[antlr-interest] simple parser lookahead problem

[Anthony Youngman]

I've got the following code in my parser ...

----------------------
// Can I distinguish between COLON and COLONPRINT here, I need to look
// ahead but not eat a SEMI or nl. It'll work if I can get catexpr to
// take priority.

printst : ( PRINT^ (expr (COLON)? )? );

catexpr : pmexpr ( COLON^ pmexpr)* ;
--------------------

plus a bit more code that effectively says

expr : catexpr ;

How do I resolve the ambiguity by doing a lookahead in printst - I
effectively want to look for an "end of statement" marker eg a newline
or semicolon. Unfortunately, ":" has three different meanings, as
exemplified in this simple line of code ...

XXX: PRINT A : B :

Where the first colon says "this is a label" (I haven't even touched
this yet!), the second says "concatenate the variables A and B
together", and the third says "don't print a newline at the end". Yeuch!
At the moment I'm disambiguating in the lexer, but I don't think that's
a good idea ... it'll be messy :-( but I really haven't got to grips
with predicates, which I think is what I need ...

Further on, I have a hiccup with my treeparser ...

printst
	: #(PRINT expr COLON) { st.write("PRINT");}
	| #(PRINT expr) { st.write("PRTLN");}
//	| #(PRINT) { st.write("CRLF");}
	;

antlr.Tool does not like the commented-out line - I'm guessing it's
incredibly simple, but it's objecting to PRINT :-(

Cheers,
Wol


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