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

Subject: Re: [boost] [gsoc-2013] Boost.Expected
From: Pierre T. (ptalbot_at_[hidden])
Date: 2013-04-27 03:39:55

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 <typename ValueType, typename ExceptionalType=exception_ptr>
>    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<exceptional_type>(except));
>        }
>      };
>      template <>
>      struct  exceptional_traits<exception_ptr>    {
>        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<ValueType, ExceptionalType> 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<typename boost::result_of<F(const expected&)>::type>
>      then(F&& fuct)
>
> Let me name
> typedef typename boost::result_of<F(const expected&) RT;
>
> 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.