[boost] [review] Boost.Outcome.v2 Review Report

Subject: [boost] [review] Boost.Outcome.v2 Review Report
From: charleyb123 . (charleyb123_at_[hidden])
Date: 2018-02-05 02:16:47

# Boost.Outcome.v2 REVIEW REPORT

In conclusion to the review for Boost.Outcome(v2) (19-Jan to 28-Jan, 2018):
 The library is ACCEPTED (conditions below).

Acceptance is principally based on:

  (1) Reviewers found the library useful to address a current recognized
need
  (2) New idioms are enabled that reviewers found compelling
  (3) The submission satisfies all Boost Library Requirements

Concerns over accepting the library raised in the review discussion include:

  (a) New idioms proposed are not compelling
  (b) Requires C++14 (prefer compatibility with C++11)
  (c) Library may evolve beyond the current design
  (d) Library is unnecessarily complex

Reviews submitted during the review period:

  *- Vinícius dos Santos Oliveira -- ACCEPT (Fri-26-Jan-2018)
  *- Andrzej Krzemienski -- ACCEPT (CONDITIONAL) (Fri-26-Jan-2018)
  *- Bjorn Reese -- ACCEPT (CONDITIONAL) (Sun-28-Jan-2018)
  *- Daniela Engart -- ACCEPT (Sun-28-Jan-2018)
  *- John P Fletcher -- ACCEPT (CONDITIONAL) (Sun-28-Jan-2018)

In addition, after the review period concluded (but within the duration had
the review period been extended) further reviews were submitted:

  *- Vinnie Falco -- REJECT (Tue-30-Jan-2018)
  *- Emil Dotchevski -- REJECT (Tue-30-Jan-2018)
  *- Glen Fernandes -- REJECT (Tue-30-Jan-2018)
  *- Paul A Bristow -- WOULD ACCEPT (?not a review?) (Wed-31-Jan-2018)
  *- Rob Stewart -- NOT ACCEPT (Sat-03-Feb-2018)

Conditions for acceptance:  It is expected:

  *- Changes are made to use Boost-standard names for macros and namespaces
  *- Docs are updated to be clear regarding:
      *- how library treats default-constructed values
      *- salient attributes of objects, and how “spare_storage” is treated
  *- Documentation is integrated into Boost framework and release process
  *- Library is distributed under the Boost Software License (BSL)

The remainder of this report contains context and analysis contributing to
this decision, including basis for why acceptance purports to be
constructive and beneficial to the Boost community, and broader C++
community.

# MOTIVATION:  REAL-WORLD USE TODAY

The prime motivation for acceptance is:

  *- Reviewers have real-world use cases _today_ for which they found
Outcome to be
     an effective and best available alternative; and which is consistent
with current-need
     and expectations; and which is consistent with ongoing C++ Standard
evolution efforts.

>From the Library Author:
  > <quote>
  > “Outcome is really an abstraction layer for setting per-namespace
  > rules for when to throw exceptions. Exception throwing is absolutely at
  > the heart of Outcome. That's why Outcome != Expected, and why it ICEs
  > older compilers, and why C++ 14 is needed.”

Exhibited behavior is two-fold:

  (1) Expected errors are handled locally with low overhead (e.g.,
deterministic / predictable with low-latency)

  (2) Unexpected errors are type-erased into an 'exception_ptr' and pushed
up the call stack (e.g., exception-throw stack-unwind)

For example, server-side code that is expected to handle a lot of failures,
where “stop-the-world” is never suitable, may always handle errors
locally.  In contrast, most other system-wide code may 'throw' an error,
which should be handled within some caller-context.

# AT ISSUE:  DESIGN AND IDIOMS

What appears to be debated in this review are the Outcome library design
and idioms; and not the quality of implementation (although the
implementation is criticized as complex -- see below).  For example, even
reviewers that voted to reject commented that the library seems sound, and
seems useful for some cases.

