add bools and nil

lib/ast/basic_expressions.rb

@@ -18,6 +18,22 @@ module Ast
     end
   end
 
+  class TrueExpression < Expression
+    def to_s
+      "true"
+    end
+  end
+  class FalseExpression < Expression
+    def to_s
+      "false"
+    end
+  end
+  class NilExpression < Expression
+    def to_s
+      "nil"
+    end
+  end
+  
   class NameExpression < Expression
     attr_reader  :name
     def initialize name

lib/ast/expression.rb

@@ -10,7 +10,10 @@
 module Ast
   class Expression
     def attributes
-      raise "abstract #{self}"
+      []
+    end
+    def inspect
+      self.class.name + ".new()"
     end
     def == other
       return false unless other.class == self.class