[swift-evolution] [Pitch] Typed throws

[Daniel Leping]

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.

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).

On Tue, 28 Feb 2017 at 18:18 Matthew Johnson <matthew at anandabits.com> wrote:

>
> On Feb 28, 2017, at 5:09 AM, Daniel Leping <daniel at crossroadlabs.xyz>
> 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 <razielim at gmail.com> wrote:
>
> 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>
> 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> 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
> <http://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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