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

[Pierre Monod-Broca]

Looking at the numbers it would make sense mostly to return from a function. And I agree with David that control flow in loops and switches should be explicit.

So maybe a function could declare a default behavior for all its `guard`

Pierre

> Le 16 déc. 2015 à 01:42, David Waite via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> 
>>> On Dec 15, 2015, at 4:53 PM, Vester Gottfried via swift-evolution <swift-evolution at swift.org> wrote:
>> <snip>
>> 
>> 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 }”
> 
> That makes sense
> 
>> --
>> 
>> func foo(x: Int) -> Int? {
>> ​    ​guard x < 10
>> ​    ​…
>> }
>> 
>> swift would implicitly add "else { return nil }”
> You would need to decide how this plays into the type system 
> - is Optional a special case
> - or, will it work with anything NilLiteralConvertible (Optional, ImplicitlyUnwrappedOptional, Pointer types, Selectors)
> 
> I thought of using the no-argument initializer, but that creates too much default behavior (empty arrays, zero doubles, etc). 
> 
> Plus, it is inconsistent with variable declarations, which require explicit initialization before use.
> 
>> --
>> 
>> 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 }"
> 
> -1 . Flow control functions need to be explicit, else I risk having (and assuming) the wrong behavior when I copy and paste code from one context to another.
> 
> Plus, why would I expect a guard in a for loop to break or continue as a default behavior? I personally know I would constantly think this guard as ‘does what I mean’, while in the debugger it seems to be doing exactly the opposite of what I assumed, every time, regardless of my assumption ;-)
> 
>> 
>> func foo(x: Int) -> Int {
>> ​    ​guard x < 10
>> ​    ​...
>> }
>> 
>> swift would provide a warning that the guard statement needs an else block
> 
> Iff your proposal was to call the no-argument initializer, this would return 0. For the other options this would need to return an error, not a warning. 
> 
> -DW
> 
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