Peter Dimov wrote:
- \ \ double - /\ f . /\ x. f(f x) - / \ / \
lambda( _f = _1 )[ bind( _f, bind( _f, _1 ) ) ] ?
FWIW, the above doesn't work for me. What is the proper Phoenix way to express this double lambda?
Here's an example: #include <iostream> #include <boost/detail/lightweight_test.hpp> #include <boost/spirit/include/phoenix_core.hpp> #include <boost/spirit/include/phoenix_operator.hpp> #include <boost/spirit/include/phoenix_function.hpp> #include <boost/spirit/include/phoenix_scope.hpp> using namespace boost::phoenix; using namespace boost::phoenix::arg_names; struct sqr_impl { template <typename Arg> struct result { typedef Arg type; }; template <typename Arg> Arg operator()(Arg n) const { return n * n; } }; function<sqr_impl> sqr; struct ffx_impl { template <typename ArgF, typename ArgX> struct result { typedef ArgX type; }; template <typename ArgF, typename ArgX> ArgX operator()(ArgF f, ArgX arg2) const { ArgX x = f(arg2); return f(x); } }; function<ffx_impl> ffx; int main() { std::cout << "Square of " << 5.0 << " is " << sqr(5)() << std::endl; std::cout << "Double Square of " << 5 << " is " << ffx(lambda[sqr(_1)], 5.0)() << std::endl; std::cout << "Double Double Square of " << 5 << " is " << ffx(lambda[ ffx(lambda[sqr(_1)], _1) ], 5.0 )() << std::endl; return 0; } This was first discussed in a post by Joel Young (2002!): http://lists.boost.org/Archives/boost/2002/03/27181.php after my naive initial response, I got something like: http://lists.boost.org/Archives/boost/2002/03/27250.php which is a template function that returns a function (doub). Notes: * Here, lambda[..] is getting rather unwieldy. I had an earlier version with implicit lambdas. With that, you write Double Double Square expression more succinctly as: ffx(ffx(sqr(_1), _1), 5.0) * Perhaps there's a terser way similar to your suggestion without using phoenix function. I'll investigate on it. Regards, -- Joel de Guzman http://www.boostpro.com http://spirit.sf.net