> On Feb 28, 2017, at 12:21 PM, Daniel Leping <[email protected]> wrote: > > Matthew, I understand your idea. Though IMO nested exceptions lead to > cluttering and to tons of boilerplate code.
I understand this and am suggesting that this boilerplate is a problem that can be addressed by language features. We should not disregard the value of a stable error contract just because it currently requires boilerplate that could be eliminated. > > What I think is that the important (logical) parts are to be provided by > custom errors (not wrapped, just custom). It's a good practice and you don't > need any typed throws for it. When the error is a result of a condition directly detected by the library this is true. But if it was originally produced by an underlying error that information should not be thrown away. It can be very useful, if for no other purpose than logging / debugging / troubleshooting. You don’t need typed throws for wrappers either, it just allows you to communicate the wrapped error type to users when you decide that is appropriate. When you decide that is providing too much concrete type information to callers you can still just throw `Error`. In the latter case, you don’t need to wrap the errors even if you call other functions that can throw different error types. `Error` is a supertype of all errors. > Anyways, though, the library developer should change the major version > (everybody is more or less bound to same major version) and warn of the API > change. It's not unreliable API, it's sticking to the specific version. > Anyways the major versions are exactly the ones that break the API. What this > means is that the logical part remains the same, but some exceptions (all of > which you will anyways treat the same way as you don't know what to do with > them) should be transparent. It’s best to minimize source-breaking changes even across major versions. If you change a dependency and you are directly propagating errors from your dependencies it is source breaking for all callers who try to do anything more than totally generic “unknown error” handling in their code. It is a bad idea to put users in the position of a) fragile error handling or b) totally generic error handling. > > It's not that I don't understand you. I do. Though I had quite some > experience in Java (it has typed throws) and I know what problems this > approach inherently has. How do you think why Scala (modern language based on > Java) dropped typed throws even being supposed having them inherently? I am not very familiar with Java and Scala but this topic has come up several times in the past and there have been several people who have commented on differences between the design in Java and the design in languages where it works better. > > I think typed throws do more harm than good for all the reasons described > above. > > TL;DR - typed throws is not a solution for the real world. It's good in > theory only. > > On Tue, Feb 28, 2017 at 7:00 PM, Matthew Johnson <[email protected] > <mailto:[email protected]>> wrote: > >> On Feb 28, 2017, at 10:44 AM, Daniel Leping <[email protected] >> <mailto:[email protected]>> wrote: >> >> Mathew, I totally understand what you're talking about. And this is exactly >> what I want to avoid. >> >> If a library developer knows how to handle certain case (i.e. recover or >> throw a special error) I'm ok with it. Though I'm not ok with completely >> hiding errors from the underlying implementation. The library developer >> might not handle EVERYTHING. I want to know what happened and where. >> Otherwise I will have to explicitly debug his library to find out what the >> issue is. In most cases I will not have time to wait developer fixing the >> library. I will fix it myself and make PR. So I consider tranparent errors >> rather a benefit. >> >> Hiding errors under library errors might have worked for proprietary >> middleware. Today the world changed. It's open source now. > > I agree with you that having *access* to the original error is important and > that a library should not be expected to handle every case. I hope I have > not said anything that suggests otherwise. > > What I’m suggesting is that if the error originated in a dependency that > might change it should be wrapped in a stable abstraction that the users of > the library can depend on. The underlying error should still be available > via a property or associated value exposed with an existential type. If you > expect users to catch errors thrown by a dependency that might change you > have a fragile API contract in the area of errors. > > This can easily lead to unreliable software - when you change the dependency > nothing alerts users to the fact that their error handling code is probably > broken. Even if the compiler told them it was broken, they now have the > burden of learning about the errors that might come from your new dependency. > An API that could lead to this consequence is badly designed IMO. > > I even suggested adding a new requirement to `Error` that includes a default > implementation to standardize how we access wrapped errors, including the > originating error underneath all wrapper layers. > >> >> My point is that I'm completely against typed throws. I'm sure there will be >> a lot of developers who would try to use the feature to hide underlying >> errors under their own. I can't consider it a good practice (reasons above). >> This is how good intentions (caring about API and library users) can lead to >> bad consequences. Sounds good in theory - works bad in practice (in real, >> non ideal world). > > What is your opinion about the problem I describe above? Do you think this > is unimportant? > >> >> On Tue, 28 Feb 2017 at 18:18 Matthew Johnson <[email protected] >> <mailto:[email protected]>> wrote: >> >>> On Feb 28, 2017, at 5:09 AM, Daniel Leping <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> When you're going to present a user with specific error