From f5443d2bf115c55900234baa083608fa219d6356 Mon Sep 17 00:00:00 2001
From: David Majda <dmajda@suse.cz>
Date: Wed, 12 Mar 2014 19:05:26 +0100
Subject: [PATCH] Complete rewrite of the arithmetics example grammar

This is a complete rewrite of the arithmetics example grammar. It now
allows whitespace between tokens, supports "-" and "/" operators, and
gets the operator associativity right. Also, rule names now match the usual
conventions (term, factor,...).

Beside this, the rewrite reflects how I write grammars today (as opposed
to few years ago) and what style I would recommend to others.
---
 examples/arithmetics.pegjs | 52 +++++++++++++++++++++++++++-----------
 1 file changed, 37 insertions(+), 15 deletions(-)

diff --git a/examples/arithmetics.pegjs b/examples/arithmetics.pegjs
index a0b7bc0..ce56d09 100644
--- a/examples/arithmetics.pegjs
+++ b/examples/arithmetics.pegjs
@@ -1,22 +1,44 @@
 /*
- * Classic example grammar, which recognizes simple arithmetic expressions like
- * "2*(3+4)". The parser generated from this grammar then computes their value.
+ * Simple Arithmetics Grammar
+ * ==========================
+ *
+ * Accepts expressions like "2 * (3 + 4)" and computes their value.
  */
 
-start
-  = additive
+{
+  function combine(first, rest, combiners) {
+    var result = first, i;
 
-additive
-  = left:multiplicative "+" right:additive { return left + right; }
-  / multiplicative
+    for (i = 0; i < rest.length; i++) {
+      result = combiners[rest[i][1]](result, rest[i][3]);
+    }
 
-multiplicative
-  = left:primary "*" right:multiplicative { return left * right; }
-  / primary
+    return result;
+  }
+}
 
-primary
-  = integer
-  / "(" additive:additive ")" { return additive; }
+Expression
+  = first:Term rest:(_ ("+" / "-") _ Term)* {
+      return combine(first, rest, {
+        "+": function(left, right) { return left + right; },
+        "-": function(left, right) { return left - right; }
+      });
+    }
 
-integer "integer"
-  = digits:$[0-9]+ { return parseInt(digits, 10); }
+Term
+  = first:Factor rest:(_ ("*" / "/") _ Factor)* {
+      return combine(first, rest, {
+        "*": function(left, right) { return left * right; },
+        "/": function(left, right) { return left / right; }
+      });
+    }
+
+Factor
+  = "(" _ expr:Expression _ ")" { return expr; }
+  / Integer
+
+Integer "integer"
+  = [0-9]+ { return parseInt(text(), 10); }
+
+_ = "whitespace"
+  [ \t\n\r]*