This merits repeating:  This review has the highly positive characteristic
(despite the accompanying discomfort) of debating the idioms that challenge
what already exists, and which forces re-evaluation of our historical
approaches.

Many of these discussions might otherwise be summarized (and which has a
fair chance of being agreed upon by all parties) as:

  *- Error handling can be done with exceptions, or with branch-testing on
error instances;
      and some algorithms or constraints may favor one over the other for
technical or
      compositional reasons.

It is this highly pragmatic observation that is at the core of the Outcome
library submission.

Outcome enables a new idiom consistent with other pattern explorations
deemed by the C++ community as useful, as demonstrated through acceptance
into the C++ standard for 'std::optional<>' and the pending
'std::expected<>'.  Outcome enables value-transport of a multi-modal data
object '<T|error|exception>' across API boundaries that provides the
benefit of static compile-time type-checking, but places the explicit
burden upon the user for 'visit / interrogation' of that multi-modal data
object.

Outcome is intended for use in program areas with harsh resource
constraints, where many inconveniences are expected, and tradeoffs are part
of the design decision for creating and handling explicit 'out' values that
include failure information.  These constraints may include:  interaction
among one-or-more modules compiled without exception-handling enabled;
deterministic or low-latency execution requirements; and use of custom
'error' and/or 'exception' objects that are expected to be populated and
tested within the local context (such as to perform explicit conditional
branching for error handling or recovery).

During the review there was some confusion regarding evaluating Outcome in
the context of “generalized use” across an entire codebase, versus
“localized use” where the algorithm intends to machine specific cases for
statically-bound stateful handling of errors.

For example, it is not expected that Outcome might be used across all APIs
across all modules within a system.  (Recall that current C++ community
guidance might otherwise suggest 'std::error_code' or 'exception-throw'
might be used across all APIs for all modules within a system, if such
consistency is permitted.)  Rather, Outcome intends to address that nexus
where an algorithm interacts with several modules, each with differing
'error' and 'exception' practices, where the library consistently transfers
'<T|error|exception>' with compile-time type enforcement and runtime
behavior and performance guarantees.

Design decisions within Outcome v2 are well-considered, and faithfully
reflect feedback raised during the (quite extensive) v1 review.  Indeed,
none of the issues raised during the v1 review were again raised within the
v2 review.  And, it is noted that two reviewers previously voting REJECT on
v1 now vote ACCEPT on v2.

Further, it should be noted that Outcome v2 has evolved into a
collaborative effort with assistance from several contributors, with
validation and feedback in multiple real-world codebases with specific
engineering constraints.

In this review, reviewers found the idioms specifically useful (beyond
alternatively available tools / idioms),  and consistent with practices in
existing codebases and in other languages.  Further, the proposed direction
appears to be consistent with evolving idiomatic practice within the C++
Standard enabling new idiomatic algorithm expression, as evidenced by
multi-mode types such as 'std::optional<>' and the (proposed and evolving)
'std::expected<>'.

Idioms provided through Outcome are also demonstrated to be useful /
successful in other languages.  As noted in the review, Outcome enables
handling of failure with idioms similar to those found in Rust, a language
which also supports RAII but which does not support exceptions (and
reviewers commented that these idioms are sufficiently successful that
exceptions are not viewed as an interesting nor desired mechanism in Rust).

One reviewer commented that it is not compelling to attempt to replicate
Rust error handling idioms, which (as a different language) naturally
offers different idioms.  And instead, offered that the natural idiom in
C++ is to throw (and catch) exceptions.  However, it can be noted that the
Outcome design approach is somewhat agnostic to the use of 'exceptions',
but rather relies upon a multi-modal data object that the user must
explicitly unpack (similar to 'std::optional<>' or 'std::expected<>'), and
which makes explicit guarantees about unified transport and handling of
'<error|exception>'.