message you don't >>> do it for all errors. First of all at the end of a call chain you will have >>> a couple of dozens of possible errors and only 2 or 3 will do really matter >>> for the end user. Would you list all the dozen in params? Probably not. >>> You'll just catch what matters and put the rest to the log with a default >>> "sorry" message. >>> >>> What I mean is it's OK to be "unaware" why everything failed. What really >>> does matter is what you can deal with and what you can't (the rest). "The >>> rest" is usually a magnitude bigger, though. >>> >>> I might agree, that listing some errors in the throws clause is useful >>> feature for documentation (even so, I think the docs should be in the >>> comments), but only and only if it can throw the listed errors + a bunch of >>> other stuff. In Java it's resolved with a terrible solution of RuntimeError >>> interface which I definitely suggest to avoid. >>> >>> As a library developer I can say that even for a library it's a nightmare >>> and libraries will start wrapping system errors with their own if the trend >>> would become to list errors in middleware. My preference though is to have >>> all errors transparently passed in both roles, being a library developer >>> and a library user. >> >> Whether to wrap or not is a judgment call. >> >> If the purpose of your library is to simply provide a cleaner experience for >> a system API that is a stable dependency that will not change. Users might >> expect to have the errors from the system API forwarded directly and that >> would be perfectly appropriate. >> >> In other cases you might have dependencies that are considered >> implementation details. In that case if you don’t wrap the errors you’re >> doing users a disservice by giving them an unstable interface for error >> handling. Changing out your dependency that is supposed to be an >> implementation detail becomes a breaking change. >> >>> >> >>> On Tue, 28 Feb 2017 at 8:29 Karl Wagner <[email protected] >>> <mailto:[email protected]>> wrote: >> >>>> On 28 Feb 2017, at 02:43, Matthew Johnson via swift-evolution >>>> <[email protected] <mailto:[email protected]>> wrote: >>>> >>>> >>>> >>>> Sent from my iPad >>>> >>>> On Feb 27, 2017, at 7:19 PM, Daniel Leping <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> >>>>> Well, as Dave pointed, you can very rarely recover from an error, which >>>>> IMO is absolutely true. >>>>> >>>>> If your operation fails you don't really care unless you can recover. And >>>>> you know your cases, which you can recover from (in reality one usually >>>>> does it in optimization phase, though). >>>>> >>>>> What else do you need the type of error for at the very end of your call >>>>> stack? In 90% you will tell the user "ah, sorry, something happened. Come >>>>> back later" and log the error (if you haven't forgot it). >>>>> >>>>> In most cases the errors are not for recovering. They neither are to be >>>>> presented to users. They are for developers to read the log/crash >>>>> report/whatever else and analyze it. Most of the errors are for debugging >>>>> purposes. >>>>> >>>>> I don't want to deal with cumbersome code the purpose of which is to just >>>>> "obey the language rules". Unless I know how to recover I would rethrow >>>>> it. Than catch at the top of the stack and log + show the user a nice >>>>> "sorry" message without getting techy. >>>> >>>> In order to provide a helpful experience to end users an app needs to know >>>> about what might have caused the error and therefore what might (or might >>>> not) resolve it, allowing the operation to succeed on a subsequent >>>> attempt. This can influence the strategy an app uses to avoid bothering >>>> the user if it might be resolvable without user intervention and can also >>>> influence the content of the message presented to users if that is >>>> necessary. >>>> >>> >>> Errors are certainly useful, but the key distinction (IMO) is that callers >>> should never rely on an error happening or not. They are typically highly >>> implementation and hence version-specific. I think that means specific >>> errors shouldn’t be part of the function signature. >>> >>> That doesn’t mean you can’t handle the error. A function that throws is one >>> that reserves the right to fail; maybe you can resolve the problem, but >>> maybe you can’t. That means your enclosing operation also needs to be >>> prepared to fail for reasons beyond _its_ control, and so on. At some point >>> your entire UI-level sub-operation (like opening the file, downloading the >>> data from the network) just fails, and you decide whether or not the user >>> needs to be told of that and how to do so. >> >>>> Indeed, many apps don't bother with this kind of detail and just treat all >>>> errors the same. Personally, I find that to be an unfortunate state of >>>> affairs both as a user and as a developer. >>>> >>>> Types can be useful in conveying this kind of information and abstracting >>>> low level details that are not helpful at higher levels in the stack. Of >>>> course types are not the only way to convey this information. But my >>>> experience is that untyped errors often result in libraries with poor >>>> documentation of error cases and not enough consideration of the error >>>> handling experience of users of the library in general. That makes it >>>> very difficult to handle errors well. I have experiences like this in >>>> several languages and on several platforms, including Apple's. >>> >> >>> I’m not sure that types are really the best abstraction for “the list of >>> errors that can be thrown by this function”. They’re fine for error >>> themselves, but not for per-function error-lists IMO. Most of our types are >>> way too rigid for this to be easy to live with. >>> >>> If I understand what you’re saying, the core problem can be summarised as: >>> "I don’t know/can’t easily communicate which errors this function throws”. >>> Really, it is a