[swift-evolution] Testing enum cases with associated values
Pierre Monod-Broca
pierremonodbroca at gmail.com
Wed Jan 18 10:57:16 CST 2017
The last time I was in this situation, I resolved it by having a shadow enum with the same cases but without any associated value.
I also created shadow enums for error enums. The shadow enum wouldn't have any associated value, and Int as RawValue, and I would also use it as an error code when converting to NSError.
But it's not really fun to maintain.
Pierre
> Le 18 janv. 2017 à 05:51, Andy Chou via swift-evolution <swift-evolution at swift.org> a écrit :
>
> I see your point about conformances. In my example, AuthenticationResponse isn't a generic type, so the conformances spec won't apply.
>
> I'll go out on a limb and say that 80%+ of the use cases for equality will be to distinguish enum cases, not the associated values.
>
> I do like your proposal though. In the thread talking about it someone mentioned a 'derived' keyword to specify conformance and derivation simultaneously. I like the idea of being able to say:
>
> enum AuthenticationResponse: derived Equatable { ... }
>
> Still, even with derived conformance, it can still be very useful to be able to test against specific enum cases when the associated values aren't Equatable. Maybe this example would make it clearer:
>
> ```
> enum OpaqueResponse {
> case success(OpaqueResult)
> case failure
> }
> ```
>
> If OpaqueResult is from a library module and its implementation uses private variables, it may not be easy to make it conform to Equatable. Yet, it would be nice to be able to say:
>
> ```
> let result: OpaqueResponse = request()
> if request == .failure { ... }
> ```
>
> A more realistic example comes from the world of Rx, where the original issue I have came from:
>
> ```
> let result: Observable<OpaqueResponse> = request()
>
> result.filter { $0 == .failure } ...
> result.filter { $0 == .success } ...
>
> // The best we can do today looks more like:
> result.filter { if case .failure = $0 { return true} else { return false } }
> result.filter { $0.isFailure } // but we have to define isFailure ourselves
>
> // There's a similar issue with assertions
> if case .failure = result { assert(false) }
> assert({ if case .failure = result { return true } else { return false } }())
> ```
>
> I agree it's less of an issue with test cases. The issue arises when we want a Bool valued expression...
>
> Andy
>
>> On Jan 17, 2017, at 6:38 PM, Tony Allevato <tony.allevato at gmail.com> wrote:
>>
>> Conditional conformances doesn't solve enum equality though, because there likely won't be a way to utter "enum Foo : Equatable where <all types across all associated value payloads are Equatable>" with the generics syntax that's available, and conformance alone wouldn't be able to derive the implementation. It'll require some work on the compiler's part to generate the right implementation—I mentioned a draft proposal I wrote a while back for auto-equality of enums and structs where all the components are equatable <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad> in another thread, but as Robert mentioned, the missing piece is how users opt in/out of it.
>>
>> If you just want to check the case of an enum in a test, what about this?
>>
>> enum Foo {
>> case bar
>> case baz(Int)
>> }
>> let foo = Foo.baz(5)
>> guard case .baz = foo else { XCTFail("expected baz"); return }
>>
>> The "return" being required isn't ideal because XCTFail doesn't return Never, but other than that it's not *horrible*. You can do whatever pattern matching you need to use or ignore associated values as part of your comparison.
>>
>>
>>> On Tue, Jan 17, 2017 at 5:59 PM Andy Chou via swift-evolution <swift-evolution at swift.org> wrote:
>>> Yes, here's a reference to the conditional conformance proposal which was accepted:
>>>
>>> https://github.com/apple/swift-evolution/blob/master/proposals/0143-conditional-conformances.md
>>>
>>> But as I mention in my post, there are cases it doesn't handle. Specifically, when the associated types for an enum aren't equatable themselves, it's still possible to define equality on the enum cases without associated values.
>>>
>>> Andy
>>>
>>>> On Jan 17, 2017, at 5:45 PM, Robert Widmann <devteam.codafi at gmail.com> wrote:
>>>>
>>>> Automatic “equatability” of aggregates that contain equatable members has been discussed on this list quite a few times. (I think I had a branch at one point that was exploring this kind of deriving mechanism… It seems to be lost to the sands of time). Everybody seems to agree that it’s a worthwhile feature, but there needs to be thought put into how it is exposed to the end user. e.g. Should we continue with silently implementing these protocols if we can, or should there be some kind of annotation to tell the compiler to only synthesize what the user wants?
>>>>
>>>>> On Jan 17, 2017, at 7:15 PM, Andy Chou via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>
>>>>> Enums with associated values can be very useful in Swift, but once you add associated values you lose some properties, especially equality:
>>>>>
>>>>> ```
>>>>> enum AuthenticationResponse {
>>>>> case success
>>>>> case alert(Alert)
>>>>> case reauthenticate
>>>>> }
>>>>> ```
>>>>>
>>>>> Testing for a specific case requires a switch statement or the if pattern match syntax:
>>>>>
>>>>> if case .success = response { … }
>>>>>
>>>>> But while this works well for control flow, it doesn’t work well for cases where we want a Bool, such as assert(). There are also common situations with lists and libraries like RxSwift where a filtering function uses a Bool valued closure. In these situations the best we can do is write functions like:
>>>>>
>>>>> ```
>>>>> enum AuthenticationResponse {
>>>>> case success
>>>>> case alert(Alert)
>>>>> case reauthenticate
>>>>>
>>>>> var isSuccess: Bool {
>>>>> if case .success = self {
>>>>> return true
>>>>> } else {
>>>>> return false
>>>>> }
>>>>> }
>>>>>
>>>>> var isReauthenticate: Bool {
>>>>> if case .reauthenticate = self {
>>>>> return true
>>>>> } else {
>>>>> return false
>>>>> }
>>>>> }
>>>>>
>>>>> var isAlert: Bool {
>>>>> if case .alert(_) = self {
>>>>> return true
>>>>> } else {
>>>>> return false
>>>>> }
>>>>> }
>>>>> }
>>>>> ```
>>>>> Any suggestions better than writing out each of these functions explicitly?
>>>>>
>>>>> The conditional conformances proposal coming in Swift 4 solves some of this issue, but not completely. If Alert isn’t Equatable, it is still useful to test whether the result is .success. For example:
>>>>>
>>>>> assert(response == .success)
>>>>>
>>>>> This is perfectly intelligible and I would argue that equality should be defined for enums with associated values omitted:
>>>>>
>>>>> assert(response == .alert)
>>>>>
>>>>> Here we are ignoring the associated values, and merely checking if the enum case is the same.
>>>>>
>>>>> Andy
>>>>>
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>
>>>
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>
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