I have the following program to print out the arguments of a function call expression. It works fine when I directly call the grammar on the expression like so : foo_tostr()(f(a,b,c)). However, if I assign the expression to a variable pr using BOOST_PROTO_AUTO and call foo_tostr()(pr), I get error of the form "conversion from ‘boost::fusion::... to non-scalar type .... requested". Can someone let me know what I am doing wrong? #include <iostream> #include #include #include <sstream> #include <string> using namespace std; namespace proto = boost::proto; unsigned ids; struct var { unsigned id; var() { id = ++ids; } }; struct foo {}; typedef proto::terminal<var>::type Var; typedef proto::terminal<foo>::type Foo; struct _var_tostr : proto::callable { typedef string result_type; result_type operator()(var const &v) const { stringstream sout; sout << "v" << v.id << " "; return sout.str(); } result_type operator()(foo const &v) const { stringstream sout; sout << "foo("; return sout.str(); } }; struct var_tostr : proto::or_< proto::when > {}; struct str_plus : std::plusstd::string, proto::callable {}; struct foo_tostr : proto::or_ < proto::when >, proto::fold > > {}; int main() { Var a, b, c; Foo f; #define COPY_AND_PRINT #ifdef COPY_AND_PRINT BOOST_PROTO_AUTO(pr, (f(a,b,c))); cout << foo_tostr()(pr) << endl; #else cout << foo_tostr()(f(a,b,c)) << endl; #endif } -- Manjunath