- Boost - lists.boost.org

[outcome] Review of Outcome
by Emil Dotchevski 29 May '17

29 May '17

I. Design There is a need in C++ for dealing with failures without using exceptions. This is not so much driven by actual inefficiencies (as opposed to perceived inefficiencies) of exception handling, but by the fact that not all C++ code in existence is exception-safe, and the fact that some C++ programmers work in environments where exception handling is disabled, as ill-advised such a decision may be. In my opinion the library should drop option<T> and result<T> altogether and should be reduced to outcome<T> only, which can be empty, or hold a T, or hold an exception (but not error code, see below.) This is critical for a library that aims to provide a _common_ general mechanism for dealing with failures in interfaces; providing a rich set of alternatives works against that goal. (It seems desirable to get rid of the empty state too but the discussions during the review period show that this might cause more problems than it solves.) Secondly, it is not a good idea to use error codes or any other value as means to dispatch on different kinds of failures because semantically such dispatch should be static: there is some _code_ which detects the error, and there is some other _code_ that needs to bind it and handle it. This is one reason why in C++ catch dispatches by the static type of the exception objects, not by some exception.what() value. Further, using static types to communicate different kinds of failures allows users to recognize and handle an entire class of errors by means of implicit type conversions, by organizing error types in a hierarchy. For example: struct io_error: virtual std::runtime_error { }; struct read_error: virtual io_error { }; struct write_error: virtual io_error { }; struct file_error: virtual io_error { }; struct file_read_error: virtual file_error, virtual read_error { }; struct parse_error: virtual std::runtime_error { }; struct syntax_error: virtual parse_error { }; With this hierarchy, we can use read_error to handle any kind of read errors (not just file-related), file_error to handle any kind of file failures (read or write), etc. That said, outcome<T> should also be able to transport values, however their purpose should not be to tell _what_ went wrong, but to provide additional information. It must be stressed that such additional information should be decoupled from the classification of the error. For example: outcome<parsed_data> parse_file( char const * file_name ); The above function reads a file and returns some parsed_data -or- stores an exception object in the outcome. What are the possible failures? It may be a file_read_error, but also types unrelated to I/O like std::bad_alloc or syntax_error. Regardless of the type of the failure, the outcome object should be able to transport the file name because it is relevant no matter _what_ went wrong. Incidentally, the need to transport contextual information independently of the error classification exists when throwing exceptions as well, so if outcome<T> can hold exception objects (and allow users to dispatch on their static type, like catch does), the exception objects themselves can take care of holding any additional information, for example by using Boost Exception. II. Implementation In general the library should not shy away from integrating with common Boost facilities in an effort to appeal to people who hate Boost. A Boost library should primarily target Boost users and address their concerns. This includes the testing framework as well as boost/config.hpp where applicable (it is okay to not always include boost/config.hpp, but it should be included to get things like BOOST_NO_EXCEPTIONS and other configuration macros.) The library should use BOOST_ASSERT, though I don't see a problem with not #including boost/assert.hpp if BOOST_ASSERT is already defined (by the user). The library should use BOOST_THROW_EXCEPTION to throw. Like in the case of BOOST_ASSERT, it's fine to not #include boost/throw_exception.hpp if BOOST_THROW_EXCEPTION is already defined. That said, avoiding coupling with individual Boost submodules is desirable in general. Also there is no problem with providing facilities to avoid all Boost coupling in order to support non-Boost users. III. Documentation The documentation makes broad claims about the inefficiency of exception handling which seem to be motivated by a desire to appeal to programmers who hold such beliefs. There is no need to make such generally untrue claims in order to justify the need for Outcome. Other than that the documentation does the job. IV. Verdict Should Outcome be accepted into Boost? NO This obviously applies only to the current state of the library.

