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