ANTLR language - Tree not created (EOF - syntax error)

Question

I have apparently a mistake in the following language definition :

grammar Hello;


object  :
ALL* NAME ALL* '{' 
    (ALL* | (ALL* NAME ALL* NAME)* | (ALL* object)*)*
'}' ALL*;

ALL     :
(~('{' | '}' | '"'))+ -> skip;      // All but braces and double quotes

NAME    :
'"' ALL* '"';

When I'm trying to read a file or add content directly (with run.bat Hello object -gui), the parser give me a syntax error :mismatched input '<EOF>' expecting NAME. And I just have a tree with a single node : 'object'.

Here is the Java source :

public static void main(String[] args) throws Exception {
    HelloLexer lexer = new HelloLexer(new ANTLRFileStream("gamemodes.txt"));
    TokenStream tokenStream = new CommonTokenStream(lexer);
    HelloParser parser = new HelloParser(tokenStream);

    System.out.println(lexer.getAllTokens().size());

    parser.setBuildParseTree(true);

    // Tree Creation
    RuleContext tree = parser.object();
    tree.inspect(parser);
}

And finally, the file structure (with some spaces/characters anywhere (without braces and double quotes)) :

...
"objName"
{
    ...
    "innerObjName1"
    {
        "firstProperty"  "firstResult"
        ...
        "secondProp"     ""
    }

    "innerObjName2"
    {
        "firstProperty"  "firstResult"
        "secondProp"     ""
    }
}
...

NOTE : I'm on Windows.

Thanks !

I think you are attempting to create parser rules (rules that start with a lowercase letter, such as the object rule) with regex. I don't think antlr can handle that. Parser rules possess much simpler mechanics. — Marcelo Zabani
I saw some examples with regex like syntax (*, +, ?) and in the official website... Btw, if you have an idea to fix my problem, I'm listening to you :) — Val
Wow, it seems like this is called EBNF, which I didn't even know existed. Very nice to know this. Sadly, though, I can't help you with your problem. — Marcelo Zabani

Sam Harwell Sam Harwell · Accepted Answer · 2013-04-02T20:03:10

reportAttemptingFullContext is not an error. It's simply letting you know that ANTLR 4 is using the section of its internal implementation that uses the complete parse context to ensure accuracy. The message is included because the full context algorithm is a bit slower than the SLL algorithm it attempts first.

Edit: If you are using a non-standard distribution, you may need to specify the following option to make sure the parse tree is built. This option is true by default in the official distribution.

parser.setBuildParseTree(true);

Edit 2: Covering grammar issues.

Here is the object rule from the original grammar with only formatting changes.

object // intermediate form 1
    :   ALL* NAME ALL* '{' 
        (   ALL*
        |   (ALL* NAME ALL* NAME)*
        |   (ALL* object)*
        )*
        '}' ALL*
    ;

This rule is highly ambiguous. As a quick example, note that the following modified grammar will actually match the same input.

object // intermediate form 2
    :   ALL* NAME ALL* '{' 
        (   ALL
        |   ALL* NAME ALL* NAME
        |   ALL* object
        )*
        '}' ALL*
    ;

Another simplification removes the Unnecessary ALL* prefixes from 2 alts.

object // final form
    :   ALL* NAME ALL* '{' 
        (   ALL
        |   NAME ALL* NAME
        |   object
        )*
        '}' ALL*
    ;

As a final change, I would create an entry rule that ends with an explicit EOF:

entry : object EOF;

To parse your input, call entry() instead of object() to ensure that all of your input gets parsed. If you need the ObjectContext object, it is available by calling EntryContext.object().

ANTLR language - Tree not created (EOF - syntax error)

2 Answers