start to add while loop

lib/ast/while_expression.rb

@@ -0,0 +1,16 @@
+module Ast
+  class WhileExpression < Expression
+    attr_reader  :condition, :body
+    def initialize condition, body
+      @condition , @body = condition , body
+    end
+    def inspect
+      self.class.name + ".new(" + condition.inspect + ", "  + body.inspect + " )"  
+    end
+    def attributes
+      [:condition, :body]
+    end
+  end
+
+  
+end