refman-handler_ref.md

type handler_ref

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Abstract reference to an active handler.

using handler_ref = ...;

A handler_ref is an abstract reference to an active handler, which can be used to tie the handler to a command, without the need to look up the handler every time we use invoke_command or static_invoke_command.

‼️ Handler references are valid if the referenced handler is active. Invoking a command with a reference to a handler of a computation that has already ended or is captured in a resumption will cause undefined behaviour.

A handler reference should not be confused with:

• A reference to an object of a class that inherits from handler, although it is not far from the truth: handler_ref can be thought of as a form of an iterator that points to an object, not a reference to the object itself.

• A label, since labels are not necessarily unique, and invoking a command with a label still involves looking up a handler with the given label on the stack.

Example

using namespace cpp_effects;

struct Ask : command<int> { };
struct AddOneMoreHandler : command<> { };

template <typename T>
class Reader : public flat_handler<T, Ask, AddOneMoreHandler> {
public:
  Reader(int val) : val(val) { }
private:
  int val;
  T handle_command(Ask, resumption<T(int)> r) override
  {
    return std::move(r).Resume(val);
  }
  T handle_command(AddOneMoreHandler, resumption<T()> r) override
  {
    return handle<Reader<void>>(300, [r = r.release()]() {
      resumption res(r);
      std::move(res).resume();
    }, 300);
  }
};

int main()
{
  handle_ref<Reader<void>>(100, [](auto href) {
    debug_print_metastack();
    std::cout << invoke_command(href, Ask{}) << std::endl;
    invoke_command(AddOneMoreHandler{});
    handle<Reader<void>>([=]() {
      debug_print_metastack();
      std::cout << invoke_command(href, Ask{}) << std::endl;
      return 0;
    }, 200);
  }, 100);
}

Example output:

100:6ReaderIvE[3Ask][17AddOneMoreHandler]
0:N10CppEffects9MetaframeE
100
-2:6ReaderIvE[3Ask][17AddOneMoreHandler]
100:6ReaderIvE[3Ask][17AddOneMoreHandler]
300:6ReaderIvE[3Ask][17AddOneMoreHandler]
0:N10CppEffects9MetaframeE
100