The core of the dispute seems to be over the value of localized reasoning
for handling errors:  Exceptions are good for “distant” handling (to
seamlessly transfer handling to a parent context), while explicit
error/result instances (possibly customized) encourage local reasoning for
discrete handling within the local algorithm context.  Complicating the
issue is that (of course) both are accepted practice in differing
environments and with differing engineering constraints:  Throwing
exceptions may be most expressive to avoid algorithmic edge cases or ensure
errors are actually handled; but alternatively it may be more expressive to
explicitly handle errors locally when it is not desirable to delegate the
failure to an upper level.

# DISAGREEMENT:  ERROR / EXCEPTION HANDLING

Even prior to considering the Outcome library submission, error handling
idioms and practices remain contentious and confusing within the C++
community (as well as within the Boost community).  Recent threads on the
Boost email-list continue to highlight the ongoing confusion and
disagreement even over the proper or idiomatic use of 'std::error_code',
which boasts a decade of real-world experience and approaches a decade in
the C++standard.  One might think such discussions should by now be
resolved; but no, we observe that even semantic agreement of
proper-or-intended behavior for 'std::error_code' was not reached on these
recent Boost email discussions.

We also observe that discussion regarding 'error' or 'exception' handling
can at times be clouded within the evolving C++ Standard itself, with
discussions of 'throw'/'no-throw' (e.g., “dual”) APIs, semantics for 'wide'
or 'narrow' contracts, and even the occasional (and “over-simplified”)
discussion of “for vs. against” exception handling (a topic that alone is
worthy of consuming a surprising number of evenings and libations).

The Library Author can be congratulated (or scolded) for exploring work or
attempting Boost community review in such a contentious space.

However, these topics are also fundamental motivations in the creation of
Outcome itself:  It provides a mechanism to unify 'error' _and_ 'exception'
handling to enable unified localized reasoning across third-party modules;
and directly exposes the resulting issues for cross-module error handling
by charging the user with explicit unpacking of the '<T|error|exception>'
object, but with the benefit of static binding (e.g., compile-time type
checking).

The implication is that in such a library review attempting to address a
portion of the C++ landscape already marked with disagreement regarding
current-and-alternative approaches, we cannot expect but to see charged
commentary where issues of “global reasoning” over a “generalized use case”
were inevitably raised to sometimes overshadow the discussion of a library
that specifically intends to address “localized reasoning” with specific
performance constraints and behavior guarantees.  (Although compared to the
v1 review, we might consider some threads to have missed opportunity to be
yet more colorful.)

The beclouded discussion appeared at times to exhibit violent agreement:

Quoting reviewer (voting to reject):
  > However it looks like Outcome provides a solution for most of the
practical
  > cases, while leaving the general case unsolved. Boost.Outcome is a set
of
  > tools (rather than just one) and you are expected to choose one that
best
  > solves your particular problem.

Quoting response by library author:
  > I was just about to say the same thing, but more pointed.  <...>
Knowing that
  > a piece of code will never, ever see stack unwinding lets you skip
handling
  > stack unwinding. Hence `noexcept`.
  >
  > Furthermore, unlike with exception specifications which were
unhelpfully checked
  > at runtime, Outcome fails at compile time. .... You can't successfully
compile code
  > if your E types don't have interop specified for them.
  >
  > Outcome is of particular use in a lower-level layer of code (closer to
bare metal,
  > or in server contexts), where explicit deterministic error handling
warrants
  > increased tedium or inconvenience in explicit checks for
success/failure.

An observer might note that the reviewer (voting to reject) and library
author have a shared view of what the library proposes to do; but the
reviewer desires a generalized solution, rather than an interop-solution
that provides specific performance constraints and behavioral guarantees.
They agree on what it does.  They disagree regarding the value and
likelihood of a future possible generalized solution to perform a similar
function, which is not currently proposed for review.

