Re: [boost] [outcome] High level summary of review feedback accepted so far

Subject: Re: [boost] [outcome] High level summary of review feedback accepted so far
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-05-27 16:28:39

>> I would assume an expected<T, E1, .., En> could only return a
>> std::variant<E1, ..., En> from its .error(). I can't think what else
>> it could do.
> 
> As the foremost authority on the nonexistent expected<T, E...>, I can
> tell you what it does:
> 
>    // F shall be in E...
>    template<class F> bool has_error() const;
>    template<class F> F error() const;
> 
>    // if sizeof...(E) == 1
>    bool has_error() const;
>    E1 error() const;
> 
> It returns the equivalent of variant<E...> not from error(), but from
> 
>    unexpected<E...> unexpected() const;
> 
> which allows you to
> 
>    expected<T, E1, E2, E3> function()
>    {
>        expected<U, E1, E2> e1 = function1();
>        if( !e1 ) return e1.unexpected();
> 
>        expected<V, E3> e2 = function2();
>        if( !e2 ) return e2.unexpected();
> 
>        return e1.value() + e2.value();
>    }

That's a second approach.

A third approach could be implementing .index() and std::get<>() for
expected.

Niall

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