#include <exception>
#include <functional>
#include <mutex>

namespace nix {

/* A helper class for lazily-initialized variables.

     Lazy<T> var([]() { return value; });

   declares a variable of type T that is initialized to 'value' (in a
   thread-safe way) on first use, that is, when var() is first
   called. If the initialiser code throws an exception, then all
   subsequent calls to var() will rethrow that exception. */
template <typename T>
class Lazy {
  typedef std::function<T()> Init;

  Init init;

  std::once_flag done;

  T value;

  std::exception_ptr ex;

 public:
  Lazy(Init init) : init(init) {}

  const T& operator()() {
    std::call_once(done, [&]() {
      try {
        value = init();
      } catch (...) {
        ex = std::current_exception();
      }
    });
    if (ex) {
      std::rethrow_exception(ex);
    }
    return value;
  }
};

}  // namespace nix