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

Niall I'm trying to see what you want to propose by difference to a hypothetical generic interface to see what is the added value. Le 27/05/2017 à 16:13, Niall Douglas via Boost a écrit :

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- outcome<T> and result<T> will have their empty state removed, and all observers gain narrow contracts. Default construction is disabled. https://github.com/ned14/boost.outcome/issues/44. Example:

``` result<Foo> v(make_errored_result(std::errc::invalid_argument)); assert(v.has_error()); // true assert(!v.has_value()); // true // As if Foo(reinterpret_cast // (error_code_extended(std::errc::invalid_argument)); Foo f(std::move(v.value())); assert(v.has_error()); // still true assert(!v.has_value()); // still true ``` Just a question I could do for the existing library also. What has_error mean for outcome, result? is that it has an EC or that it has no value? .has_error() is true if and only if the current state is an error_code_extended instance. Okay. Why don't you name it has_error_code? For me having an exception_ptr stored was also an error.

Why not name the getter error function get_error_code, .... as the function doesn't returns a generic Error but one of them. I don't think using the same name for different things is helping in generic programming.

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And now that we don't have empty, what is the sens of has_error for result?

There will be no longer an empty state in result<T>, but for generic programming I'll be retaining a .has_error() so end users can more easily swap a result<T> for a result_e<T>.

:( I see generic programming in other terms.

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Maybe, outcome should have a get_state function that returns an enum so that the user can do a switch. I had previously a .visit(callable) for that.

Any sum type should provide a visit function. I've implemented it in the std_make repository. I will add it to Expected proposal once we have a SumType type of classes.

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(NOTE: expected will track whatever LEWG Expected does, but it differs from what will become result<T> by having a wide contract on .value() and narrow contracts on operator*(), .error(), operator->(). result<T> will have narrow contracts on everything, it is basically a thin wrap of std::variant except with strong never empty warranty) What will be the differences between result<T> and expected?

The wide contracts for the observers? Can not we provide wide and narrow contracts or don't reuse the same name with different meaning? The description above quite literally tells you the differences.

Why we cannot provide wide and narrow observers? If this is the single difference I don't see the need for having two types.

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If we had a expected what will be the differences between outcome<T> and expected? I would assume an expected could only return a std::variant from its .error(). I can't think what else it could do.

outcome<T> continues to provide: - T& .value() - error_code_extended .error() - std::exception_ptr .exception()

i.e. hard coded. Ok, so the single difference would be about the error observers returning by value and having an explicit name.

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- New typedefs outcome_e<T> and result_e<T> are identical to outcome<T> and result<T> except for adding a formal empty state. Observer contract slightly widens, an attempt to use an empty object throws a bad_outcome_access exception. Implicit conversion from non-empty-capable varieties is permitted to empty-capable varieties, but not the other way round. Default construction is to **empty**. https://github.com/ned14/boost.outcome/issues/44 Okay this corresponds to what others are naming optional_outcome, optional_result. I'm not wedded to result_e<T> etc. Ok, I'll change result_e<T> and outcome_e<T> to optional_result<T> and optional_outcome<T>. Done at https://github.com/ned14/boost.outcome/issues/44 Neither me. Just wanted to name the types in a more explicit way. result_e doesn't tells me the intent. If we had a optional_expected what will be the differences between result_e<T> and optional_expected? what will be the differences between outcome_e<T> and optional_expected? I am not sure what semantics you have chosen for optional_expected<>.

This function *could* return variant or variant or any sum type that represents the error, that is the Not-A-Value. Maybe, expected::get_error<Ek> could be more useful. In any case expected should provide a visit() function. Having more than one Error makes this absolutely necessary. When having more than one error in expected, having access to each one of them using a different interface would make the user code more complex, isn't it? optional_expected should be an optimize version of optional>.

optional_outcome<T> is exactly like outcome<T> but with an added empty state and a default constructor which constructs to empty. Otherwise identical.

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- New typedefs checked_outcome<T>/checked_result<T>, checked_outcome_e<T>/checked_result_e<T> are added. These mirror the editions just described, but checks and default actions occur on all observer usage so hidden reinterpret_cast<> never occurs. Implicit conversion from non-checked varieties is permitted to checked varieties, but not the other way round. https://github.com/ned14/boost.outcome/issues/47. Examples:

``` // Note result<T> implicitly converts to checked_result<T>, but not // the other way round. So we can use same make_errored_result(). checked_result<Foo> v(make_errored_result(std::errc::invalid_argument)); assert(v.has_error()); // true assert(!v.has_value()); // true // .value() throws std::system_error( // std::make_error_code(std::errc::invalid_argument)); Foo f(std::move(v.value())); ```

``` checked_result<Foo> v(make_valued_result(Foo())); assert(!v.has_error()); // true assert(v.has_value()); // true // .error() returns a default constructed (null) error_code_extended // when result is valued to indicate "no error here" error_code_extended ec(std::move(v.error())); assert(!ec); // true ``` I will need more rationale about the need of this classes and why we need to do an action while observing them. Please, could you elaborate? checked_optional_outcome<T> equals exactly the outcome<T> in the presented library.

checked_optional_result<T> equals exactly the result<T> in the presented library.

All the discussion of the presented library to date applies to the checked_*() functions.

if error returns the stored error or a default error when valued, why we don't use another name for the function? Why do we want to change the signature and the semantic of an existing function (when I say existing, I'm referring so std::experimental::expected, or for ). Why not have always the same interface and adding an error_or function as suggested in the Expected proposal? This function could be generic as far as we have common interface. What your checked_outcome<T>::error will return if the is a exception_ptr?

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Still to be decided: ==================== - Should *_e() varieties provide convenience .get(), .get_error(), .get_exception() which returns T, error_code_extended and std::exception_ptr by value moved from internal state, resetting state afterwards to empty? These would mirror future.get()'s single shot observers. We need a valid use case to introduce them and even more as member functions. In any case, these functions can be defined on to of the provided interface, and could be non-member functions, isn't it? There is an argument to make all the observers of Expected or Outcome free functions. It certainly would fit how std::begin() etc work. So:

expected foo(T());

if(has_value(foo)) ...

T x(std::move(value(foo)));

T y(std::move(get(foo)));

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I see a difference between the minimal interface and other functions. You can not define has_value in function of other functions. You could define a wide value() function using has_value() and deref()/operartor*. If you go the future::get way, don't forget that you cannot set twice a promise. Vicente