The (perhaps unstated) concept is that no generalized solution may be
possible with today's language, or perhaps ever:  Both 'error' objects and
'exception' throws fundamentally serve different use cases:

  (a) 'error / status' instance (i.e., “opt-in”):  A discrete object that
can be inspected (or ignored), such as to perform conditional processing

  (b) 'exception' throw (i.e., “opt-out”):  A control transfer-to-caller
that avoids accidental instruction execution (e.g., stack-unwind) and which
cannot be ignored

Proponents of (a) talk of contract and execution simplicity, and of
determinism / efficiency.  Proponents of (b) talk of composition
simplicity, and avoidance of edge cases due to liberation from local tedium.

A further complication is due to how arbitrary (implementation-specific)
data is returned for failure handling:  It can be encoded into the type
system (increasing coupling across APIs), or may be type-erased (such as
done by 'std::error_code').  Noted in the review is that 'exception' throws
are a special case of type erasure, as the C++ runtime performs the type
erasure without impacting the declared API (thus providing a very large
part of the convenience for using exceptions).

The Outcome library attempts to unify '<error|exception>' handling for
localized reasoning, and (re-)throw only in specific well-defined
contexts.  It is not a generalized pattern, but a unified mechanism for
discrete handling of those scenarios where the design choice is explicitly
made to perform localized reasoning of failure from dependence upon
heterogeneous modules that may perform surprising (and evolving) behavioral
changes for failure-notification.

A generalized solution intends to provide uniformly simpler code, or
generalized idioms.  This might not (ever) be possible when we explicitly
talk about localized reasoning of error handling within the local context:
 By definition, we want to enumerate and handle discrete failure cases
specific to the local algorithm; so the best we can do is to utilize
(compile-time) type-checks that verify our unwrapping-and-interrogation of
our '<T|error|exception>' instance that bubbles up from within some
far-away dependency that chooses the most inopportune time to exhibit
surprising changes in behavior.  Also by definition, localized reasoning
requires localized tedium to _handle_ specific error cases, which are also
enumerated locally.  In such cases, a generalized solution does not apply.

Much of the concern over Outcome acceptance appears to be based on concerns
of the effects of the library on the ecosystem.  It is true that users
might wrongly apply localized reasoning tools to (widespread) generalized
use.  However, that same issue already exists with the C++ community's
dichotomous split between 'error' or 'exception' handling, and with our
Groundhog-Day revisiting of the same irreconcilable patterns that bring us
the “dual-APIs” for “throw/no-throw” in the C++ Networking TS and
Filesystem TS (and presumedly, many more Standard libraries to come).

Of particular note is the assertion raised during the review that:

  *- The error being exceptional or not depends upon context, and not the
algorithm.

For example, connection failure on a game client and a game server are both
backed by the same function; but Outcome permits the context to convert an
'error' into an 'exception', or not (based on caller-context).

To quote one reviewer (voting to accept):
  > Boost.Outcome is not to “replace exception handling in your programs”:
 It is
  > used to cover those isolated places where exception handling proves
inferior
  > to manual control flows.

We might now expand our Matrix Of Confusion for preferring 'error' or
'exception' to include:

  (a) “Generalized-pattern” vs. “Localized-reasoning”
  (b) Expected vs. Non-Expected failure
  (c) Dependency upon subsystem providing 'error' vs. subsystem providing
'exception'

While technically a “matrix”, commonly this is (quite) multi-dimensional:
 Frequently we depend upon many subsystems, each of which make very
different decisions for how they relay disappointment, and where each
individual subsystem will *change* that decision in surprising ways merely
when we perform a version update.  This is the brittleness that Outcome
intends to address.

Outcome is merely a mechanism to enable 3rd party module inter-operation.
It does not make the decision for whether 'error' or 'exception' instances
are provided by some subsystem, nor does it care.  Rather, it is a
unification mechanism that enables an algorithm to be _authored_ with
specific performance and behavioral guarantees when reliance is upon
one-or-more modules that _made_ that 'error' vs. 'exception' decision in a
manner your specific use case finds unfortunate.

