[swift-evolution] Adding Result to the Standard Library

[Tony Allevato]

On Thu, Nov 2, 2017 at 11:58 AM Jon Shier <jon at jonshier.com> wrote:

> You would continue to be free to discourage the usage of Result for
> whatever you want. For the rest of us, Result isn’t intended to replace
> throws or do/catch, but provide a way to accomplish things in a much more
> compact and sometimes natural way. As with any API it could be used
> stupidly. But frankly, what developers what to do to wrap their errors is
> up to them.
>

And it still is, with the Result implementations that are available to
third-parties today.

My concerns regarding Result aren't about my personal discouragement of its
use, but the *reasons* why I discourage its use. The Swift language very
deliberately draws a line between result outcomes that are return values
and error outcomes that are thrown, and it implements not only standard
library types but also language syntactic sugar to support those.

If someone wants to depend on a third-party Result<> to conflate successful
outcomes and error outcomes in their own code, that's absolutely their
right. But entry into the standard library has a much higher bar, and what
we're talking about here is adding a feature that now would give users two
disparate and incompatible ways (without explicit transformations, or other
syntactic sugar) of handling errors. That makes me uneasy from the point of
view of both an API designer and consumer, and just restating that it's a
common pattern and people want it doesn't address those concerns.




> Adding Result is just a way of blessing a result/error representation,
> since it has become a rather common pattern. If you’ve looked at the
> implementation I showed, you’ll see that there’s far more functionality
> than just a Result type, including API for converting back and forth from
> throwing functions, as well as functional transforms. Result is a
> complement to try do catch, not a replacement.
>
>
>
> Jon
>
>
>
>
> On Nov 2, 2017, at 2:48 PM, Tony Allevato <tony.allevato at gmail.com> wrote:
>
>
>
> On Thu, Nov 2, 2017 at 11:32 AM Jon Shier <jon at jonshier.com> wrote:
>
>> That’s been an argument against Result for 2 years now. The usefulness of
>> the type, even outside of whatever asynchronous language support the core
>> team comes up with, perhaps this year, perhaps next year, is still very
>> high. Even as something that just wraps throwing functions, or otherwise
>> exists as a local, synchronous value, it’s still very useful as way to
>> encapsulate the value/error pattern.
>>
>
> This is one of the parts that concerns me, actually. The beauty of Swift's
> error design is that function results denote expected/successful outcomes
> and thrown errors denote unexpected/erroneous outcomes. Since they are
> different, each is handled through its own language constructs, and since
> the language itself supports it (rather than being entirely type-based),
> you don't have the proliferation of unwrapping boilerplate that you have
> with Result<>.
>
> In our own code bases, I actively discourage the use of Result<> in that
> way, because it tries to cram both of those concepts into the
> expected/successful outcomes slot in the language. For asynchronous APIs
> that's somewhat unavoidable today, but if that's going to change, I'd
> rather the language focus on a way that's consistent with other error
> handling already present in Swift.
>
> Adding an API to the standard library is the core team saying "this is
> blessed as something around which we support APIs being designed." IMO, I'd
> prefer it if the language did *not* bless two disparate ways of
> communicating error outcomes but rather converged on one.
>
> IMO, "things aren't happening fast enough" isn't great motivation for
> putting something permanently into the standard library or the language
> without considering the context of other things going on around it. If
> you're going to propose something that overlaps with asynchronous APIs, it
> only helps your case if you can discuss how it can integrate—rather than
> collide—with those efforts.
>
>
>
>
>> That pattern will likely never go away. Additionally, having the Result
>> type in the standard library removes a source of conflict between all other
>> Result implementations, which are becoming more common.
>>
>>
>> On Nov 2, 2017, at 2:26 PM, Tony Allevato <tony.allevato at gmail.com>
>> wrote:
>>
>> Given that the Swift team is currently working on laying the groundwork
>> for asynchronous APIs using an async/await model, which would presumably
>> tie the throwing cases more naturally into the language than what is
>> possible using completion-closures today, are we sure that this wouldn't
>> duplicate any efforts there or be made obsolete through other means?
>>
>> In other words, while Result<> can be a very useful foundational
>> component on its own, I think any proposal for it can't be made in
>> isolation, but very much needs to consider other asynchronous work going on
>> in the language.
>>
>>
>> On Thu, Nov 2, 2017 at 11:15 AM Jon Shier via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> You don’t lose it, it’s just behind `Error`. You can cast out whatever
>>> strong error type you need without having to bind an entire type to it
>>> generically. If getting a common error type out happens a lot, I usually
>>> add a convenience property to `Error` to do the cast for me. Plus, having
>>> to expose an entire new error wrapper is just a non starter for me and
>>> doesn’t seem necessary, given how Result is currently used in the community.
>>>
>>>
>>> Jon
>>>
>>>
>>> On Nov 2, 2017, at 2:12 PM, Dave DeLong <swift at davedelong.com> wrote:
>>>
>>> I think I’d personally rather see this done with a generic error as
>>> well, like:
>>>
>>> enum GenericResult<T, E: Error> {
>>> case success(T)
>>> case failure(E)
>>> }
>>>
>>> And a typealias:
>>>
>>> typealias Result<T> = GenericResult<T, AnyError>
>>>
>>> This would require an “AnyError” type to type-erase a specific Error,
>>> but I’ve come across many situations where a strongly-typed error is *incredibly
>>> *useful, and I’d be reluctant to see that thrown away.
>>>
>>> Dave
>>>
>>> On Nov 2, 2017, at 12:08 PM, Jon Shier via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> Swift-Evolution:
>>> I’ve written a first draft of a proposal to add Result<T> to the
>>> standard library by directly porting the Result<T> type used in Alamofire
>>> to the standard library. I’d be happy to implement it (type and tests for
>>> free!) if someone could point me to the right place to do so. I’m not
>>> including it directly in this email, since it includes the full
>>> implementation and is therefore quite long. (Discourse, please!)
>>>
>>>
>>> https://github.com/jshier/swift-evolution/blob/master/proposals/0187-add-result-to-the-standard-library.md
>>>
>>>
>>> Thanks,
>>>
>>> Jon Shier
>>>
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>>
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20171102/2c59940e/attachment.html>