Allow user-defined context fields

lib/propane/generator.rb

@@ -265,6 +265,15 @@ class Propane
           "context.user_terminate_code = (#{user_terminate_code}); return #{retval};"
         end
       end
+      code = code.gsub(/\$\{context\.(\w+)\}/) do |match|
+        fieldname = $1
+        case @language
+        when "c"
+          "context->#{fieldname}"
+        when "d"
+          "context.#{fieldname}"
+        end
+      end
       if parser
         code = code.gsub(/\$\$/) do |match|
           case @language

lib/propane/grammar.rb

@@ -5,6 +5,7 @@ class Propane
     # Reserve identifiers beginning with a double-underscore for internal use.
     IDENTIFIER_REGEX = /(?:[a-zA-Z]|_[a-zA-Z0-9])[a-zA-Z_0-9]*/
 
+    attr_reader :context_user_code
     attr_reader :tree
     attr_reader :tree_prefix
     attr_reader :tree_suffix
@@ -34,6 +35,7 @@ class Propane
       @tree_prefix = ""
       @tree_suffix = ""
       @free_token_node = nil
+      @context_user_code = ""
       parse_grammar!
       @start_rules << "Start" if @start_rules.empty?
     end
@@ -62,6 +64,7 @@ class Propane
       if parse_white_space!
       elsif parse_comment_line!
       elsif @modeline.nil? && parse_mode_label!
+      elsif parse_context_statement!
       elsif parse_tree_statement!
       elsif parse_tree_prefix_statement!
       elsif parse_tree_suffix_statement!
@@ -98,6 +101,15 @@ class Propane
       consume!(/#.*\n/)
     end
 
+    def parse_context_statement!
+      if md = consume!(/context\b\s*/)
+        unless code = parse_code_block!
+          raise Error.new("Line #{@line_number}: expected code block")
+        end
+        @context_user_code += code
+      end
+    end
+
     def parse_tree_statement!
       if consume!(/tree\s*;/)
         @tree = true