Lastly, reviewers most critical of Outcome point to 'exceptions' as the C++
language mechanism to be leveraged in design, such as to enforce strong
guarantees of object invariants (e.g., RAII permits ctors to throw to
ensure invariants are not violated), and thus the Outcome library is not
needed.  However, despite this language feature, alternative idioms such as
private ctors and 'make_xxx()' factory functions are not uncommon to also
enable successful instantiation with internal state upholding invariant
values; and we again must concede that modules employ differing decisions
to compile with exception handling enabled.  Thus, it seems reasonable that
a library such as Outcome might exist to present a unifying interface
across 3rd party code with differing design decisions, rather than hope for
a simpler world where a single (exception-based) approach is mandated
system-wide across modules.

# CONCERN:  CUSTOMIZATION POINTS (COMPLEXITY)

Outcome attempts to bridge vocabulary types for '<error|exception>'
handling among subsystems, including:  C++ with or without exceptions
enabled, C subsystems, and modules providing customized errors and/or
customized exceptions.  These goals necessarily complicate the design
beyond a simple 'variant<>' value-type with no such customization points.

This complexity was commented on by reviewers that voted to reject,
suggesting:  (1) Complexity is too high for the features provided; (2)
Additional complexity to support C compatibility is unnecessary; and (3)
Complexity is unnecessary if the library required subsystems not using
exceptions to be wrapped behind a C-style API and to compile only those
wrapped-subsystems with exception handling disabled.  Further, other
indirect comments suggested:  (4) Complexity is lessened if Outcome assumed
use of only 'std::error_code' (not user-customized error instances), and
the type-erased payload provided through an exception (e.g., 'throw').

In response, the Outcome library premise is that a simpler world does not
exist (where such customization is unnecessary):  Despite nearly a decade
of experience, the C++ community has yet to adopt 'std::error_code' as the
ubiquitous mechanism to identify error (where instead bespoke error types
remain in widespread use); and even in modern C++ codebases, it is not
uncommon to compile with exceptions disabled.  A heterogeneous landscape
exists, and likely will continue to exist.

For both practical and technical reasons, it is expected that
domain-specific customization for error and exception handling will
continue to be required, such as to address engineering constraints and
needs for 'constexpr'; for small-embedded environments with severe
restrictions on memory allocation; and for context-specific payload
transferred or accumulated into journals / logs as a part of error
handling.  It is even expected that the C++ Standard itself will adapt to
its own evolving idioms exposed through new language features, and (future)
standard types that we might assume will be consistently applied across
standard library APIs.

This underscores the need for such customization points, where proposals
such as Outcome attempt to provide an effective mechanism within a rather
bleak current landscape with guaranteed future changes.

In the face of this (currently-seen and continually expected) evolution, we
have today's practical real-world issues:  Commonly today's systems are
composed of 3rd party modules that must interoperate in a well-behaved and
deterministic manner, where '<error|exception>' customization becomes a
system requirement.

Of particular note is that in large codebase environments, current practice
is to adopt customization policies to impose consistent and bespoke rules
across the entire codebase, and across modules.  Prior to Outcome, this
customization is often done with preprocessor macros.  In these
environments, Outcome is viewed as a positive evolution.

Today's Outcome customization points exist “out-of-the-box” to be
compatible with existing module-specific 'error' or 'exception' needs, and
to provide API/source-compatibility with a possible future-compatible
'std2::error_code'.

Reviewers critical of the provided customization points want type-erased
error state, but the library author asserts that this is hard to do in a
manner that is (1) efficient, and (2) permits evolution (such as to support
user-specific error values, or a possible future 'std2::error_code').

Further, the library author asserts that the library is not in fact
complex; but that it can be overwhelming to the uninitiated merely because
it enables customization of '<E>', and customization enables possibilities
(which by definition implies complexity); and that such customization is
characteristic of most (all?) vocabulary libraries.

Despite concerns over complexity, v2 is greatly simplified over v1 where
changes were directed through v1 review feedback; and provides a
single-header version with reasonable compile-time overhead.  Using
(integrating) the library should not be difficult, and explicit efforts to
establish ABI stability should make extended use over evolving codebases
possible, and not-hard.

