[swift-evolution] Proposal: Add implicit/default else-behaviour for the guard statement

Radosław Pietruszewski radexpl at gmail.com
Wed Dec 16 17:40:09 CST 2015


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> 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
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