[antlr-interest] Resolving a First/Follow Conflict in Logo.

[Michael Matera]

Hi Everyone,

For kicks I'm writing a parser for the Logo programming language.  Logo 
has an ambiguity in its syntax in that:

  1. Statements are values (Like in C)
  2. Subroutine's arguments are not marked with delimiters

Here's a simplified example of a parser rule for Logo:

statement
   : 'fd' statement
   | 'bk' statement
   | IDENTIFIER statement*
   ;

As you can see, this rule is ambiguous.  Logo parsers must determine how 
many arguments are taken by 'IDENTIFIER' and use that to resolve the 
conflict.  I've brainstormed a couple of solutions for this problem but 
I need help with things that I was unable to figure out.

Here's my first solution:

In the scanner, lookup 'IDENTIFIER' in a table to determine the number 
of arguments it requires, then create a fake token to inform the parser. 
  The rules would look something like this:

statement
   : 'fd' statement
   | 'bk' statement
   | ZERO_ARG IDENTIFIER
   | ONE_ARG IDENTIFIER statement
   | TWO_ARG IDENTIFIER statement statement
   | THREE_ARG IDENTIFIER statement statement statement
   ;

IDENTIFIER
   : ('a' .. 'z')+
   {
      lookup(getText());
      <<INSERT_FAKE_TOKEN>>
   }
   ;

Question: How do I create and insert a fake token in the scanner?

Here's my second solution:

Use a predicate to inform the First/Follow conflict.  I've tried to use 
a semantic predicate to control the match without success.  The 
following is a non-working attempt to show what I mean:

statement
   : 'fd' statement
   | 'bk' statement
   | IDENTIFIER {args=lookup()} (statement {count<args}? {count++})*
   ;

Question: Is it possible to resolve a First/Follow conflict this way in 
ANTLR?  If so, how do I do it?

~~~~

If there's a solution I've missed please let me know, and thanks for 
your time and consideration.

Cheers
./m

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