3 4

[move] BOOST_RV_REF(TypeWithoutMoveEmulation) parameters
by Groke, Paul 29 May '17

29 May '17

Hi, I'd like to use BOOST_RV_REF, with both C++98 and C++11 compilers, for some sink-functions, with types that are not "move-enabled". I.e. types that the sink can "consume"/"move from" by other means (swapping, calling .release(), ...). Some of them I can - eventually - change to be fully move-enabled, others I can not (e.g. STL classes). I just assumed that this was possible by simply passing the argument with boost::move(arg). Apparently it's not though, because boost::move will return a plain T& for types without move emulation. For the time being I have defined my own helper function template "move_any" that will always return a rv<T>&. Is there such a function that I missed? And if not, would you think it'd make sense to provide one (I certainly think it would)? Regards, Paul Groke

3 5

[outcome] On the design and documentation
by Thomas Heller 28 May '17

28 May '17

Hi Niall (and also probably Vicente) I was following the discussion about expected/outcome/result very closely and I can absolutely see the usefulness of such a library (I explicitly discarded option here). I am not sure if this post should count as a review, mainly because I disagree with the fundamental design decisions (that includes expected as defined in D0323R2) and therefor would cast my vote as in "not ready yet", I hope I can convey my points below. First of all, I don't agree with the strong (conceptual) relationship between optional (be it boost:: or std::experimental) in such a way that expected is a generalization of it. From what I understand, the purpose of expected/outcome/ result is to be used as a mechanism to return the result of a function. As such it should also expose the semantics of it. Fortunately, we already have a (asynchronous) return object standardized (std::future). And this is my basic disagreement here: Why not model expected in the lines of future? With the main points being: - expected being movable only - expected<T,E>::value() should always return by value and invalidate this. - (I would really prefer the color .get though ;)) So the question to Vicente/Niall is: what is the motivation to make it "optional-ish"? Do we have use cases which make this design undesirable? By having these constraints, expected of course needs to have an uninitialized state. As such we'd have the three observers: valid(): true when has_value() || has_error(), false otherwise (for example default constructed, invalidated), has_value() and has_error(). Second, I think Niall raised a valid about outcome being a framework for interoperability (completely orthogonal to the first point). However, I totally miss this from the proposed library, most pressing are non intrusive mechanisms. For that purpose I postulate, that a mechanism to transform between different unexpected results, that is: various error codes etc. However, for that to work, one would of course need a properly defined concept, for example, as Vicente suggested "EitherValue", and a mechanism to coerce one error type into another, maybe through ADL, or traits specialization or whatever. With that in place, one could simply define the different EitherValue types, there is no need that everything needs to be in the form of "basic_XXX". For the library under review, this would be perfectly sufficient: template <typename T, typename E> class expected; template <typename T> using result = expected<T, extended_error_info>; template <typename T> using outcome = expected<T, variant<extended_error_info, exception_ptr>>; That is, given that we have either a value or unexpected, we can convert expected<T, E1> to expected<U, E2> if T is convertible to U and E1 "coercable" (with whichever mechanism) to E2. If we then have a generic mechanism to get from a (possibly user defined "E") to an exception, I completely miss the point of the outcome template. Furthermore, I believe that .value/.error should really have a narrow contract, that is that it is UB to call those functions if the respective types are not held (easy to catch this in a debug build...). Why? Probably just a micro optimization, but consider the canonical usage: auto r = some_function_returning_expected(...); if (r.has_value()) // Do something with the value else // An error occurred, PANIC In both branches, we know exactly what's in there ... so why check again when getting the state out? I don't get the "reinterpret_cast" argument. I am one of the persons that believe that UB is a necessary evil for some optimizations... All optimizations can then easily be put as implementation details and the generic expected<T, E> will probably suffice for most use cases, for everything else, we can implement special types which conform to our concepts and implement the error conversion mechanisms. This will most likely also work with different APIs/ABIs. Last but not least, I wanted to give some feedback on the proposed Boost.Outcome documentation. At the very least, I find it highly confusing. When I open its "landing page", I get no idea about the following: - What is the purpose of the library? - Why/when should I use it? - What should I look into next? >From the front page, it looks like a implementation of expected, but only talks about the outcome template, it is very hard to find the documentation for the other three types result, outcome and option. Yet, the documentation explicitly warns about it being unstable and suggests to use its refinements. Useful information is spread across the whole documentation, there is no easy way to find the requirements on T or E. The reference section is anything but complete and doesn't convey any useful information. I did not really look at the actual implementation of the proposed library. I cloned the git repository and had a quick glance, but to be honest, I got lost very early, I have no idea where to start looking first. Cheers, Thomas

