[antlr-interest] lexing tips needed... (or, could I use predicate in lexer rule?)

Micheal J open.zone at virgin.net
Sat Jul 14 18:29:03 PDT 2007


You may be trying to do too much in the lexer Lloyd. Seems you can simply
change the lexer to emit say NUM_FLOAT_DOT for floats that end with the
decimal point. The lexer seems to handle your other NUM_FLOAT examples just
fine. 
 
You can then deal with the NUM_FLOAT_DOT ID sequence in the parser.
 
NUM_INT
  :  DIGITS 
     (
          '.' { $type = NUM_FLOAT_DOT; } 
          ( fraction_digits=DIGITS { $type = NUM_FLOAT; } )? 
          ( exponent_part=EXPONENT_PART { $type = NUM_FLOAT; } )? 
          ( float_suffix=FLOAT_TYPE_SUFFIX { $type = NUM_FLOAT; } )?
     )?
 
  | ..................... other match
;

 
Micheal

-----------------------
The best way to contact me is via the list/forum. My time is very limited. 

-----Original Message-----
From: antlr-interest-bounces at antlr.org
[mailto:antlr-interest-bounces at antlr.org] On Behalf Of Lloyd Dupont
Sent: 15 July 2007 02:02
To: antlr-interest at antlr.org
Subject: [antlr-interest] lexing tips needed... (or,could I use predicate in
lexer rule?)


For my parser I reaped of the lexer rules from the Java exemple.
Particularly you've got this lexer token:
NUM_FLOAT
 : DIGITS '.' (DIGITS)? (EXPONENT_PART)? (FLOAT_TYPE_SUFFIX)?
 | ............ other match
 ;
 
Now there is a problem with that!....
in the language I target everything is an object.
 
so:

"2."  is NUM_FLOAT["2."] 
"2.1"  is NUM_FLOAT["2.1"]
"2.ToString()" is NUM_INT["2"] DOT["."] ID["ToString"] LPAREN["("]
RPAREN[")]
 
So I was trying to disambiguate the lexer with a construct like that (trying
predicate in the lexer):
NUM_FLOAT
 : (
   ( DIGITS '.' EXPONENT_PART )=> DIGITS '.' (EXPONENT_PART)
(FLOAT_TYPE_SUFFIX)?
  | ( DIGITS '.' ID )=> DIGITS
  | DIGITS '.' (DIGITS)? (EXPONENT_PART)? (FLOAT_TYPE_SUFFIX)?
  )
  |    ..................... other match
;
 
but this doesn't seem to work,
1. it told me the NUM_INT rule is now not accessible anymore (I guess the
NUM_FLOAT rule absorbs it with my second alternative)
2. but more importantly an input such as "2.a" return "2." followed by
MismatchTokenException.
 
can I use predicate in lexer rule?
how could I disambiguate the NUM_FLOAT lexer rule?
I.e. being both able to read "2.3" (NUM_FLOAT) and "2.ToString()" (NUM_INT
DOT ID LPAREN RPAREN)?
 

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