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    <p>I'm assuming that the types you are interested in are <code>int</code> and <code>Foo</code>, if this is not what you are after please disregard this answer. Looking over the documentation I haven't been able to find an easy way to get these types. But if you look at the type of the proto expression stored in <code>f</code> you can see that <code>int</code> and <code>Foo</code> can be found in a vector of actors inside the first child. The steps you need to make to finally get to the interesting types can be seen in the output and following that you can easily create a metafunction that does what you want. In this simple case <code>get_local_type</code> uses an index to access the type in question. If you want to access it via name (using <code>_a</code>) you should be able to get the index associated with a name using the data in <code>map_local_index_to_tuple</code> in the second child of the lambda expression. Using <code>phoenix::detail::get_index</code> that is defined <a href="http://www.boost.org/boost/phoenix/scope/detail/local_variable.hpp" rel="nofollow">here</a> implementing <code>get_local_type_from_name</code> is also pretty easy. This metafunction expects the map mentioned above as its first argument and the type of the placeholder you want information from (more specifically it needs <code>phoenix::detail::local&lt;phoenix::local_names::_a_key&gt;</code>, and you can get that using <code>proto::result_of::value</code> on the type of the placeholder) as its second.</p> <pre><code>#include &lt;iostream&gt; #include &lt;typeinfo&gt; #include &lt;string&gt; #include &lt;cxxabi.h&gt; #include &lt;type_traits&gt; #include &lt;boost/proto/proto.hpp&gt; #include &lt;boost/phoenix.hpp&gt; namespace proto = boost::proto; namespace phoenix = boost::phoenix; using namespace phoenix::local_names; namespace fusion = boost::fusion; struct Foo { const char str[6] = " Ok.\n"; }; std::string demangle(const char* mangledName) { int status; char* result = abi::__cxa_demangle(mangledName, nullptr, nullptr, &amp;status); switch(status) { case -1: std::cerr &lt;&lt; "Out of memory!" &lt;&lt; std::endl; exit(1); case -2: return mangledName; case -3: // Should never happen, but just in case? return mangledName; } std::string name = result; free(result); return name; } template &lt;typename Lambda, int N&gt; struct get_local_type { typedef typename proto::result_of::value&lt;typename proto::result_of::child_c&lt;Lambda,0&gt;::type &gt;::type vector_of_locals_type; typedef typename proto::result_of::value&lt;typename fusion::result_of::at_c&lt;vector_of_locals_type,N&gt;::type &gt;::type ref_type; typedef typename std::remove_reference&lt;ref_type&gt;::type type; }; template &lt;typename Lambda, typename Arg&gt; struct get_local_type_from_name { typedef typename proto::result_of::value&lt;Arg&gt;::type local_name; typedef typename proto::result_of::value&lt;typename proto::result_of::child_c&lt;Lambda,1&gt;::type &gt;::type map_type; typedef typename phoenix::detail::get_index&lt;map_type,local_name&gt;::type index; typedef typename get_local_type&lt;Lambda,index::value&gt;::type type; }; int main(int argc, char *argv[]) { auto f = phoenix::lambda(_b = 17, _a = Foo()) [ std::cout &lt;&lt; _b &lt;&lt; phoenix::bind(&amp;Foo::str,_a) ]; std::cout &lt;&lt; std::endl &lt;&lt; "This is the whole lambda expression:" &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; demangle(typeid(f).name()) &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; "Take the first child:" &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; demangle(typeid(proto::child_c&lt;0&gt;(f)).name()) &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; "Then its value (this is a vector that contains the types you want):" &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; demangle(typeid(proto::value(proto::child_c&lt;0&gt;(f))).name()) &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; "Take the first element of that vector:" &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; demangle(typeid(fusion::at_c&lt;0&gt;(proto::value(proto::child_c&lt;0&gt;(f)))).name()) &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; "Take the value of that element:" &lt;&lt; std::endl; std::cout &lt;&lt; std::endl &lt;&lt; demangle(typeid(proto::value(fusion::at_c&lt;0&gt;(proto::value(proto::child_c&lt;0&gt;(f))))).name()) &lt;&lt; std::endl; typedef typename proto::tag_of&lt;decltype( f )&gt;::type tag; typedef typename proto::tag_of&lt;decltype(proto::child_c&lt;2&gt;(f))&gt;::type tagc; static_assert(proto::arity_of&lt;decltype(f)&gt;::value==3,""); static_assert(std::is_same&lt;tag, phoenix::tag::lambda_actor&gt;::value,""); static_assert(std::is_same&lt;tagc, proto::tag::shift_left&gt;::value,""); typedef typename get_local_type&lt;decltype(f),0&gt;::type type_of_1st; typedef typename get_local_type&lt;decltype(f),1&gt;::type type_of_2nd; typedef typename get_local_type_from_name&lt;decltype(f),_a_type&gt;::type type_of_a; typedef typename get_local_type_from_name&lt;decltype(f),decltype(_b)&gt;::type type_of_b; static_assert(std::is_same&lt;type_of_1st,int&gt;::value,""); static_assert(std::is_same&lt;type_of_2nd,Foo&gt;::value,""); static_assert(std::is_same&lt;type_of_a,Foo&gt;::value,""); static_assert(std::is_same&lt;type_of_b,int&gt;::value,""); return 0; } </code></pre>
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    1. CO+100 Many thanks, that's exactly what I was after. Showing your working, so to speak, with the use of `abi::__cxa_demangle` is also total class; now I can fish for myself ;)
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    2. COGlad it helped. I have added a way to get the type from the name of the placeholder instead of just its index and changed slightly the lambda expression to use both metafunctions. Even though I like this answer, it is a little lame/shoddy (sorry I don't know how to express this in English). I hope this doesn't deter other people from giving a better answer. And in the case someone does you should accept that answer.
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    3. CONote that the demangling is obviously proprietary. You might find it more convenient to just print typeid().name() and pipe your program's output through c++filt. This is usually a lot more convenient and supports more compilers.
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