On Thursday, 18 February 2016, Dennis Weissmann via swift-evolution < swift-evolution@swift.org> wrote:
Can’t the compiler generate them?
@requiredSuper(end) func foo3()
If the compiler can tell you that you need to call super() at the very
beginning (or the very end) of a method, it should be able to do that for
you.
What I imagine is a syntax like this:
Note: I’ve changed the syntax to # because I think this is compiler-magic
and as Chris mentioned the following in the property behavior thread:
Where [lazy] is currently used, could the syntax instead be
#behavior(lazy)? That prevents a possible future naming clash, keeps the #
meaning compiler-magic, and doesn't use the , which is contentious.
class Base {
#requireSuperStart // nitpicking, but I’d like #requireSuperAtBeginning
actually better
func mustCallSuperMethod() {
// Some code
}
}
class Subclass: Base {
// All subclasses automatically (implicitly) inherit #requireSuperX,
this is visible across module boundaries
override func mustCallSuperMethod() {
// Do your setup
}
}
The compiler would inject (invisible to the dev) the super() call at the
very beginning. The same is true for #requireSuperEnd.
I don’t think we need a #requireSuper because it means we don’t care when
it’s called so the compiler can decide where to put it.
The only problematic point might be a super call with arguments but I have
never ever changed the argument of a super call (e.g. in
viewDidAppear(animated: Bool) I never called the super implementation
other than super.viewDidAppear(animated)).
- Dennis, who is sick => please forgive any mistakes or oversights 
On Feb 17, 2016, at 11:26 PM, Kyle Sherman via swift-evolution < > swift-evolution@swift.org> wrote:
Thanks for the replies.
Kenny: After thinking about it more, discussing with Peter, and looking
Haravikk’s comments, I think the best thing would be for this to be a
warning as suggested. I respectfully disagree that as a library creator you
would not be able to know that a call to super should be required. A
perfect example of this is the one stated in the proposal: viewDidLoad,
viewWillAppear, etc. In these cases, the library writers know that the
super version must be called and no matter what the subclasser does, they
will not be able to have correct behavior without calling super. This is
present in many places throughout the iOS SDK as example. In the static
analyzer in Xcode, for ObjC code, it warns when the developer doesn’t call
super in certain cases. Having these annotations would allow for developers
to specify this for their own code. Being able to suppress the warning
could also be a good feature, but I definitely don’t feel it would be
necessary for the implementation of the feature.
Haravikk: I don’t agree with using “(required)” as that will be a sort of
overloaded usage of the keyword. Also, I think that simply not specifying
anything would be a better way to go about it. So, @super would mean you
require the super method to be called, and simply not having an annotation
means that you do not have to call the super method. Peter and I have a
separate proposal that we will put up here soon for extending the
“required” keyword for all methods that I think will solve the problem of
abstract classes. I don’t think that we should conflate that issue, which
we feel is a separate issue, in with the issue of requiring the super
method to be called. Along the same lines, final by default was already
being discussed much earlier and I believe there was already a conclusion
to that saying it would not be implemented. I think the final keyword
should stay separate, especially since it is a compiler error vs this which
we are now saying would be a warning.
-Kyle
On Feb 17, 2016, at 12:31 PM, Haravikk via swift-evolution < > swift-evolution@swift.org> wrote:
Since this proposal is along the same lines as another current thread I’m
contributing to I’m of course very much for the basic functionality ;-)
From that other discussion my current preference is towards either an
attribute named @super, or allow the super keyword to be used in the method
declaration. So a declaration might look like:
@super(required)
func someMethod() { … }
Or (if you prefer):
super(required) func someMethod() { … }
The main ones needed are required and optional, with optional being what
we have now, and required simply ensuring that the super method is called
(no requirement on where). Options for before and after will be more useful
if we get abstract classes whose sole purpose is to be extended, as they
may have more specific requirements. The other possibly useful option would
@super(replace), in which case super may not be called by extending methods
at all, as it’s implementation may be very tightly coupled to the specific
implementation at that level, thus requiring a sub-class to re-implement
it; the parent method could also be some kind of stub for a feature it
doesn’t support, but which was required by a protocol for example.
Howard Lovatt also mentioned another interesting extension to the idea
which is that methods would effectively become final by default, requiring
a @super attribute if they are to be overridable, which I think would be
good for ensuring that only classes designed with extension in mind can
actually be extended. For this reason the @super attribute could also be
used on a class to set a new default for all methods. If we go with this
aspect then the final keyword would probably be moved into the @super
attribute for methods (properties would still have access to it though I
think).
On 17 Feb 2016, at 19:55, Kenny Leung via swift-evolution < > swift-evolution@swift.org> wrote:
There is just no way to know what may be required by clients of a
framework when it comes time to write actual shipping code. To require
users to call super - and to even extend that to knowing whether they
should call super at the beginning or end of their methods is too
restrictive. If this feature were to go forward, I would limit it to a
warning. It may belong more in a linter.
A lot of people have reacted negatively to the idea of before and after
requirements, but I think it’s important to note that they’re unlikely to
be commonly added; in most cases it won’t matter when the super method is
called so required or optional should be used as appropriate. Before and
after requirements are more useful for abstract or abstract-style classes
that offer partial implementations specifically designed to be extended, in
which case the order may be more important. Most of the time the deciding
factor will be whether your method performs some kind of updates to cached
values, in which case it will be important that it is called; the
requirement can also be used to serve as a reminder to read the
documentation about how exactly the parent method should be used by
extending classes, if it has any more unusual caveats.
