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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