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 

lib/parser/basic_types.rb

@@ -43,6 +43,7 @@ module Parser
     
     rule(:float) { integer >>  dot >> integer >> 
                             (exponent >> sign.maybe >> digit.repeat(1,3)).maybe >> space?}
-    rule(:basic_type){ integer | name | string | float | instance_variable | module_name }
+    rule(:basic_type){ integer | name | string | float | instance_variable | module_name | 
+                       keyword_true | keyword_false | keyword_nil }
   end
 end

lib/parser/transform.rb

@@ -6,7 +6,10 @@ module Parser
     rule(:string => sequence(:chars)) { Ast::StringExpression.new chars.join }
     rule(:esc => simple(:esc)) { '\\' +  esc }
     rule(char: simple(:char)) { char }
-    
+
+    rule(:true => simple(:true)) { Ast::TrueExpression.new() }
+    rule(:false => simple(:false)) { Ast::FalseExpression.new() }
+    rule(:nil => simple(:nil)) { Ast::NilExpression.new() }
     rule(:integer => simple(:value)) { Ast::IntegerExpression.new(value.to_i) }
     rule(:name   => simple(:name))  { Ast::NameExpression.new(name.to_s) }
     rule(:instance_variable   => simple(:instance_variable))  { Ast::VariableExpression.new(instance_variable.name) }