I’ve been using IntelliJ IDEA a lot lately, and they basically have a live
linter that they call “code inspections”. I like this a lot. It goes waaay
beyond compiler-level warnings to offering you suggestions to improve your
code, finding sections of duplicated code, anything under the sun. They
also allow you to suppress any individual warning by putting an @suppress
in your code. Maybe Swift could benefit from another layer like this in
general, where you could be warned about a lot of stuff, but not be locked
into it.
It could make sense to have an option for whether breaking a requirement
produces a warning or an error? For example @super(required, warn) will
warn the developer that they are going against the parent class’
requirement to include the super method, but won’t actually stop them from
doing so if they really want to. This could provide a useful middle-ground
between optional and required, plus if the code of the parent class isn’t
under your control it gives you a fallback until there’s an update if the
warning behaviour were the default, with @super(required, error) style
declarations for developers who really know what their parent class needs
from sub-classes.
It’s also worth considering that most code will be under your control, so
you’ll just need to tweak (or add) the requirements, in the latter case it
forces you to consider how that class may be extended which I think is a
good thing.
On Feb 17, 2016, at 10:02 AM, Kyle Sherman via swift-evolution < > swift-evolution@swift.org> wrote:
I just saw that there was a discussion started about this topic just
recently while I was developing this idea with my colleague Peter Livesey.
So, I figured I would submit this proposal for discussion.
The link to the original discussion is here:
[swift-evolution] Replace the override keyword by 'extend' and 'replace' or add an annotation like @SuppressSuperCall
The subject was: “Replace the override keyword by ‘extend’ and ‘replace’
or add an annotation like @SuppressSuperCall”
-Kyle
# Enforcing Calling Super
* Proposal: [SE-NNNN](
https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md
)
* Author(s): [Swift Developer](https://github.com/swiftdev\)
* Status: **Awaiting review**
* Review manager: TBD
## Introduction
Many times when creating a subclass the superclass has reasons for certain
overridden methods to call the superclass’s version of the method. This
change would enforce that the subclass called the superclass's method in
its overridden version at compile time. Also, it would optionally enforce
that the superclass's version would be called before any other
implementation in the method (similar to initialization rules).
Swift-evolution thread: [link to the discussion thread for that proposal](
https://lists.swift.org/pipermail/swift-evolution\)
## Motivation
A concrete example of the type of problem this solves can be taken from
simple iOS code. When creating a subclass of UIViewController, you often
need to override methods like viewDidLoad or viewWillAppear. You are
supposed to call super.viewDidLoad or super.viewWillAppear, respectively,
in your overridden implementation. If you don't, you will have undefined
behavior and run into issues. Of course, this type of situation can be
extrapolated to any class created in Swift.
Currently, the only way this can be enforced is by commenting the
superclass's code and making a note in the documentation. Quite obviously
this can cause many issues as mistakes can be made by new developers quite
easily who didn't look at the documentation for the method or even seasoned
developers who simply overlooked this small detail.
## Proposed solution
The solution proposed here would be to use an annotation similar to
@available and @noescape in order to convey this information. Optionally,
the developer can also choose to specify that the super method must be
called as the first line or last line of the overridden method.
The compiler would use the information from the annotation to ensure that
any overridden version of the method must call super at the appropriate
time according to the information given in the annotation. The compiler
would also need to ensure that any method that was going to use this
annotation had the same access control level as the class that contains it.
This solution will be much safer than what is currently available, because
there is currently no way to enforce super being called in an overridden
method. This bug happens constantly for iOS developers.
## Detailed design
A possible implementation of this may look like this:
class MyClass {
@requiredSuper func foo1() { }
@requiredSuper(start) func foo2() { }
@requiredSuper(end) func foo3() { }
}
Now, if the developer were to create a subclass and not call the super
method, the compiler should display an error. The errors that should be
displayed should be similar to:
• Overridden method must call the superclass’s implementation
• Overridden method must call the superclass’s implementation as the first
line of the method.
• Overridden method must call the superclass’s implementation as the last
line of the method.
for the cases of `@requiredSuper`, `@requiredSuper(start)`, and
`@requiredSuper(end)` respectively.
The compiler would also need to display an error in this case where the
access control of the method is stricter than that of the class:
public class MyClass {
@requiredSuper func foo() { }
}
The compiler should show an error, such as “A method using @requiredSuper
must have access control set to be at least as accessible as the class that
contains it”.
## Impact on existing code
Implementation of this feature by the developer is completely optional.
Therefore, existing code will be unaffected and no migration of code will
be necessary. However, when APIs are updated to use this new feature, some
code will not compile if the developer did not use the APIs correctly. This
should be a welcomed compilation error as it will result in less buggy code
at runtime. The impact of this change is similar to adding nullability
annotations to Objective-C.
It will be impossible to migrate code automatically, because this
information cannot be derived in any way aside from reading comments if and
only if the API author documented it.
## Alternatives considered
The alternative would simply be to not implement this feature.
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