6 15

[outcome] outcome without empty state?
by Andrzej Krzemienski 28 May '17

28 May '17

Niall, What do you think about the following alternate interface of outcome types? 1. Remove option<T> altogether. 2. result<T> (and outcome<T>) does not have a separate empty state. 3. A default-constructed result<T> is initialized as if `result<T>{error_code_extended{}}`. 4. Function r.empty() is implemented as `r.has_error() && !error()`. 5. Framework requires that type E in `expected<T, E>` is default-constructible, and its default-constructed value represents no error condition (already the case for exception_ptr, error_code). The rationale behind this is my assumption that `option<T>` is not as useful as the other types. (Or can you give an example to the contrary?) That there was no initial business need for an empty state, and the examples that illustrate its usefulness are just exploring how we can make use of it now that it is there. (Or am I wrong here?) I noticed a number of people report negative feedback about this empty state. Getting rid of it would take result<T> closer to "either T or reason for failure". What do you think? Regards, &rzej;

6 27

[review] Review of Outcome (starts Fri-19-May)
by charleyb123 . 28 May '17

28 May '17

Hi Everyone, The formal review of Niall Douglas' Outcome library starts today (Fri-19-May) and continues until Sun-28-May. Your participation is encouraged, as the proposed library is uncoupled and focused, and reviewers don't need to be domain experts to appreciate the potential usefulness of the library and to propose improvements. Everyone needs (and has suffered) error handling, and can compose an opinion on that topic. Outcome is a header-only C++14 library providing expressive and type-safe ultra-lightweight error handling, suitable for low-latency code bases. Key features: *- Makes using std::error_code from C++11's <system_error> more convenient and safe *- Provides high-quality implementation of proposed std::expected<T,E> (on C++20 standards track) *- Good focus on low-latency (with tests and benchmarks) *- Error-handling algorithmic composition with-or-without C++ exceptions enabled *- No dependencies (not even on Boost) This review is timely, as C++17 brings us std::optional<T>. The upcoming std::expected<T,E> (an implementation provided in Outcome) is a generalization of std::optional<T> that provides a <success|fail> value, where the unhappy result is a 'std::error_code' or an instance of "your-chosen-error-type". The library further provides 'outcome<T,error-code,exception-ptr>' for handling <success|error|exception> to safely wrap throwing APIs. Documentation: https://ned14.github.io/boost.outcome/index.html ACCU 2017 talk including design rationale: https://www.youtube.com/watch?v=XVofgKH-uu4 GitHub: https://github.com/ned14/boost.outcome Latest tarball: https://github.com/ned14/boost.outcome/releases/download/ boost_peer_review3/boost.outcome-v1.0-source-201705111650.tar.xz Note: Tarball might be easiest; but if you want to clone from GitHub directly, after the clone you should run the following command to get the source zip exactly: git submodule update --init --recursive Please post your comments and review to the boost mailing list (preferably), or privately to the Review Manager (to me ;-). Here are some questions you might want to answer in your review: - What is your evaluation of the design? - What is your evaluation of the implementation? - What is your evaluation of the documentation? - What is your evaluation of the potential usefulness of the library? - Did you try to use the library? With what compiler? Did you have any problems? - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? - Are you knowledgeable about the problem domain? And most importantly: - Do you think the library should be accepted as a Boost library? For more information about Boost Formal Review Process, see: http://www.boost.org/community/reviews.html Thank you very much for your time and efforts. --charley

