left to right evaluation

examples/lambda_calculus/arithmetic.treetop

@@ -4,7 +4,10 @@ grammar Arithmetic
   end
 
   rule comparative
-    operand_1:additive space operator:equality_op space operand_2:additive <BinaryOperation>
+    head:additive
+    tail:(
+      space operator:equality_op
+      space operand:additive)* <BinaryOperation>
   end
 
   rule equality_op
@@ -16,11 +19,10 @@ grammar Arithmetic
   end
 
   rule additive
-    operand_1:multitive
-    space operator:additive_op space
-    operand_2:additive <BinaryOperation>
-    /
-    multitive
+    head:multitive
+    tail:(
+      space operator:additive_op
+      space operand:multitive)* <BinaryOperation>
   end
 
   rule additive_op
@@ -38,11 +40,10 @@ grammar Arithmetic
   end
 
   rule multitive
-    operand_1:primary
-    space operator:multitive_op space
-    operand_2:multitive <BinaryOperation>
-    /
-    primary
+    head:primary
+    tail:(
+      space operator:multitive_op
+      space operand:primary)* <BinaryOperation>
   end
 
   rule multitive_op
@@ -94,4 +95,4 @@ grammar Arithmetic
   rule space
     ' '*
   end
-end
+end

examples/lambda_calculus/arithmetic_node_classes.rb

@@ -1,7 +1,9 @@
 module Arithmetic
   class BinaryOperation < Treetop::Runtime::SyntaxNode
     def eval(env={})
-      operator.apply(operand_1.eval(env), operand_2.eval(env))      
+      tail.elements.inject(head.eval(env)) do |value, element|
+        element.operator.apply(value, element.operand.eval(env))
+      end
     end
   end
-end
+end

examples/lambda_calculus/arithmetic_test.rb

@@ -43,6 +43,10 @@ def test_order_of_operations
     assert_equal 11, parse('1 + 2 * 3 + 4').eval
   end
 
+  def test_left_to_right
+    assert_equal 2, parse('5 - 2 - 1').eval
+  end
+
   def test_parentheses
     assert_equal 25, parse('(5 + x) * (10 - y)').eval('x' => 0, 'y' => 5)
   end