[swift-evolution] Proposal: Add implicit/default else-behaviour for the guard statement
Erica Sadun
erica at ericasadun.com
Wed Dec 16 18:30:55 CST 2015
Count me in as a "no". Despite the redundancy, actually spelling out how the else clause leaves scope is valuable to reading the code.
-- E
> On Dec 16, 2015, at 4:40 PM, Radosław Pietruszewski via swift-evolution <swift-evolution at swift.org> wrote:
>
> I’m also against, for two reasons:
>
> - looking at my code, `return` and `return nil` are most common, but not super-overwhelmingly so. I’m all for common-sense default behavior, but here, I’m just not convinced it’s worth it. Likewise, I hate noise in code, but I’m not bothered by explicit `else` blocks at all.
> - guard is already confusing enough until you truly, fully get it. Having an `else` block makes it far more clearer that it’s a (specialized) branching operation. Having something implicitly return from my method sounds almost scary.
>
> — Radek
>
>> On 17 Dec 2015, at 00:26, Stephen Celis via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>> I'm opposed. I don't think `else { return }` is enough of a mouthful to add a default behavior that must be learned and reasoned with.
>>
>> Stephen
>>
>> On Wed, Dec 16, 2015 at 5:36 PM, Vester Gottfried via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> I just skipped through 200+ guard statements that return Void, nil or a value and no statement was inside a scope that can be exited with break.
>>
>> I am also thinking about the proposals regarding default values to return, but I am not sure if this would even be necessary. In my case 80% of all guard statements end with "else { return }" or "else { return nil }".
>>
>> On Wed, Dec 16, 2015 at 11:20 PM, Etan Kissling via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> If guard defaults to something context-dependent, it's also counterintuitive.
>>
>>
>> Me personally is fine with guard x > 0 returning from the function in this case.
>>
>> I use guard mainly as a glorified assert that allows safe exit from the function instead of crashing the program on fail.
>> If you think about it that way, it's perfectly reasonable that it returns in all cases.
>>
>>
>>
>> On the other hand, the implicit default else behaviour could only trigger if there is no outer scope that can be exited with break.
>> Maybe OP could post additional statistics of the number cases where "guard ... else { return }" is used inside a breakable scope.
>>
>>
>> Etan
>>
>>
>>
>>
>>
>>> On 16 Dec 2015, at 23:15, Vinicius Vendramini <vinivendra at gmail.com <mailto:vinivendra at gmail.com>> wrote:
>>>
>>> Bringing up a possible edge case:
>>>
>>> func foo() {
>>> while(…) {
>>> guard x > 0
>>> }
>>> }
>>>
>>> if guard defaulted to return even inside a while (as was suggested), this might be counterintuitive.
>>>
>>>> On Dec 16, 2015, at 5:06 PM, Etan Kissling via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>
>>>> Also +1 on default return, -1 on default continue / break, and -1 for removing braces
>>>>
>>>> Instead of VoidLiteralConvertible, one could extend on the idea with something that is not specifically tailored to nil, like
>>>>
>>>> func foo(x: Int) -> Int = 5 {
>>>> guard x < 10 // Would return default 5 for x >= 10
>>>>
>>>> if x > 5 {
>>>> return // Would return default 5
>>>> }
>>>> return x
>>>> }
>>>>
>>>>
>>>>
>>>> Etan
>>>>
>>>>
>>>>> On 16 Dec 2015, at 19:39, Ian Ynda-Hummel via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>
>>>>> I am also +1 for implicit return, but -1 on continue/break for the reasons already stated.
>>>>>
>>>>> I'm -1 for removing braces for one-liners. I think maintaining braces around blocks helps distinguish them from expressions. For example,
>>>>>
>>>>> guard x < 10 else return
>>>>>
>>>>> would catch me off guard (pardon the pun).
>>>>>
>>>>> I think I'm -1 on VoidLiteralConvertible, but I'm somewhat undecided. I think that separating the return value from the actual return point could lead to a lot of confusion, and would subsequently also make it easy to accidentally return the default value when you didn't intend to as the compiler wouldn't complain about a missing return value. I don't think I have totally convinced myself that the latter is a non-trivial problem, but I thought it was worth mentioning.
>>>>>
>>>>> On Wed, Dec 16, 2015 at 11:59 AM ilya via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> Actually I thought about VoidLiteralConvertible some more and now I think if we include it the only nontrivial case in the standard library case should be Optional.Nonel. Empty arrays and dictionaries are different
>>>>> from nothing, so it's best to always return them explicitly.
