[swift-evolution] Replace the override keyword by 'extend' and 'replace' or add an annotation like @SuppressSuperCall

Tue Feb 16 08:51:45 CST 2016

Agreed - attempting to enforce the ordering is too fiddly and complicated, IMO. If the superclass must ensure a specific order, then it should do so itself by first calling a final function that then calls the overridable function at the right time.

Personally I’d like to see the syntax using the “required” keyword that already exists for constructors - so something more like this:

class Base {
  required(super) foo() {
	// etc
  }
}

This would then require any subclasses to call super.foo() if they override foo, but the subclass would not need to do anything new - just use the existing override syntax (with the complier ensuring super is called if it is required).

Then I’d like to see something like this:

class Base {
  required foo()
}

Which implicitly turns “Base” into an abstract base class that cannot be instantiated on its own, but requires a subclass supply a body for the required missing functions.

l8r
Sean

> On Feb 16, 2016, at 8:43 AM, Jean-Daniel Dupas via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I disagree with the introduction of a method to specify if super must be call first or last. Defining that super must be call is fine, but I don’t see why the operation order should be enforced.
> 
> If the super class declares a method overridable but require that some code must be perform first, it can simply execute that code before calling the overridable method instead of putting it in the super class definition then force the subclass to call super first.
> 
> Moreover, I’m not fond of declaring that requirement in the overriding classes. That is the superclass that should define if a super implementation is required, not the children classes.
> 
> 
>> Le 16 févr. 2016 à 15:21, Ross O'Brien via swift-evolution <swift-evolution at swift.org> a écrit :
>> 
>> Jean-Daniel: can you clarify which aspect you're disagreeing with?
>> 
>> For example: should standard library types prefer the 'override' form which enforces calling super but doesn't enforce when it is called?
>> Perhaps 'override(before)' should be an indication that, if the overriding method doesn't explicitly declare the super call, then it should be called implicitly at the start of the function?
>> 
>> On Tue, Feb 16, 2016 at 2:12 PM, Jean-Daniel Dupas via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> -1 for enforcing the super call place.
>> While we usually call super at the beginning of viewDidLoad for instance, there is absolutely no need to enforce it and it may result in problem when some work have to be done before the super method is executed.
>> 
>> I have some use cases that rely on performing setup before calling the super.viewDidLoad().
>> 
>> > Le 15 févr. 2016 à 23:06, Alexey Demedetskiy via swift-evolution <swift-evolution at swift.org> a écrit :
>> >
>> > Hi
>> >
>> > I would like to suggest you to extend your proposal.
>> >
>> > In my practice, overriding super functions can have several semantics.
>> > 1) Replace - simple case for abstract classes which implementation do nothing, or throw an exceptions.
>> > 2) After super - things like viewDidLoad and viewWillAppear, setUp etc. All cases where super expect to be called before child code.
>> > 3) Before super - opposite to 2.
>> > 4) Override - no rules about order, but super call must be done.
>> >
>> > So code can look like:
>> >
>> > override(after) func viewDidLoad() {
>> >    // super.viewDidLoad() <— no need to call super at first line.
>> >    // child code
>> > }
>> >
>> > override(before) func tearDown() {
>> >    // clean code
>> >    // super… inserted by compiler
>> > }
>> >
>> > override(instead) func loadView() {
>> >    // super.loadView() <— marked as an error with appropriate fix-up to remove instead modifier
>> > }
>> >
>> > override func refillHealthBar() {
>> >   // absent call to super will cause an error with fix-up to add (instead) modifier
>> > }
>> >
>> > I am not sure about exposing this in a public interface and limit child override options.
>> >
>> > But in general - what is your thoughts about this approach to problem that you mention?
>> >
>> >
>> >> Hi!
>> >>
>> >> I would like to suggest to replace the override keyword for functions by something like extend and replace or to add an annotation like @SuppressSuperCall (don’t know a good name for it).
>> >> The reason for this is, that it might happen, that one forgets to call the super’s implementation in an overridden function or if one reads the code it might not be obvious why the super’s implementation is not called:
>> >>
>> >> class View {
>> >> func viewDidLoad() {
>> >> // does something
>> >> }
>> >> }
>> >>
>> >> class Button: View {
>> >> override func viewDidLoad() {
>> >> super.viewDidLoad() //<— this might be forgotten
>> >> // do something other
>> >> }
>> >> }
>> >>
>> >> The compiler will accept if one overrides a superclass’s function but does not call the superclass’s implementation which is often ok. The developer should clearly state that he doesn’t want to call the superclass’s implementation, otherwise the compiler should throw an error.
>> >>
>> >> // Example for extending a function
>> >> class Button: View {
>> >> extend func viewDidLoad() {
>> >> super.viewDidLoad()
>> >> // do something
>> >> }
>> >>
>> >> extend func viewDidAppear() {
>> >> // do something
>> >> } //<— the compiler should throw an error here.
>> >> }
>> >>
>> >> // Example for replacing a function
>> >> class Geometry {
>> >> func volume() ->Double {
>> >> return 0;
>> >> }
>> >> }
>> >>
>> >> class Cube: Geometry {
>> >> var length: Double = 0.0
>> >> replace func volume() ->Double {
>> >> let v = length * length * length
>> >> return v
>> >> }
>> >> }
>> >>
>> >> Cheers,
>> >> Florian
>> >>
>> >>
>> >>
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