documentation problem; it's too onerous to document every >>> individual throwing function, even with our very pretty markdown-like >>> syntax. I’m just not sure that the relatively rigid type-system is the >>> solution. >>> >>> Now, if we look at this from a documentation perspective: obviously, the >>> compiler _typically_ can't generate documentation for you, because it >>> doesn’t know what your types/functions are meant for. Error documentation, >>> though, is different: the compiler *can* often know the specific errors >>> which get thrown (or rethrown from a call to another function from whom it >>> can get that information); and even when it can’t know that, often it can >>> at least know the specific type of error. We could enhance the compiler >>> libraries to track that; I spent half a day modifying the existing >>> error-checker in Sema to prove the concept and it’s close. >>> >>> Swift libraries don’t have header files, so third-parties will only ever >>> see your documentation as generated by some tool which integrates the >>> compiler. That means the compiler can “improve” the documentation when it >>> is lacking. For yourself, if you care about explicitly writing out the >>> errors which get thrown and you’re not working with an IDE, you could >>> enable compiler-checking of your documentation comments. We can invent new >>> shorthands to make it easier to list all of your various errors (individual >>> cases, entire types, and maybe some private “other errors”), isolated from >>> the rest of the language. For example: >>> >>> /// - throws: MyError.{errorOne, errorThree}, AnotherError.*, * >>> func doSomething() throws >>> >>> Maybe you can think of other ways we could help the documentation problem >>> from the compiler side? >> >>>> Typed errors are certainly no panacea but I believe they can be an >>>> important tool. Experience in other languages has shown that to be the >>>> case for many people writing many kinds of software. >>>> >>>> If you don't believe typed errors will improve your code or if you just >>>> don't want to deal with typed errors, just don't use them! You will be >>>> able to continue using untyped throws just as you do today. You can even >>>> do this if you use libraries that throw typed errors. >>>> >> >>> If it’s the wrong solution to the problem, it does make the language worse >>> overall. >> >>>>> On Tue, 28 Feb 2017 at 1:12 Matthew Johnson via swift-evolution >>>>> <[email protected] <mailto:[email protected]>> wrote: >>>>> >>>>> > On Feb 27, 2017, at 5:01 PM, Dave Abrahams <[email protected] >>>>> > <mailto:[email protected]>> wrote: >>>>> > >>>>> > >>>>> > on Mon Feb 27 2017, Matthew Johnson <matthew-AT-anandabits.com >>>>> > <http://matthew-at-anandabits.com/>> wrote: >>>>> > >>>>> >>> On Feb 27, 2017, at 4:20 PM, Dave Abrahams <[email protected] >>>>> >>> <mailto:[email protected]>> wrote: >>>>> >>> >>>>> >>> >>>>> >>> I'm sorry, I don't see any substantive difference, based on what >>>>> >>> you've >>>>> >>> written here, between this feature and const. >>>>> >> >>>>> >> Let me give it one more shot and then I’ll drop it. :) >>>>> >> >>>>> >> Const is viral because if an API does not declare its arguments const >>>>> >> it cannot be used by a caller who has a const argument. >>>>> > >>>>> > Unless the caller casts away const, thus erasing information that was >>>>> > previously encoded in the type system. >>>>> >>>>> Yes, of course. >>>>> >>>>> > >>>>> >> It is required in order to make an API as generally useful as >>>>> >> possible. >>>>> >> >>>>> >> Typed errors are not viral in this way because no callers are >>>>> >> prevented from calling an API regardless of whether it declares error >>>>> >> types or just throws Error like we have today. >>>>> > >>>>> > Unless the caller can recover (which is *very* rare) or it catches and >>>>> > rethrows one of the errors *it* declares, thus erasing information that >>>>> > was previously encoded in the type system. >>>>> >>>>> I view this as being fundamentally different than casting away const. >>>>> Casting away const says “I know better than the types”. >>>>> >>>>> Converting an error to a different type is extremely different. It much >>>>> more similar to any other kind of value wrapper a library might create in >>>>> order to shield its users from being coupled to its implementation >>>>> details / dependencies. This is not a way *around* the type system in >>>>> the sense that casting away const is. It is a way of *using* the type >>>>> system (hopefully) to your advantage. >>>>> >>>>> >>>>> >>>>> > >>>>> >> Pressure to declare error types in your signature in order to make >>>>> >> your function as generally useful as possible does not exist. Each >>>>> >> function is free to declare error types or not according to the >>>>> >> contract it wishes to expose. >>>>> >> >>>>> >> An argument can be made that community expectations might develop that >>>>> >> good APIs should declare error types and they could be considered >>>>> >> viral in this sense because any API that is simply declared `throws` >>>>> >> is dropping type information. But I think this overstates the case. >>>>> >> The community appears to be very sensitive to the problems that can >>>>> >> arise when error types are too concrete, especially across module >>>>> >> boundaries. I think we can learn to use the tool where it works well >>>>> >> and to avoid it where it causes problems. >>>>> >> >>>>> >>> >>>>> >>> -- >>>>> >>> -Dave >>>>> >> >>>>> > >>>>> > -- >>>>> > -Dave >>>>> >>>>> _______________________________________________ >>>>> swift-evolution mailing list >>>>> [email protected] <mailto:[email protected]> >>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >> >>>> _______________________________________________ >>>> swift-evolution mailing list >>>> [email protected] <mailto:[email protected]> >>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >
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