>>>>>
>>>>>
>>>>>
>>>>> Oh, and it would help with default values, e.g.
>>>>>
>>>>>
>>>>>
>>>>> var something:Something?
>>>>>
>>>>> // where does the default value come from? VoidLiteralConvertible!
>>>>>
>>>>>
>>>>>
>>>>> I want all default values for String in this scope be "none":
>>>>>
>>>>>
>>>>>
>>>>> private extension String: VoidLiteralConvertible { ... return "None" ... }
>>>>>
>>>>> On Wed, Dec 16, 2015 at 10:49 ilya <ilya.nikokoshev at gmail.com <mailto:ilya.nikokoshev at gmail.com>> wrote:
>>>>> +1 on default return
>>>>>
>>>>>
>>>>>
>>>>> -1 on default continue or break, this is ambiguous.
>>>>>
>>>>> Even inside switch it's not clear if guard should break or return, so let's not make people guess. .
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Also can we stop requiring braces for simple one-liners:
>>>>>
>>>>>
>>>>>
>>>>> guard x<10 else return 5
>>>>>
>>>>>
>>>>>
>>>>> As for default return values, we could create a VoidLiteralConvertible, so that default return automatically becomes return nil or return [] in an Optional or Array context respectively. As a bonus, it will be technically possible to override this behavior
>>>>> inside a specific function scope.
>>>>>
>>>>>
>>>>>
>>>>> > swift would provide a warning that the guard statement needs an else block
>>>>>
>>>>>
>>>>>
>>>>> In this specific case the compiler basically has to guess, so an error seems more appropriate.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Ilya.
>>>>>
>>>>> On Wed, Dec 16, 2015 at 02:53 Vester Gottfried via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> I find myself writing the same else blocks for guard statements over and over again, so it might be reasonable to think about a default behaviour.
>>>>>
>>>>> In a current project I found 217 guard statements from which 183 have repetitive else clauses
>>>>>
>>>>> From which:
>>>>> 131 end with "else { return nil }"
>>>>> 44 "else { return }"
>>>>> 6 "else { continue }"
>>>>> 2 "else { break }"
>>>>>
>>>>> My proposal would be to make the else block optional and define a default behaviour.
>>>>>
>>>>> For example:
>>>>>
>>>>> func foo(x: Int) {
>>>>> guard x < 10
>>>>> ...
>>>>> }
>>>>>
>>>>> swift would implicitly add "else { return }"
>>>>>
>>>>> --
>>>>>
>>>>> func foo(x: Int) -> Int? {
>>>>> guard x < 10
>>>>> ...
>>>>> }
>>>>>
>>>>> swift would implicitly add "else { return nil }"
>>>>>
>>>>> --
>>>>>
>>>>> for i in 0..<10 {
>>>>> guard i%2 == 0
>>>>> }
>>>>>
>>>>> swift would implicitly add "else { continue }"
>>>>>
>>>>> --
>>>>>
>>>>> switch {
>>>>> case a :
>>>>> guard x != y
>>>>> case b :
>>>>> ...
>>>>> }
>>>>>
>>>>> swift would implicitly add "else { break }"
>>>>>
>>>>> --
>>>>>
>>>>> func foo(x: Int) -> Int {
>>>>> guard x < 10
>>>>> ...
>>>>> }
>>>>>
>>>>> swift would provide a warning that the guard statement needs an else block
>>>>>
>>>>> --
>>>>>
>>>>> Possible advantages
>>>>> - Less code to write
>>>>> - visually cleaner
>>>>> - In code with multiple guard statements you would not have to repeat the else block
>>>>>
>>>>>
>>>>> Possible Disadvantages
>>>>> - Different behaviour in different contexts (func/return, for/continue, switch/break, …) needs to be learned and understood
>>>>> - programmers might forget that guard + else {} is an option
>>>>> _______________________________________________
>>>>> 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>
>>>>> _______________________________________________
>>>>> 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>
>>>>> _______________________________________________
>>>>> 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>
>>>> _______________________________________________
>>>> 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>
>>>
>>
>>
>> _______________________________________________
>> 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>
>>
>>
>>
>> _______________________________________________
>> 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>
>>
>>
>> _______________________________________________
>> 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>
>
> _______________________________________________
> 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>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20151216/24972308/attachment.html>
More information about the swift-evolution
mailing list