I want to test if a callable lambda can be called with particular arguments by using Boost.Hana. The compiler complains that callable expects 0 arguments but 1 were provided. Isn't the if_ a compile time if that does compile depending on the condition is_callable?
Maybe someone has an idea of how to get the code to work.
#include <boost/hana.hpp>
int main(){
namespace bh = boost::hana;
auto is_callable = bh::is_valid([](auto&& callable, auto&& args) -> decltype(bh::unpack(args, callable)){});
auto callable = [](){};
auto arg = 1;
auto args = bh::make_tuple(arg);
bh::if_(is_callable(callable, args),
[arg](auto callable){ callable(arg);},
[](auto callable){})(callable);
return 0;
}
As Jason Rice mentioned in the comments, the reason this fails is that hana::unpack is not SFINAE friendly. The easiest way to work around this is to call the callable ourselves for hana::is_valid, using hana::unpack to unpack the arguments into the result of hana::is_valid:
auto const is_callable = [](auto&& callable, auto&& args) {
return bh::unpack(args,
bh::is_valid([&callable](auto&&... args)
-> decltype(callable(args...))
{})
);
};