Give a better error message when a referenced ptype has not been declared

lib/propane/generator.rb

@@ -51,6 +51,7 @@ class Propane
       unless found_default
         raise Error.new("No patterns found for default mode")
       end
+      check_ptypes!
       # Add EOF token.
       @grammar.tokens << Token.new("$EOF", nil, nil)
       tokens_by_name = {}
@@ -131,6 +132,17 @@ class Propane
       @parser = Parser.new(@grammar, rule_sets, @log)
     end
 
+    # Check that any referenced ptypes have been defined.
+    def check_ptypes!
+      (@grammar.patterns + @grammar.tokens + @grammar.rules).each do |potor|
+        if potor.ptypename
+          unless @grammar.ptypes.include?(potor.ptypename)
+            raise Error.new("Error: Line #{potor.line_number}: ptype #{potor.ptypename} not declared. Declare with `ptype` statement.")
+          end
+        end
+      end
+    end
+
     # Generate and add rules for any optional components.
     def generate_optional_component_rules!(tokens_by_name)
       optional_rules_added = Set.new