17 70

[review] Review of Outcome (ends Sun-28-May)
by charleyb123 . 27 May '17

27 May '17

Hi, Everyone, First a (very-)big "thank-you!" to all participating in the ongoing (and vigorous) debate and review for the Outcome library. The spirited discussion touches on tricky issues for composition and error handling (with and without C++ exceptions enabled), where the community is clearly searching for best convention and common ground. Thus far: (a)- some 300+ emails discussing Outcome (and more emails off-list) (b)- participation from: *- Andrzej Krzemienski *- D25fe0be *- Deniz Bahadir *- Emil Dotchevski *- Gavin Lambert *- Glen Fernandes *- Gottlob Frege (Tony) *- Hartmut Kaiser *- Ion Gaztanaga *- Jonathan Muller *- Niall Douglas *- Paul Bristow *- Pete Bartlett *- Peter Dimov *- Robert Ramey *- Thomas Heller *- Vicente J. Botet Escriba *- Vinnie Falco *- ...(apologies if I've missed anyone) (c)- Some points-of-discussion relate to: *- Outcome efficiency (copy/move) on today’s compilers *- Outcome speed/overhead (exceptions) *- Outcome purpose/motivation *- Outcome Tutorials, documentation *- Outcome “formal-empty-state”, default-initialization *- Outcome compiling, compiler support *- Outcome ABI, namespace usage, use of preprocessor *- Outcome alternative APIs *- std::expected proposal, possible changes (d)- Reviews to date (sent publicly to the list): *- Paul Bristow -- accept, conditional (Tue-23-May) *- Deniz Bahadir -- accept, unconditional (Wed-24-May) *- Thomas Heller -- (almost a review), ?reject, "not-ready-yet?" (Wed-24-May) (e)- Significant other discussion also contributes to evaluation of Outcome as a Boost library. However, I encourage further reviews to make clear any conclusions from these discussions that I might have missed. The review continues for several more days, ending Sun-28-May. Please consider posting a review to the boost mailing list, or privately to the Review Manager (to me). Here are some questions you might want to answer in your review: - What is your evaluation of the design? - What is your evaluation of the implementation? - What is your evaluation of the documentation? - What is your evaluation of the potential usefulness of the library? - Did you try to use the library? With what compiler? Did you have any problems? - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? - Are you knowledgeable about the problem domain? And most importantly: - Do you think the library should be accepted as a Boost library? For more information about Boost Formal Review Process, see: http://www.boost.org/community/reviews.html Thank you very much for your time and efforts. --charley

8 16

Detail headers moved
by Belcourt, Kenneth 26 May '17

26 May '17

Hi, I’m trying to build Boost.Graph and running into missing header files that used to live in boost/detail and now live elsewhere. Here’s a couple of examples of headers that moved and the code that still expects to find them there: detail/container_fwd.hpp libs/functional/include/boost/functional/hash/extensions.hpp: #include <boost/detail/container_fwd.hpp> libs/lambda/include/boost/lambda/detail/operator_return_type_traits.hpp: #include "boost/detail/container_fwd.hpp" libs/phoenix/include/boost/phoenix/stl/algorithm/detail/is_std_list.hpp: #include <boost/detail/container_fwd.hpp> libs/phoenix/include/boost/phoenix/stl/algorithm/detail/is_std_map.hpp: #include <boost/detail/container_fwd.hpp> libs/phoenix/include/boost/phoenix/stl/algorithm/detail/is_std_set.hpp: #include <boost/detail/container_fwd.hpp> detail/indirect_traits.hpp libs/iterator/include/boost/iterator/detail/facade_iterator_category.hpp: #include <boost/detail/indirect_traits.hpp> libs/iterator/include/boost/iterator/indirect_iterator.hpp: #include <boost/detail/indirect_traits.hpp> It seems that indirect_traits.hpp was moved from detail into python/detail, and container_fwd.hpp into container. Just curious but was it intentional to make many Boost libraries depend on Python through boost/iterator/iterator_facade.hpp? Noel

2 4

[outcome] some problems compiling
by Andrzej Krzemienski 26 May '17

26 May '17

So, I am trying to run my first test program with Boost.Outcome. The library fails to compile with GCC 6.3.0 on MinGW on Windows 7. I have filed the issue: https://github.com/ned14/boost.outcome/issues/36 But also, I can see in file <boost/outcome/outcome_v1.0.hpp>, lines 1267 to 1279 ``` #ifdef __cplusplus namespace { #endif typedef struct _IMAGEHLP_LINE64 { unsigned long SizeOfStruct; void *Key; unsigned long LineNumber; wchar_t *FileName; unsigned long long int Address; } IMAGEHLP_LINE64, *PIMAGEHLP_LINE64; ``` 1. Why do you check for __cplusplus? Is it a copy-and-paste of some Windows headers? 2. This class and subsequent functions are defined in anonymous namespace. This means they are redefined and recompiled in each translation unit, and remain a per TU definition. Is that intent? Regards, &rzej;

5 11

Re: [boost] [outcome] Possible extensions/changes to std::experimental::expected
by Andrzej Krzemienski 26 May '17

