Help with left factoring a grammar to remove left recursion

Question

I have a small custom scripting language, and I am trying to update it to allow boolean expressions such as a > 2 and a > 2 and (b < 3 or c >

Answer 1

Boolean expressions are just the same as the additive- and multiplicative expression, and should therefore not be separated from them. Here's how to account for all types of expressions:

grammar test;

parse
  :  expression EOF
  ;

expression 
  :  or
  ;

or
  :  and (OR and)*
  ;

and
  :  rel (AND rel)*
  ;

rel
  :  add (('=' | '>' | '<') add)*
  ;

add
  :  mult (('+' | '-') mult)*
  ;

mult
  :  unary (('*' | '/') unary)*
  ;

unary 
  :  '-' term
  |  '+' term
  |  NOT term
  |  term
  ;

term 
  :  INTEGER  
  |  IDENT       
  |  list
  |  '(' expression ')'
  ;

list 
  :  '{' (expression (',' expression)*)? '}'
  ;

AND     :  'and';
OR      :  'or';
NOT     :  'not';
IDENT   :  LETTER LETTER*;
INTEGER :  DIGIT+;
WS      :  (' ' | '\n' | '\r' | '\t')+  { $channel = HIDDEN; };

fragment DIGIT   : '0'..'9';
fragment LETTER  : ('a'..'z' | 'A'..'Z');

which will parse the example input:

abc > 3 and (xyz < 5 or xyz > {1, 2, 3})

into the following parse tree: