Sent from my iPad
> On Aug 19, 2017, at 12:43 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>
>
>> On Sat, Aug 19, 2017 at 08:29 Matthew Johnson <matt...@anandabits.com> wrote:
>>
>>
>> Sent from my iPad
>>
>>> On Aug 18, 2017, at 9:19 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>>>
>>>
>>>
>>>> On Fri, Aug 18, 2017 at 8:11 PM, Matthew Johnson <matt...@anandabits.com>
>>>> wrote:
>>>>
>>>>
>>>> Sent from my iPad
>>>>
>>>>> On Aug 18, 2017, at 6:56 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>>>>>
>>>>> Joe Groff wrote:
>>>>>
>>>>> An alternative approach that embraces the open nature of errors could be
>>>>> to represent domains as independent protocols, and extend the error types
>>>>> that are relevant to that domain to conform to the protocol. That way,
>>>>> you don't obscure the structure of the underlying error value with
>>>>> wrappers. If you expect to exhaustively handle all errors in a domain,
>>>>> well, you'd almost certainly going to need to have a fallback case in
>>>>> your wrapper type for miscellaneous errors, but you could represent that
>>>>> instead without wrapping via a catch-all, and as?-casting to your domain
>>>>> protocol with a ??-default for errors that don't conform to the protocol.
>>>>> For example, instead of attempting something like this:
>>>>>
>>>>> enum DatabaseError {
>>>>> case queryError(QueryError)
>>>>> case ioError(IOError)
>>>>> case other(Error)
>>>>>
>>>>> var errorKind: String {
>>>>> switch self {
>>>>> case .queryError(let q): return "query error: \(q.query)"
>>>>> case .ioError(let i): return "io error: \(i.filename)"
>>>>> case .other(let e): return "\(e)"
>>>>> }
>>>>> }
>>>>> }
>>>>>
>>>>> func queryDatabase(_ query: String) throws /*DatabaseError*/ -> Table
>>>>>
>>>>> do {
>>>>> queryDatabase("delete * from users")
>>>>> } catch let d as DatabaseError {
>>>>> os_log(d.errorKind)
>>>>> } catch {
>>>>> fatalError("unexpected non-database error")
>>>>> }
>>>>>
>>>>> You could do this:
>>>>>
>>>>> protocol DatabaseError {
>>>>> var errorKind: String { get }
>>>>> }
>>>>>
>>>>> extension QueryError: DatabaseError {
>>>>> var errorKind: String { return "query error: \(q.query)" }
>>>>> }
>>>>> extension IOError: DatabaseError {
>>>>> var errorKind: String ( return "io error: \(i.filename)" }
>>>>> }
>>>>>
>>>>> extension Error {
>>>>> var databaseErrorKind: String {
>>>>> return (error as? DatabaseError)?.errorKind ?? "unexpected
>>>>> non-database error"
>>>>> }
>>>>> }
>>>>>
>>>>> func queryDatabase(_ query: String) throws -> Table
>>>>>
>>>>> do {
>>>>> queryDatabase("delete * from users")
>>>>> } catch {
>>>>> os_log(error.databaseErrorKind)
>>>>> }
>>>>
>>>> This approach isn't sufficient for several reasons. Notably, it requires
>>>> the underlying errors to already have a distinct type for every category
>>>> we wish to place them in. If all network errors have the same type and I
>>>> want to categorize them based on network availability, authentication,
>>>> dropped connection, etc I am not able to do that.
>>>
>>> Sorry, how does the presence or absence of typed throws play into this?
>>
>> It provides a convenient way to drive an error conversion mechanism during
>> propagation, whether in a library function used to wrap the throwing
>> expression or ideally with language support. If I call a function that
>> throws FooError and my function throws BarError and we have a way to go from
>> FooError to BarError we can invoke that conversion without needing to catch
>> and rethrow the wrapped error.
>
> But isn't that an argument *against* typed errors? You need this
> language-level support to automatically convert FooErrors to BarErrors
> *because* you've restricted yourself to throwing BarErrors and the function
> you call is restricted to throwing FooErrors. Currently, without typed
> errors, there is no need to convert a FooError to a BarError.
No, this is a misunderstanding of the point of the conversion. In that
example, the point of performing a conversion is not because the types are
arbitrarily chosen. It is because the initializer of BarError includes an
algorithm that categorizes errors. It may place different values of FooError
into different cases. What I am after is language-level support for
categorizing errors during propagation and making the categories easily visible
to anyone who looks at the signature of a function that chooses to categorize
its errors. Using types and the initializer is one way (the most obvious way)
to do this.
>
> As mentioned above, it's difficult even internally to design a single
> ontology of errors that works throughout a library, so compiler support for
> typed errors would be tantamount to a compiler-enforced facility that
> pervasively requires this laborious classification and re-classification of
> errrors whenever a function rethrows, much of which may be ultimately
> unnecessary. In other words, if you are a library vendor and wrap every
> FooError from an upstream dependency into a BarError, your user is still
> likely to have their own classification of errors and decide to handle
> different groups of BarError cases differently anyway, so what was the point
> of your laborious conversion of FooErrors to BarErrors?
I am not suggesting that categorization be performed everywhere. What I am
suggesting is that there are many cases where it can be done in a meaningful
way. If you're writing a library and anticipate that there is no way to
categorize errors that is meaningful to all users of your library you should
not perform categorization. On the other hand, continuing with the line of
examples we've already seen, if you're writing an internal networking library
and intend to have similar recovery paths throughout your app categorization
can be extremely useful. This is especially true as a team grows and you are
trying to encourage common practices throughout the app.
>
>> It also provides convenient documentation of the categorization along with a
>> straightforward way to match the cases (with code completion as Chris
>> pointed out). IMO, making this information immediately clear and with easy
>> matching at call sites is crucial to improving how people handle errors in
>> practice.
>
> Again, I don't see documentation as a sufficient argument for this feature;
> there is no reason why the Swift compiler could not extract comprehensive
> information about what errors are thrown at compile time without typed
> errors--and with more granularity than can be documented via types (since
> only specific enum cases may ever be thrown in a particular function).
I am not arguing that documentation alone is a sufficient argument. That said,
while the compiler *could* do such things I don't think it would be a priority
any time soon (core team, please correct me if I'm wrong here).
>
>> Error handling is an afterthought all too often. The value of making it
>> immediately clear how to match important categories of errors should not be
>> understated.
>
> See, this is probably where I'm failing to understand you. Every library that
> has its own Error-conforming types offers an ontology of errors that, at
> least to its authors, make some sort of sense. At the call site, you can
> `catch` specific categories of errors or `switch` over specific errors. Yes,
> this can become a little annoying if your own classification of errors
> differs from the library authors' classification. However, I fail to see how
> typed errors makes this any better, other than that you'd `catch` only one
> type of error but have to `switch` over cases and then `switch` over the
> underlying error. Only now, you've introduced this issue where, for the
> library authors, FooErrors have to be reclassified into BarErrors, and then
> into BazErrors, and then into BooErrors--to what end? It seems only to
> accomplish the goal of making error handling not an afterthought by causing
> the compiler to make it more of a nuisance.
If the only use of this was to move from throwing independent types to throwing
a single type with a bunch of cases you are right, there wouldn't be much
value. However, the contract for individual functions in a library may vary
and there are also errors from dependencies of the library that may also be
thrown.
Let's assume for a moment that the error types of a library are carefully
designed to offer meaningful categorization. Even then it is still very useful
to know which categories may occur at a given call site. Yes, this is only
documentation so we'll set it aside and not give it too much weight.
