CODEWITHSUNDEEP
c++sfinaecompile-timeboost-hana
erikzenker
erikzenker

Reputation: 752

Boost.Hana test if lambda is callable with particular argument

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;
}

Upvotes: 2

Views: 164

Answers (1)

Justin
Justin

Reputation: 25367

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...))
        {})
    );
};

Live demo on Compiler Explorer

Upvotes: 2

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