[swift-evolution] The Non-Exhaustive Enums proposal kills one of Swift's top features - change proposal

[Thomas Roughton]

Hi Slava,

I think we may be referring to different things. For whatever it’s worth, I agree with your reasoning on all the points you brought up. I also don’t think having a 'default: fatalError()’ case is a good idea because then a library change can cause crashes in a running version of an application.

What I mean by some sort of ‘complete switch’ statement is that it would be compiled as per a normal ‘switch’ but error at compile time if it’s not complete against the known set of cases as compile time. Assuming an enum with known cases [a, b] at compile time,

switch nonExhaustiveEnum {
	case a:
		print(“a”)
	case b:
		print(“b”)
	default:
		break
}

would be exactly equivalent to:

complete switch nonExhaustiveEnum {
	case a:
		print(“a”)
	case b:
		print(“b”)
	unknown:  // the ‘unknown’ case would only be required for non-exhaustive enums
		break
}

where the keywords ‘complete’ and ‘unknown’ are up for debate. If, however, the programmer wrote:

complete switch nonExhaustiveEnum {
	case a:
		print(“a”)
	unknown:
		break
}

the compiler would give an error that there are unhandled cases in the switch statement, whereas

switch nonExhaustiveEnum {
	case a:
		print(“a”)
	default:
		break
}

would compile without issue. If a user didn’t know about the existence of the ‘complete switch’ construct, they could just use normal ‘switch’ statements and miss out on the completeness checking.

Thomas

> On 24/12/2017, at 1:15 PM, Slava Pestov <spestov at apple.com> wrote:
> 
> 
> 
>> On Dec 23, 2017, at 3:47 PM, Thomas Roughton via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>> On 24/12/2017, at 9:40 AM, Cheyo Jimenez via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> What are your thoughts on `final switch` as a way to treat any enum as exhaustible?
>>> https://dlang.org/spec/statement.html#FinalSwitchStatement <https://dlang.org/spec/statement.html#FinalSwitchStatement>_______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>> 
>> I’d be very much in favour of this (qualms about the naming of the ‘final’ keyword aside - ‘complete’ or ‘exhaustive’ reads better to me). 
>> 
>> Looking back at the proposal, I noticed that something similar was mentioned that I earlier missed. In the proposal, it says:
>> 
>>> However, this results in some of your code being impossible to test, since you can't write a test that passes an unknown value to this switch.
>> 
>> Is that strictly true? Would it be theoretically possible for the compiler to emit or make accessible a special ‘test’ case for non-exhaustive enums that can only be used in test modules or e.g. by a ‘EnumName(testCaseNamed:)’, constructor? There is  potential for abuse there but it would address that particular issue. 
>> 
>> Regardless, I still feel something like a ‘final switch’ is necessary if this proposal is introduced, and that it fits with the ‘progressive disclosure’ notion; once you learn this keyword you have a means to check for completeness, but people unaware of it could just use a ‘default’ case as per usual and not be concerned with exhaustiveness checking. 
> 
> My general philosophy with syntax sugar is that it should do more than just remove a constant number of tokens. Basically you’re saying that
> 
> final switch x {}
> 
> just expands to
> 
> swift x {
> default: fatalError()
> }
> 
> I don’t think a language construct like this carries its weight.
> 
> For example, generics have a multiplicative effect on code size — they prevent you from having to write an arbitrary number of versions of the same algorithm for different concrete types.
> 
> Another example is optionals — while optionals don’t necessarily make code shorter, they make it more understandable, and having optionals in the language rules out entire classes of errors at compile time.
> 
> On the other hand, a language feature that just reduces the number of tokens without any second-order effects makes code harder to read, the language harder to learn, and the compiler buggier and harder to maintain without much benefit. So I think for the long term health of the language we should avoid ‘shortcuts’ like this.
> 
> Slava

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