More importantly, if I depend on library X it may depend on library Y as an
implementation detail which is not intended to be part of its API contract. As
a user of X I should not need to be aware that Y even exists. I certainly am
not going to import it and have access to its error types - I don't want a
direct dependency on Y. Untyped errors are likely to lead to errors from Y
leaking through the interface of X. They may be errors which I would have a
meaningful recovery strategy if my code was able to properly understand them,
but it won't because I my code doesn't know anything about Y. Yes, these are
bugs in X but they are bugs that the language could help to prevent.
As I have said before, none of this *requires* typed errors, they are just a
natural way to approach the problem. Any solution that allows for
categorization during propagation that can be matched at call sites would be
acceptable.
>
>> I really believe language support of some kind is warranted and would have
>> an impact on the quality of software. Maybe types aren't the right
>> solution, but we do need one.
>>
>> Deciding what categories are important is obviously subjective, but I do
>> believe that libraries focused on a specific domain can often make
>> reasonable guesses that are pretty close in the majority of use cases. This
>> is especially true for internal libraries where part of the purpose of the
>> library may be to establish conventions for the app that are intended to be
>> used (almost) everywhere.
>>
>>>
>>>> The kind of categorization I want to be able to do requires a custom
>>>> algorithm. The specific algorithm is used to categorize errors depends on
>>>> the dynamic context (i.e. the function that is propagating it). The way I
>>>> usually think about this categorization is as a conversion initializer as
>>>> I showed in the example, but it certainly wouldn't need to be accomplished
>>>> that way. The most important thing IMO is the ability to categorize
>>>> during error propagation and make information about that categorization
>>>> easy for callers to discover.
>>>>
>>>> The output of the algorithm could use various mechanisms for
>>>> categorization - an enum is one mechanism, distinct types conforming to
>>>> appropriate categorization protocols is another. Attaching some kind of
>>>> category value to the original error or propagating the category along
>>>> with it might also work (although might be rather clunky).
>>>>
>>>> It is trivial to make the original error immediately available via an
>>>> `underlyingError` property so I really don't understand the resistance to
>>>> wrapping errors. The categorization can easily be ignored at the catch
>>>> site if desired. That said, if we figure out some other mechanism for
>>>> categorizing errors, including placing different error values of the same
>>>> type into different categories, and matching them based on this
>>>> categorization I think I would be ok with that. Using wrapper types is
>>>> not essential to solving the problem.
>>>>
>>>> Setting all of this aside, surely you had you had your own reasons for
>>>> supporting typed errors in the past. What were those and why do you no
>>>> longer consider them important?
>>>
>>> My memory is certainly spotty, but as far as I can recall, I had no
>>> distinct reasons; it just seemed like a reasonable and "natural" next step
>>> that other people wanted for which I had no use case of my own in mind.
>>> Having seen the argumentation that there aren't very many use cases in
>>> general, I'm warming to the view that it's probably not such a great next
>>> step.
>>>
>>>
>>>>>> On Fri, Aug 18, 2017 at 6:46 PM, Matthew Johnson
>>>>>> <matt...@anandabits.com> wrote:
>>>>>>
>>>>>>> On Aug 18, 2017, at 6:29 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>>>>>>>
>>>>>>>> On Fri, Aug 18, 2017 at 6:19 PM, Matthew Johnson
>>>>>>>> <matt...@anandabits.com> wrote:
>>>>>>>>
>>>>>>>>> On Aug 18, 2017, at 6:15 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> On Fri, Aug 18, 2017 at 09:20 Matthew Johnson via swift-evolution
>>>>>>>>>> <swift-evolution@swift.org> wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Sent from my iPad
>>>>>>>>>>
>>>>>>>>>> On Aug 18, 2017, at 1:27 AM, John McCall <rjmcc...@apple.com> wrote:
>>>>>>>>>>
>>>>>>>>>> >> On Aug 18, 2017, at 12:58 AM, Chris Lattner via swift-evolution
>>>>>>>>>> >> <swift-evolution@swift.org> wrote:
>>>>>>>>>> >> Splitting this off into its own thread:
>>>>>>>>>> >>
>>>>>>>>>> >>> On Aug 17, 2017, at 7:39 PM, Matthew Johnson
>>>>>>>>>> >>> <matt...@anandabits.com> wrote:
>>>>>>>>>> >>> One related topic that isn’t discussed is type errors. Many
>>>>>>>>>> >>> third party libraries use a Result type with typed errors.
>>>>>>>>>> >>> Moving to an async / await model without also introducing typed
>>>>>>>>>> >>> errors into Swift would require giving up something that is
>>>>>>>>>> >>> highly valued by many Swift developers. Maybe Swift 5 is the
>>>>>>>>>> >>> right time to tackle typed errors as well. I would be happy to
>>>>>>>>>> >>> help with design and drafting a proposal but would need
>>>>>>>>>> >>> collaborators on the implementation side.
>>>>>>>>>> >>
>>>>>>>>>> >> Typed throws is something we need to settle one way or the other,
>>>>>>>>>> >> and I agree it would be nice to do that in the Swift 5 cycle.
>>>>>>>>>> >>
>>>>>>>>>> >> For the purposes of this sub-discussion, I think there are three
>>>>>>>>>> >> kinds of code to think about:
>>>>>>>>>> >> 1) large scale API like Cocoa which evolve (adding significant
>>>>>>>>>> >> functionality) over the course of many years and can’t break
>>>>>>>>>> >> clients.
>>>>>>>>>> >> 2) the public API of shared swiftpm packages, whose lifecycle may
>>>>>>>>>> >> rise and fall - being obsoleted and replaced by better packages
>>>>>>>>>> >> if they encounter a design problem.
>>>>>>>>>> >> 3) internal APIs and applications, which are easy to change
>>>>>>>>>> >> because the implementations and clients of the APIs are owned by
>>>>>>>>>> >> the same people.
>>>>>>>>>> >>
>>>>>>>>>> >> These each have different sorts of concerns, and we hope that
>>>>>>>>>> >> something can start out as #3 but work its way up the stack
>>>>>>>>>> >> gracefully.
>>>>>>>>>> >>
>>>>>>>>>> >> Here is where I think things stand on it:
>>>>>>>>>> >> - There is consensus that untyped throws is the right thing for a
>>>>>>>>>> >> large scale API like Cocoa. NSError is effectively proven here.
>>>>>>>>>> >> Even if typed throws is introduced, Apple is unlikely to adopt it
>>>>>>>>>> >> in their APIs for this reason.
>>>>>>>>>> >> - There is consensus that untyped throws is the right default for
>>>>>>>>>> >> people to reach for for public package (#2).
>>>>>>>>>> >> - There is consensus that Java and other systems that encourage
>>>>>>>>>> >> lists of throws error types lead to problematic APIs for a
>>>>>>>>>> >> variety of reasons.
>>>>>>>>>> >> - There is disagreement about whether internal APIs (#3) should
>>>>>>>>>> >> use it. It seems perfect to be able to write exhaustive catches
>>>>>>>>>> >> in this situation, since everything in knowable. OTOH, this could
>>>>>>>>>> >> encourage abuse of error handling in cases where you really
>>>>>>>>>> >> should return an enum instead of using throws.
>>>>>>>>>> >> - Some people are concerned that introducing typed throws would
>>>>>>>>>> >> cause people to reach for it instead of using untyped throws for
>>>>>>>>>> >> public package APIs.
>>>>>>>>>> >
>>>>>>>>>> > Even for non-public code. The only practical merit of typed
>>>>>>>>>> > throws I have ever seen someone demonstrate is that it would let
>>>>>>>>>> > them use contextual lookup in a throw or catch. People always say
>>>>>>>>>> > "I'll be able to exhaustively switch over my errors", and then I
>>>>>>>>>> > ask them to show me where they want to do that, and they show me
>>>>>>>>>> > something that just logs the error, which of course does not
>>>>>>>>>> > require typed throws. Every. Single. Time.
>>>>>>>>>>
>>>>>>>>>> I agree that exhaustive switching over errors is something that
>>>>>>>>>> people are extremely likely to actually want to do. I also think
>>>>>>>>>> it's a bit of a red herring. The value of typed errors is *not* in
>>>>>>>>>> exhaustive switching. It is in categorization and verified
>>>>>>>>>> documentation.
>>>>>>>>>>
>>>>>>>>>> Here is a concrete example that applies to almost every app. When
>>>>>>>>>> you make a network request there are many things that could go wrong
>>>>>>>>>> to which you may want to respond differently:
>>>>>>>>>> * There might be no network available. You might recover by
>>>>>>>>>> updating the UI to indicate that and start monitoring for a
>>>>>>>>>> reachability change.
>>>>>>>>>> * There might have been a server error that should eventually be
>>>>>>>>>> resolved (500). You might update the UI and provide the user the
>>>>>>>>>> ability to retry.
>>>>>>>>>> * There might have been an unrecoverable server error (404). You
>>>>>>>>>> will update the UI.
>>>>>>>>>> * There might have been a low level parsing error (bad JSON, etc).
>>>>>>>>>> Recovery is perhaps similar in nature to #2, but the problem is less
>>>>>>>>>> likely to be resolved quickly so you may not provide a retry option.
>>>>>>>>>> You might also want to do something to notify your dev team that
>>>>>>>>>> the server is returning JSON that can't be parsed.
>>>>>>>>>> * There might have been a higher-level parsing error (converting
>>>>>>>>>> JSON to model types). This might be treated the same as bad JSON.
>>>>>>>>>> On the other hand, depending on the specifics of the app, you might
>>>>>>>>>> take an alternate path that only parses the most essential model
>>>>>>>>>> data in hopes that the problem was somewhere else and this parse
>>>>>>>>>> will succeed.
>>>>>>>>>>
>>>>>>>>>> All of this can obviously be accomplished with untyped errors. That
>>>>>>>>>> said, using types to categorize errors would significantly improve
>>>>>>>>>> the clarity of such code. More importantly, I believe that by
>>>>>>>>>> categorizing errors in ways that are most relevant to a specific
>>>>>>>>>> domain a library (perhaps internal to an app) can encourage
>>>>>>>>>> developers to think carefully about how to respond.
>>>>>>>>>
>>>>>>>>> I used to be rather in favor of adding typed errors, thinking that it
>>>>>>>>> can only benefit and seemed reasonable. However, given the very
>>>>>>>>> interesting discussion here, I'm inclined to think that what you
>>>>>>>>> articulate above is actually a very good argument _against_ adding
>>>>>>>>> typed errors.
>>>>>>>>>
>>>>>>>>> If I may simplify, the gist of the argument advanced by Tino,
>>>>>>>>> Charlie, and you is that the primary goal is documentation, and that
>>>>>>>>> documentation in the form of prose is insufficient because it can be
>>>>>>>>> unreliable. Therefore, you want a way for the compiler to enforce
>>>>>>>>> said documentation. (The categorization use case, I think, is well
>>>>>>>>> addressed by the protocol-based design discussed already in this
>>>>>>>>> thread.)
>>>>>>>>
>>>>>>>> Actually documentation is only one of the goals I have and it is the
>>>>>>>> least important. Please see my subsequent reply to John where I
>>>>>>>> articulate the four primary goals I have for improved error handling,
>>>>>>>> whether it be typed errors or some other mechanism. I am curious to
>>>>>>>> see what you think of the goals, as well as what mechanism might best
>>>>>>>> address those goals.
>>>>>>>
>>>>>>> Your other three goals have to do with what you term categorization,
>>>>>>> unless I misunderstand. Are those not adequately addressed by Joe
>>>>>>> Groff's protocol-based design?
>>>>>>
>>>>>> Can you elaborate on what you mean by Joe Gross’s protocol-based design?
