A recent thread in spirit.general NG suggested to me the need for a 'tagged variant' extension to boost::variant in order to emulate the tagged-union mentioned in this post in the thread: http://article.gmane.org/gmane.comp.parsers.spirit.general/12611 A prototype of such an extension is available in variant_tagged.zip here: http://www.boost-consulting.com/vault/index.php?&directory=Data%20Structures Use of the variant_tagged could be used whenever non-string processing is required in a spirit parser. Each tag in the using corresponds to a token-id and each value associated with that token-id is the attribute for that token-id. This closely emulates the way yacc does it, IIRC, except it just uses a union and the tag is somehow "implicit". Is there any interest in using this in spirit or boost in general?