Sorry about all these tests and thanks for your patience. Here's another one which uses a slightly different method that I think should will better: #include <boost/preprocessor/seq/seq.hpp> #include <boost/preprocessor/detail/split.hpp> #include <boost/preprocessor/punctuation/comma.hpp> #include <boost/preprocessor/punctuation/paren.hpp> #include <boost/preprocessor/tuple/eat.hpp> #define IS_NULLARY_IMPL(x) \ BOOST_PP_SPLIT(1, IS_NULLARY_C x BOOST_PP_COMMA() (0)) \ /**/ #define IS_NULLARY_C() \ ~, (1) BOOST_PP_RPAREN() BOOST_PP_LPAREN() ~ \ /**/ #define IS_NULLARY(x) BOOST_PP_SEQ_HEAD(IS_NULLARY_IMPL(x)) IS_NULLARY(()) // 1 IS_NULLARY(()abc) // 1 IS_NULLARY(+) // 0 IS_NULLARY(xyz) // 0 #define IS_UNARY(x) IS_NULLARY(IS_UNARY_C x) #define IS_UNARY_C(a) () IS_UNARY((a)) // 1 IS_UNARY((a)abc) // 1 IS_UNARY(+) // 0 IS_UNARY(xyz) // 0