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From: Robert Jacobson <rljacobson@gmail.com>
Newsgroups: comp.compilers
Subject: Grammar with low-precedence postfix operator?
Date: Thu, 5 Feb 2015 13:27:31 -0500 (EST)
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I'm trying to write an ANTLR4 grammar for a language with a low precedence
postfix operator (Wolfram Language with '&', but I use a simple toy grammar
below). I'm struggling with finding the right pattern to express this
language.

Consider my (incorrect) grammar below that parses math expressions:

<snip>
grammar MathExpression;

//Parser Rules
term
	:	INT
	|	'(' expr ')' //parentheses
	|	<assoc=right> term '^' term //exponentiation
	|	term term //implicit multiplication
	;

expr
	:	term
	|	('+' | '-') expr //unary plus/minus
	|	expr ('/' | '*') expr //division and explicit multiplication
	|	expr ('+' | '-') expr //addition/subtraction
	|	expr '++' //increment
	;

//Lexer Rules
INT :   [0-9]+ ;
WS  :   [ \t\r\n]+ -> skip ;
<snip>

The problem with this grammar is that the generated parser chokes on, for
example, the input '2++^3', as '^' cannot follow '++' in this grammar. If I
get rid of implicit multiplication I can collapse the grammar into a single
rule that parses the language I am wanting, but I don't want to give up
implicit multiplication.

What's the pattern for this situation?

Incidentally, this is my first post to this group, and I am charmed to see
several authors of my textbooks among the members here.

Best,

Robert