sqrt-either-usage.cpp

// And for the caller, the usage changes from this... 
try {
  float x = sqrt(-1);
  std::cout << "sqrt(x) = " << x << std::endl;
} catch(std::string x) {
  std::cout << "error occurred: " << x << std::endl;
}

// ... to this... 
std::string msg = sqrt(-1)
  .leftMap([](auto msg) { return "error occurred: " + msg; })
  .rightMap([](auto result) { return "sqrt(x) = " + to_string(result); })
  .join();

std::cout << msg << std::endl;