Because enabling cross-module '<error|exception>' handling is fundamental
to the library, it is conceivable that the library offers the minimal
interface with the minimal possible complexity to do this job.  And, this
is a job that is (1) needed by real-world users; and (2) unavailable
through alternative mechanisms (and which would not be provided through a
simplified value-type composed of 'variant<T,error,exception>').

>From the library author:
  > As the last section of the tutorial covers, there is a
non-source-intrusive mechanism
  > for externally specifying interoperation rules to handle libraries
using one policy
  > interoperating with libraries with different policies. This lets Eve
stitch together the
  > Alice and Bob libraries without having to modify their source code, and
without
  > affecting any other libraries. I personally think this Outcome's coup
de grace and
  > why it's the only scalable choice for large programs considering using
this sort of
  > error handling.

In this context where Outcome may possibly exhibit minimal sufficient
complexity to address its intended target of cross-module
'<error|exception>' transport, one might recall the Fred Brooks quote:

  > “The complexity of software is an essential property, not an accidental
one.
  > Hence, descriptions of a software entity that abstract away its
complexity
  > often abstracts away its essence.”  -- Fred Brooks, “No Silver Bullet”
(1986)

# DISAGREEMENT:  REQUIRES C++14 (versus C++11)

Outcome requires C++14 and is identified as failing on some toolchains
exhibiting non-conforming C++ Standard behavior.  The review raised
concerns that requiring C++14 (rather than C++11) would limit the library's
suitability for Boost inclusion.

In this context, it might be suggested that an error variant is needed by
everyone, so we might reject a C++14 implementation in the hope that a
future C++11 compatible library will be proposed.

It is noted that Outcome v1 supported old compilers back to clang 3.1 and
gcc 4.9, and had many workarounds to suppress non-conforming compiler
behavior; and used preprocessor metaprogramming to work around compile-time
selected CRTP (because of compile-time costs, and issues on older
compilers).  However, these approaches were rejected in the v1 review as
too complex; and reviewers were not persuaded by concerns raised by the
library author that their removal would demand dropping compatibility with
older toolchains.  Outcome v2 removed these workarounds based on v1
feedback, resulting in a requirement for newer toolchains.

Similar discussions have been raised in other contexts regarding Boost
distribution packaging, and the possibility of forking “pre/post” C++11
(e.g., “modern C++”) libraries into separate Boost distributions.  This is
an unfortunately complex issue, as Boost already contains libraries
supporting varying levels of “minimum” requirements including varying
support for compilers exhibiting non-conforming behavior for C++98, C++03,
C++11, C++14, and C++17 (and others).  Some libraries such as Boost.Hana or
those reliant upon heavy 'constexpr' behavior demand the very-latest C++17
toolchains.

Further, it is recognized that in some cases backward-compatibility can be
undesirable, or fundamentally limiting to the library's usefulness.  For
example, at a recent 'BoostCon' / 'C++Now' conference an attempt was made
to evolve the 'Boost.Date_Time' API to use the seemingly highly suitable
C++17 'structured binding' declaration to decompose 'date / time' objects.

Unfortunately, this work was abandoned:  Due to use by other Boost
libraries restricted to a previous C++ Standard, and the discovery that
'Boost.Date_Time' library semantics fundamentally changed when using this
new language feature, it was concluded to not be feasible to support both
the legacy API and new API using structured bindings.  It appears in this
case a new library must be authored without consideration of
backward-compatibility to enable this evolution for what appears to be an
otherwise obvious or natural expressiveness to decompose 'date / time'
objects using 'structured binding' declarations.

