Add support for logical expressions

lib/liquid/binary_expression.rb

@@ -13,6 +13,14 @@ module Liquid
 
     def evaluate(context)
       left = value(left_node, context)
+
+      # logical relation short circuiting
+      if operator == 'and'
+        return left && value(right_node, context)
+      elsif operator == 'or'
+        return left || value(right_node, context)
+      end
+
       right = value(right_node, context)
 
       case operator
@@ -38,7 +46,7 @@ module Liquid
     end
 
     def to_s
-      "(#{left_node} #{operator} #{right_node})"
+      "(#{left_node.inspect} #{operator} #{right_node.inspect})"
     end
 
     private

lib/liquid/parser.rb

@@ -47,12 +47,30 @@ module Liquid
       tok[0] == type
     end
 
-    # expression := equality
+    # expression := logical
+    # logical    := equality (("and" | "or") equality)*
     # equality   := comparison (("==" | "!=" | "<>") comparison)*
     # comparison := primary ((">=" | ">" | "<" | "<=" | ... ) primary)*
     # primary    := string | number | variable_lookup | range | boolean
     def expression
-      equality
+      logical
+    end
+
+    # Logical relations in Liquid, unlike other languages, are right-to-left
+    # associative. This creates a right-leaning tree and is why the method
+    # looks a bit more complicated
+    #
+    # `a == b and b or c` is evaluated like (a and (b or c))
+    def logical
+      expr = equality
+      while (operator = id?('and') || id?('or'))
+        if expr.is_a?(BinaryExpression) && (expr.operator == 'and' || expr.operator == 'or')
+          expr.right_node = BinaryExpression.new(expr.right_node, operator, equality)
+        else
+          expr = BinaryExpression.new(expr, operator, equality)
+        end
+      end
+      expr
     end
 
     def equality

lib/liquid/tags/case.rb

@@ -99,7 +99,7 @@ module Liquid
       parser = @parse_context.new_parser(markup)
 
       loop do
-        expr = BinaryExpression.new(@left, '==', Condition.parse_expression(parser))
+        expr = BinaryExpression.new(@left, '==', parser.equality)
         block = Condition.new(expr)
         block.attach(body)
         @blocks << block