[swift-evolution] [Pitch] Typed throws

Tue Feb 28 00:29:12 CST 2017

> On 28 Feb 2017, at 02:43, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> 
> Sent from my iPad
> 
> On Feb 27, 2017, at 7:19 PM, Daniel Leping <daniel at crossroadlabs.xyz <mailto:daniel at crossroadlabs.xyz>> 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 <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> > On Feb 27, 2017, at 5:01 PM, Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>> 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 <dabrahams at apple.com <mailto:dabrahams at apple.com>> 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
>> 
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