[swift-evolution] Adding Result to the Standard Library

[Tony Allevato]

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
>>
>>
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>>
>>
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>
>
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