>>>>>> I certainly haven’t seen anything that I believe addresses those goals
>>>>>> well.
>>>>>>
>>>>>>>
>>>>>>>>>
>>>>>>>>> However, the compiler itself cannot reward, only punish in the form
>>>>>>>>> of errors or warnings; if exhaustive switching is a red herring and
>>>>>>>>> the payoff for typed errors is correct documentation, the
>>>>>>>>> effectiveness of this kind of compiler enforcement must be directly
>>>>>>>>> proportional to the degree of extrinsic punishment inflicted by the
>>>>>>>>> compiler (since the intrinsic reward of correct documentation is the
>>>>>>>>> same whether it's spelled using doc comments or the type system).
>>>>>>>>> This seems like a heavy-handed way to enforce documentation of only
>>>>>>>>> one specific aspect of a throwing function; moreover, if this use
>>>>>>>>> case were to be sufficiently compelling, then it's certainly a better
>>>>>>>>> argument for SourceKit (or some other builtin tool) to automatically
>>>>>>>>> generate information on all errors thrown than for the compiler to
>>>>>>>>> require that users declare it themselves--even if opt-in.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Bad error handling is pervasive. The fact that everyone shows you
>>>>>>>>>> code that just logs the error is a prime example of this. It should
>>>>>>>>>> be considered a symptom of a problem, not an acceptable status quo
>>>>>>>>>> to be maintained. We need all the tools at our disposal to
>>>>>>>>>> encourage better thinking about and handling of errors. Most
>>>>>>>>>> importantly, I think we need a middle ground between completely
>>>>>>>>>> untyped errors and an exhaustive list of every possible error that
>>>>>>>>>> might happen. I believe a well designed mechanism for categorizing
>>>>>>>>>> errors in a compiler-verified way can do exactly this.
>>>>>>>>>>
>>>>>>>>>> In many respects, there are similarities to this in the design of
>>>>>>>>>> `NSError` which provides categorization via the error domain. This
>>>>>>>>>> categorization is a bit more broad than I think is useful in many
>>>>>>>>>> cases, but it is the best example I'm aware of.
>>>>>>>>>>
>>>>>>>>>> The primary difference between error domains and the kind of
>>>>>>>>>> categorization I am proposing is that error domains categorize based
>>>>>>>>>> on the source of an error whereas I am proposing categorization
>>>>>>>>>> driven by likely recovery strategies. Recovery is obviously
>>>>>>>>>> application dependent, but I think the example above demonstrates
>>>>>>>>>> that there are some useful generalizations that can be made
>>>>>>>>>> (especially in an app-specific library), even if they don't apply
>>>>>>>>>> everywhere.
>>>>>>>>>>
>>>>>>>>>> > Sometimes we then go on to have a conversation about wrapping
>>>>>>>>>> > errors in other error types, and that can be interesting, but now
>>>>>>>>>> > we're talking about adding a big, messy feature just to get
>>>>>>>>>> > "safety" guarantees for a fairly minor need.
>>>>>>>>>>
>>>>>>>>>> I think you're right that wrapping errors is tightly related to an
>>>>>>>>>> effective use of typed errors. You can do a reasonable job without
>>>>>>>>>> language support (as has been discussed on the list in the past).
>>>>>>>>>> On the other hand, if we're going to introduce typed errors we
>>>>>>>>>> should do it in a way that *encourages* effective use of them. My
>>>>>>>>>> opinion is that encouraging effect use means categorizing (wrapping)
>>>>>>>>>> errors without requiring any additional syntax beyond the simple
>>>>>>>>>> `try` used by untyped errors. In practice, this means we should not
>>>>>>>>>> need to catch and rethrow an error if all we want to do is
>>>>>>>>>> categorize it. Rust provides good prior art in this area.
>>>>>>>>>>
>>>>>>>>>> >
>>>>>>>>>> > Programmers often have an instinct to obsess over error taxonomies
>>>>>>>>>> > that is very rarely directed at solving any real problem; it is
>>>>>>>>>> > just self-imposed busy-work.
