Re: [boost] [gsoc-2013] Boost.Expected

On 04/26/2013 08:17 PM, Vicente J. Botet Escriba wrote:

Le 26/04/13 14:22, Pierre T. a écrit :

...

Hello,

I wrote a proposal for the Boost.Expected project. You can find a pdf version here:

http://hyc.io/boost/boost-expected-proposal.pdf

It's about the Boost.Expected project, I'll add my own informations by the end of the week.

Finally, I propose a single class design, and facilities for custom error code. It's based on the Alexendrescu idea upgraded by Vicente J. Botet Escriba and myself. I also took into account the Boost.Optional class. If needed for this proposal, I can add documentation per method (such as in official proposal), but I'm not sure it's useful without a frozen interface.

Please do not hesitate to comment it and request further clarification,

Hi,

first of all, thanks for writing this proposal.

I have some concerns

* Single class: I'm all for a single class; but having an ExceptionalType template parameter which defaults to exception_ptr.

template class expected;

We could define the traits that make the difference between having an exception_ptr and another error

template <typename ExceptionalType> struct exceptional_traits { typedef ExceptionalType exceptional_type; template <class E> static exceptional_type make_exceptional(E const& except) { return exceptional_type(except) } static exceptional_type current_exceptional() { return exceptional_type() } static void rethrow(exceptional_type except) { boost::throw_exception(bad_expected_access(except)); } }; template <> struct exceptional_traits { typedef ExceptionalType exceptional_type;

template <class E> static exceptional_type make_exceptional(E const& except) { return boost::make_exception_ptr(except); } static exceptional_type current_exceptional() { return boost::current_exception() } static void rethrow(exceptional_type except); { boost::rethrow_exception(except); } template <class E> static bool has_exception(bool valid, exceptional_type except) const BOOST_NOEXCEPT { try { if (!valid) rethrow(except); } catch (const E& ex) { return true; } catch (...) { } return false; } }; }

With these traits we are able to define the functions that have a specific behavior as e.g.

typedef detail::exceptional_traits traits;

expected() BOOST_NOEXCEPT : except_(traits::current_exceptional()) , has_value_(false) {} const T& get() const { if (!valid()) traits::rethrow(except_); return value_; } template <class E> bool has_exception() const BOOST_NOEXCEPT { return traits::has_exception(except_); }

I like this proposition but at first, it seems a bit "overkill". I would like to have more opinions about it. Through it seems to be less compatible with the "visitor" idea. The visitor resolves on types and most of the error code have a same type. So it would visit only one type.

* Default Constructor or constructor from nullexpect What is the advantage of having a expected instance that doesn't have neither a value nor an exception? How would the user manages with this possibility? Are you looking to make expect movable?

Basically, I noticed that classes without default constructor (or default state) are burdensome to use. Indeed, you cannot store an expected in a class as a member if not initialized in the constructor. Or doing something like: expected<int> e; if(…) else(…) return e; Through, I removed the default constructor because I found it unclear. I use a nullexcept because it was a good idea in Boost.Optional with nullopt. Finally, I'm not sure to understand how it's related to the movable ability of Boost.Expected.

* then/otherwise issues

Humm, I don't agree here with the proposed design (even if I made the then suggestion). What returns the then function if the instance has a value? I declared it as

template <typename F> expected::type> then(F&& fuct)

Let me name typedef typename boost::result_of

so the result would return the expected<RT>(fuct(value_)). Chaining this temporary with an otherwise call will work with the new temporary, and not with the original expected. I don't think this was what was expected ;-)

E.g.

string f(int); int f(string);

expected<int> e= 1; e.then(f).otherwise(g);

Here g will be called with expected<string>(fuct(value_)).except_. Hrrr

expected<int> e= make_exceptional_expect(X); e.then(f).otherwise(g);

Here g will be called with the exception_ptr stored in e. Hrrr

That is g don't know if it is called with an exception stored on e or on the temporary resulting for the call to f.

But the design error is not on the otherwise function but on the then function.

Resuming, I'm not more for the 'then' function.

I'm totally agree with you. But I think that I misnamed the "otherwise" method. I think the "then" method is really useful, it's like an automaton. With e.then().then().then(), the treatment is stopped anytime when an error occurs in a then method. Also, I think that the function taken by then should return void or an expected. In fact the otherwise method is the error visitor. A common usage would be to chain it in the end: e.then(...).then(...).then(...).visit_error(error_visitor); visit_error is called if any "then" return an error. I found it wonderfully useful.

Using a visitor would as you proposed initially would be better.

template <class V> void accept_visitor(V& visitor) const BOOST_NOEXCEPT { if (valid()) visitor.visit(value_); else visitor.visit(expectional, except_); }

I'm not sure that we should use the visitor pattern for the value and the error. Initially, I though at the visitor pattern because error would be exception-based, so with some types to visit. And also to provide a way to visit exception without throwing them with the get() method.

HTH, Vicente

As usual, these discussions help a lot, but this time, I'm not sure if our idea are still good ideas, that's why more opinions would help. Thanks, Pierre T.