[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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