>>>>>>>>>>
>>>>>>>>>> I agree that obsessing over intricate taxonomies is
>>>>>>>>>> counter-productive and should be discouraged. On the other hand, I
>>>>>>>>>> hope the example I provided above can help to focus the discussion
>>>>>>>>>> on a practical use of types to categorize errors in a way that helps
>>>>>>>>>> guide *thinking* and therefore improves error handling in practice.
>>>>>>>>>>
>>>>>>>>>> >
>>>>>>>>>> >> - Some people think that while it might be useful in some narrow
>>>>>>>>>> >> cases, the utility isn’t high enough to justify making the
>>>>>>>>>> >> language more complex (complexity that would intrude on the APIs
>>>>>>>>>> >> of result types, futures, etc)
>>>>>>>>>> >>
>>>>>>>>>> >> I’m sure there are other points in the discussion that I’m
>>>>>>>>>> >> forgetting.
>>>>>>>>>> >>
>>>>>>>>>> >> One thing that I’m personally very concerned about is in the
>>>>>>>>>> >> systems programming domain. Systems code is sort of the classic
>>>>>>>>>> >> example of code that is low-level enough and finely specified
>>>>>>>>>> >> enough that there are lots of knowable things, including the
>>>>>>>>>> >> failure modes.
>>>>>>>>>> >
>>>>>>>>>> > Here we are using "systems" to mean "embedded systems and
>>>>>>>>>> > kernels". And frankly even a kernel is a large enough system that
>>>>>>>>>> > they don't want to exhaustively switch over failures; they just
>>>>>>>>>> > want the static guarantees that go along with a constrained error
>>>>>>>>>> > type.
>>>>>>>>>> >
>>>>>>>>>> >> Beyond expressivity though, our current model involves boxing
>>>>>>>>>> >> thrown values into an Error existential, something that forces an
>>>>>>>>>> >> implicit memory allocation when the value is large. Unless this
>>>>>>>>>> >> is fixed, I’m very concerned that we’ll end up with a situation
>>>>>>>>>> >> where certain kinds of systems code (i.e., that which cares about
>>>>>>>>>> >> real time guarantees) will not be able to use error handling at
>>>>>>>>>> >> all.
>>>>>>>>>> >>
>>>>>>>>>> >> JohnMC has some ideas on how to change code generation for
>>>>>>>>>> >> ‘throws’ to avoid this problem, but I don’t understand his ideas
>>>>>>>>>> >> enough to know if they are practical and likely to happen or not.
>>>>>>>>>> >
>>>>>>>>>> > Essentially, you give Error a tagged-pointer representation to
>>>>>>>>>> > allow payload-less errors on non-generic error types to be
>>>>>>>>>> > allocated globally, and then you can (1) tell people to not throw
>>>>>>>>>> > errors that require allocation if it's vital to avoid allocation
>>>>>>>>>> > (just like we would tell them today not to construct classes or
>>>>>>>>>> > indirect enum cases) and (2) allow a special global payload-less
>>>>>>>>>> > error to be substituted if error allocation fails.
>>>>>>>>>> >
>>>>>>>>>> > Of course, we could also say that systems code is required to use
>>>>>>>>>> > a typed-throws feature that we add down the line for their
>>>>>>>>>> > purposes. Or just tell them to not use payloads. Or force them
>>>>>>>>>> > to constrain their error types to fit within some given size.
>>>>>>>>>> > (Note that obsessive error taxonomies tend to end up with a bunch
>>>>>>>>>> > of indirect enum cases anyway, because they get recursive, so the
>>>>>>>>>> > allocation problem is very real whatever we do.)
>>>>>>>>>> >
>>>>>>>>>> > John.
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> swift-evolution mailing list
>>>>>>>>>> swift-evolution@swift.org
>>>>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>
>>>>>
>>>
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