26 May '17

It looks like part of the discussion accidentally went into a private conversation. So, here we are back. 2017-05-25 22:39 GMT+02:00 Vicente J. Botet Escriba < vicente.botet(a)wanadoo.fr>: > Le 25/05/2017 à 18:40, Andrzej Krzemienski a écrit : > > > > 2017-05-25 17:32 GMT+02:00 Vicente J. Botet Escriba < > vicente.botet(a)wanadoo.fr>: > >> Le 25/05/2017 à 13:18, Andrzej Krzemienski via Boost a écrit : >> >>> 2017-05-25 9:44 GMT+02:00 Vicente J. Botet Escriba via Boost < >>> boost(a)lists.boost.org>: >>> >>> We have now the possibility for uninitialized variables, but static >>> analysis tools will help here. >>> >>> So, more specifically, I understand that you propose the following: >>> >>> - Default constructor works: no T or no E is construced (similar what >>> outcome<> does) >>> >> I was not aware of this. Do you know that from this reference >> documentation? >> >> constexpr outcome () noexcept(is_nothrow_default_constructible) >> Default constructor. >> > > Well, I just assumed that since outcome<> has this empty state, it is > exactly for the purpose of giving semantics to the default constructor. But > now that you have asked, I looked at the docs. They say nothing about what > happens in the default constructor. I tried to look in the code, but was > confused with these policies. So, I just run a small test program, and yes: > default constructor creates an empty state. > > Niall, please could you add an issue in order to document default > constructors (if it doesn't already exists). > This is already logged in https://github.com/ned14/boost.outcome/issues/26 > > >> >> Sorrty, I couldn't interpret it as you. >> But, if it is the case, yes, as outcome does. >> >>> - You can assing to and and destroy such an objetc (similar what >>> outcome<> does) >>> >> I don't catch what you mean here. Are you referring to the conversion >> between expected with convertible value type and error types? >> > > No, I am saying that the following should work: > outcome<T> o; // default-constructed > o = some_outcome<T>(); // this should be safe to execute > > This works already with expected even if it doesn't defaults to empty. > Ok, and you expect it to continue to work. > > >> - You will probably need to add an observer function that checks for >>> this singular state, like `is_singular()`. If not for anything else >>> it >>> would be used for assisting the static analysis tools. (again, >>> similar what >>> outcome<> does) >>> >> No. There will be no such observer. This is essential. There is no >> visible empty state. >> Or do you consider that chrono::duration should tell you if it was >> initialized or not? >> The user know if it is initialized or not, but there is no tool to check >> it. >> This could be against safe programming, but we are here in C++ and we >> should provide first the raw tools and then build on top of them when we >> want more. >> >>> - other observer functions (has_value(), value(), has_error(), >>> error()) >>> cause UB when `is_singular() == true. (this is the only difference >>> from >>> outcome<>) >>> >> Not the only one. >> outcome fixes its error types. I'm proposing an extension to the current >> expected so that it can take care of the Outcome use cases and needs. >> >>> >>> Did I understand your intentions correctly? >>> >> Not completly. I don't want an observer that tells me, attention you have >> not initialized your variable, or you have moved from. This is essential. > > > Very daring. It would be easier to accept if you could show a tool that is > capable of statically analying this kind of "used unintialized" bug for > `expected`. > > > Static analysis to check if uninitialized variables are used (read) works > very well already . I don't remember if it si with clang-tidy that I have > seen this kind of warnings. > Ok. Now I am thinking, another thing you can do, is to internally store something like varient<T, Y, Empty> but do not expose this Empty in the interface. Instead, rewrite observer functions like this: ``` bool has_value() const { if (BOOST_UNLIKELY(_is_in_empty_state())) __builtin_unreachable(); return _is_in_valued_state; } ```

3 12

[outcome] expected<T&>, result<T&>
by Andrzej Krzemienski 25 May '17

25 May '17

Niall, Are Boost.Outcome monads intended to work with references (at least lvalue references)? If I try to compile the following program that tries to use an `result<T&>` (on GCC 6.3.1): ``` #include <boost/outcome.hpp> namespace out = BOOST_OUTCOME_V1_NAMESPACE; int g_count = 0; out::result<int&> get_ref() { return out::result<int&>(g_count); } int main () {} ``` I get lots of compiler errors. But the messages ("constructor cannot be overloaded") indicate that it was just an omission and not a design decision. If I try it with `outcome::expected` I also get these messages, but I also get a static assert saying that as per LWG decision T must be default constructible. Vicente, does this mean you cannot use `std::expected` to return references? Regards, &rzej;

5 8