[swift-evolution] [Pitch] Typed throws

[Matthew Johnson]

Sent from my iPad

> On Feb 27, 2017, at 7:19 PM, Daniel Leping <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.  

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.  

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.


> 
>> On Tue, 28 Feb 2017 at 1:12 Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> > On Feb 27, 2017, at 5:01 PM, Dave Abrahams <dabrahams at apple.com> wrote:
>> >
>> >
>> > on Mon Feb 27 2017, Matthew Johnson <matthew-AT-anandabits.com> wrote:
>> >
>> >>> On Feb 27, 2017, at 4:20 PM, Dave Abrahams <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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