    else:
        self.emit_token(self.common_token(self.NEWLINE, new_line))

        indent = self.get_indentation_count(spaces)
        if len(self.indents) == 0:
            previous = 0
        else:
            previous = self.indents[-1]

        if indent == previous:
            self.skip()
        elif indent > previous:
            self.indents.append(indent)
            self.emit_token(self.common_token(MyParser.INDENT, spaces))
        else:
            while len(self.indents) > 0 and self.indents[-1] > indent:
                self.emit_token(self.create_dedent())
                del self.indents[-1]
   };

The SPACES lexer rule fragment that NEWLINE uses is here:

 fragment SPACES
 : [ \t]+
 ;

I feel I should also add that both SPACES and COMMENTS are ultimately being skipped by the grammar, but only after the NEWLINE lexer rule is declared, which, as far as I know, should mean that there are no adverse effects from that, but I wanted to include it just in case.

SKIP_
 : ( SPACES | COMMENT ) -> skip
 ;

When the input file is run without any empty lines between statements, everything runs as it should. However, if there are empty lines in my file (such as between import statements and variable assignement), I get the following errors:

line 15:4 extraneous input '\r\n    ' expecting {<EOF>, 'from', 'import', NEWLINE, NAME}
line 15:0 extraneous input '\r\n' expecting {<EOF>, 'from', 'import', NEWLINE, NAME}

As I said before, when line feeds are omitted in my input file, the grammar and my ExtendedListener perform as they should, so the problem is definitely with the \r\n not being matched by the NEWLINE lexer rule - even the error statement I get says that it does not match alternative NEWLINE.

The AST produced by my grammar looks like this:

I would really appreciate any help with this since I cannot see why my NEWLINE lexer rule woud fail to match \r\n as it should and I would like to allow empty lines in my DSL.

Answer 1

so the problem is definitely with the \r\n not being matched by the NEWLINE lexer rule

There is another explanation. An LL(1) parser would stop at the first mismatch, but ANTLR4 is a very smart LL(*) : it tries to match the input past the mismatch.

As I don't have your statement rule and your input around line 15, I'll demonstrate a possible case with the following grammar :

grammar Question;

/* Extraneous input parsing NL and spaces. */

@lexer::members {
  public boolean at_start_of_input() {return true;}; // even if it always returns true, it's not the cause of the problem
}

question
@init {System.out.println("Question last update 2108");}
    :   ( NEWLINE
    |     statement
              {System.out.println("found <<" + $statement.text + ">>");}
        )* EOF
    ;

statement
    :   'line ' NUMBER NEWLINE 'something else' NEWLINE
    ;

NUMBER : [0-9]+ ;
NEWLINE
    : ( {at_start_of_input()}? SPACES
       | ( '\r'? '\n' | '\r' | '\f' ) SPACES?
      )
   ;

SKIP_
    :   SPACES -> skip
    ;

fragment SPACES
    :   [ \t]+
    ;

Input file t.text :

line 1
   something else

Execution :

$ export CLASSPATH=".:/usr/local/lib/antlr-4.6-complete.jar"
$ alias
alias a4='java -jar /usr/local/lib/antlr-4.6-complete.jar'
alias grun='java org.antlr.v4.gui.TestRig'
$ hexdump -C t.text 
00000000  6c 69 6e 65 20 31 0a 20  20 20 73 6f 6d 65 74 68  |line 1.   someth|
00000010  69 6e 67 20 65 6c 73 65  0a                       |ing else.|
00000019
$ a4 Question.g4
$ javac Q*.java
$ grun Question question -tokens -diagnostics t.text
[@0,0:4='line ',<'line '>,1:0]
[@1,5:5='1',<NUMBER>,1:5]
[@2,6:9='\n   ',<NEWLINE>,1:6]
[@3,10:23='something else',<'something else'>,2:3]
[@4,24:24='\n',<NEWLINE>,2:17]
[@5,25:24='<EOF>',<EOF>,3:0]
Question last update 2108
found <<line 1
   something else
>>

Now change statement like so :

statement
//  :   'line ' NUMBER NEWLINE 'something else' NEWLINE
    :   'line ' NUMBER         'something else' NEWLINE // now NL will be extraneous
    ;

and execute again :

$ a4 Question.g4
$ javac Q*.java
$ grun Question question -tokens -diagnostics t.text
[@0,0:4='line ',<'line '>,1:0]
[@1,5:5='1',<NUMBER>,1:5]
[@2,6:9='\n   ',<NEWLINE>,1:6]
[@3,10:23='something else',<'something else'>,2:3]
[@4,24:24='\n',<NEWLINE>,2:17]
[@5,25:24='<EOF>',<EOF>,3:0]
Question last update 2114
line 1:6 extraneous input '\n   ' expecting 'something else'
found <<line 1
   something else
>>

Note that the NL character and spaces have been correctly matched by the NEWLINE lexer rule.

You can find the explanation in section 9.1 of The Definitive ANTLR 4 Reference :

$ grun Simple prog ➾ class T ; { int i; } ➾EOF ❮ line 1:8 extraneous input ';' expecting '{'

A Parade of Errors • 153

The parser reports an error at the ; but gives a slightly more informative answer because it knows that the next token is what it was actually looking for. This feature is called single-token deletion because the parser can simply pretend the extraneous token isn’t there and keep going.

Similarly, the parser can do single-token insertion when it detects a missing token.

In other word, ANTLR4 is so powerful that it can resynchronize the input with the grammar even if several tokens are mismatching. If you run with the -gui option

$ grun Question question -gui t.text

you can see that ANTLR4 has parsed the whole file, despite the fact that a NEWLINE is missing in the statement rule, and that the input does not match exactly the grammar.

To summary : extraneous input is quite a common error when developing a grammar. It can come from a mismatch between input to parse and rule expectations, or also because some piece of input has been interpreted by another token than the one we believe, which can be detected by examining the list of tokens produced by the -tokens option.