[swift-evolution] typed throws

Xiaodi Wu xiaodi.wu at gmail.com
Sat Aug 19 12:43:27 CDT 2017

On Sat, Aug 19, 2017 at 08:29 Matthew Johnson <matthew at anandabits.com>

> Sent from my iPad
> On Aug 18, 2017, at 9:19 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> On Fri, Aug 18, 2017 at 8:11 PM, Matthew Johnson <matthew at anandabits.com>
> wrote:
>> Sent from my iPad
>> On Aug 18, 2017, at 6:56 PM, Xiaodi Wu <xiaodi.wu at 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.

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?

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).

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.

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 <matthew at anandabits.com>
>> wrote:
>>> On Aug 18, 2017, at 6:29 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> On Fri, Aug 18, 2017 at 6:19 PM, Matthew Johnson <matthew at anandabits.com
>>> > wrote:
>>>> On Aug 18, 2017, at 6:15 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>> On Fri, Aug 18, 2017 at 09:20 Matthew Johnson via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>>> Sent from my iPad
>>>>> On Aug 18, 2017, at 1:27 AM, John McCall <rjmccall at apple.com> wrote:
>>>>> >> On Aug 18, 2017, at 12:58 AM, Chris Lattner via swift-evolution <
>>>>> swift-evolution at swift.org> wrote:
>>>>> >> Splitting this off into its own thread:
>>>>> >>
>>>>> >>> On Aug 17, 2017, at 7:39 PM, Matthew Johnson <
>>>>> matthew at 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 at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
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