It is noted that C++11 is already eclipsed by C++14 and C++17, and that
current Boost library requirements merely require C++ Standard conforming
behavior, whereby the library must clearly document those platforms that
are supported.  Further, it is noted that a v1 implementation supporting
older tool chains was rejected (so v2 now requires C++14).  As this is a
complex issue that demands consideration of many tradeoffs (including
feasibility and behavior fundamental to the library itself), it is expected
that the Library Maintainer constantly review and evolve these decisions as
the C++ language evolves; toolchains are updated; and users raise issues or
provide contributions.

# CONCERN:  LIBRARY FUTURE EVOLUTION

A concern is raised that upon acceptance into Boost, Outcome may evolve
beyond the design reviewed for acceptance.  Further, a specific concern was
raised regarding Outcome exploring evolution of an 'std::error_code' (which
we might call a proposed 'std2::error_code').

Within the context of Outcome, the 'error' type exists as a mere
customization point by which the user may supply a bespoke definition, or
incidentally use the 'std::error_code' provided by the C++11 Standard.
Both of these are seen today in common practice.  It is expected that users
(and perhaps the library author) will continue to explore possible 'error'
implementations for specific purposes (such as exploration of an 'error'
for small-embedded that does not rely upon allocation machinery).  This is
viewed as a necessary and healthy advancement of the science, for which the
library (by design) conveniently empowers the end-user to parameterize
domain-specific 'error' types into 'result<>' or 'outcome<>'.

Indeed, it is hoped (and expected) that a greater amount of
user-experimentation or flirtation with 'error' value transfer will be
performed (not less) in the context of user-specific needs or engineering
constraints:  This is a fundamental benefit from having Outcome as a design
option for authoring interfaces across module boundaries.

Regarding a possible future “drift” from its clearly-stated mission, this
review considers the library as submitted; and defers to the Boost
community regarding policies and procedures for handling libraries included
in the Boost distribution (which exist in varying states of evolution and
maintenance).

# FINAL THOUGHTS

The Boost community is stronger for enabling and exploring idioms, and for
not shying away from the difficult (and sometimes contentious) effort to
discover new approaches that address the dark corners that stress our
real-world systems.  These are (perhaps) the only noble efforts that
someday may lead to new best practices.

If experience (otherwise known as, “painful memory of limitation”) is a
prime driver for the design and implementation choices made by developers,
then it is unreasonable in a review such as this to expect agreement on all
fronts.  Indeed, our systems have differing constraints and evolutionary /
scaling prospects, and we fear different things.  However, it seems
important to keep in mind that this disagreement continues to be necessary
and expected:  Rather than pretending to hide within an echo chamber, the
Boost Community does the “hard work” of challenging perspectives, pushing
the envelope, and questioning assertions in the face of a constantly
shifting technological landscape and evolving C++ language.  Dissenting or
critical reviews are essential, and desired.

The long Boost history includes many examples of speculative and risky
approaches, which in hindsight are now considered common and "best"
practice.  Few C++ developers today can practice professionally without at
least passing knowledge of template-metaprogramming, which used to be an
esoteric ritual only within the confines of Boost.

Outcome was designed under the proposition that cross-module 'error' /
'exception' handling in today's systems is unnecessarily brittle and
problematic.  Reviewers found Outcome to effectively address a serious
concern of transporting different '<error|exception>' instances across
modules.  It attempts to solve a hard problem, which spans 3rd-party module
composition, while adhering to specific performance constraints and
behavioral guarantees.  Its use is demonstrated to be effective in some
environments exhibiting severe engineering restrictions.  As such, it is
not expected that all users everywhere will have direct need for Outcome.
However, it is hoped that evolution of similar idioms will eventually
permit the broader C++ community to consider this domain of cross-module
stateful control transfer to be a “solved problem” (or certainly
“less-brittle” or “less-problematic”).

Great thanks to the Boost Community for their tremendous efforts,
disciplined review, and detailed exploration of topics in considering this
library submission.

Sincerest appreciation to Niall Douglas (Author of the Outcome Library) and
other contributors for pushing the boundary for '<error|exception>'
handling and for submitting this work to the Boost Review process.

--charley
Boost.Outcome (v2) Review Manager