This, is actually the wrong conclusion. The entire purpose of the “inner class” is to provide value semantics while maintaining effeciency of implementation.
Is that an opinion? Or was there a meeting when the "entire purpose" of inner classes was established and I wasn't invited? Sure, they can be used for efficiency, but they can also be used for other purposes, including inner mutation.
Just for starters, Mike Ash seems to have uncovered that inner classes are used inside the standard library to provide destructors:
Destruction can be solved by using a class, which provides deinit. The pointer can be destroyed there. class doesn't have value semantics, but we can solve this by using class for the implementation of the struct, and exposing the struct as the external interface to the array.
So to say that the "entire purpose" of inner classes is efficiency is I think plainly false; they are used for many reasons. Efficiency may be one.
Moreover, following Mike Ash's outline in which Swift.Array
calls malloc
and free
(read: wraps the kernel, read: mutates the free block chain), Swift.Array
is directly analagous to my KernelSender
. They both hide inner mutation "somewhere else" inside a struct, by moving the mutation to a place where you don't see it. So not only is inner-mutation-inside-structs an actual thing, it's being used inside the standard library.
I agree with you that inner mutability "feels" wrong in the CanSend
example. But the suggestion that inner mutability should never ever be used is wrong, because of Swift.Array
. So we need some rule to distinguish when inner-mutability-with-structs is bad from when it is okay. If the rule was "never do any mutation inside a struct" then we would not have value-typed-arrays to begin with.
(In fact, we didn't have value-typed-arrays to begin with. I have a serious theory that it was exactly the rigid application of "never have inner mutation" inside Swift that initially led to the silly behavior. Recall that in early betas, the value/reference behavior of arrays used to hinge on whether or not the operation caused a resize--which is exactly those cases in which the array implementation needed to mutate the free block chain.)
No, do not write the “struct with inner class wrapper” so you can simply make it pseudo-mutable.
I will look forward to the patches you will submit to the standard library on this topic when it lands on GitHub later this year.
You see, MockSender does indeed have a lifecycle; it has a history of all messages sent that varies over time.
A lifecycle is not merely a description of something that varies over time. Lifecycle here is
In object-oriented programming (OOP), the object lifetime (or life cycle) of an object is the time between an object's creation and its destruction.
...and this is what the SO poster giving the advice meant: that if our type "has a lifecycle" (read: needs a destructor, much like Mike Ash's Array), then it should be a class, because only classes have destructors. I agree with you that this rule would be stupid (just for starters, it forbids Swift.Array
), but let's be clear about what kind of stupid it is. He is not making some broad point about things that change over time, he is talking about the implementation details of struct's deinit
.
Even if he were saying that things that change over time should be classes, that would be wrong. We can and do implement value types that change over time (see: the mutating func
) so if we say that whether to use a value type hinges on whether something changes over time would be inconsistent with e.g. the standard library.
This solution does exactly what we want. It also does it by maintaining the protocol can be conformed by immutable value-types and my mutable reference types. This is a validation of the “Structs Philosophy™”; `MockServer is not a value-type, don't try and make it one.
I agree with the solution, but it is hardly a validation of the Structs Philosophy™. If we have two choices and the oracle says "take this path and if it doesn't out work (like it won't most of the time) try the other one" then it is a shitty oracle. You had one job.
We went down an initial path (implementing CanSend with a struct) which was a dead end. In this particular case (because it is a contrived example that fits on our screen) the path to the dead-end was short, but in the general case, long paths to dead-ends are possible, and they motivated the original essay. An algorithm that gives us the wrong-answer-first is a shitty algorithm.
Even if we adopt the rule "always chose classes" (which I do not advocate, but I know smart people who do), that would be strictly better than the Structs Philsophy™ because "most custom data constructs should be classes" (Apple) and an oracle that picks the most likely path is far superior to an oracle that picks the unlikely path first.
Then of course there's the oracle I actually advocate (the Jeff Trick) which I think agrees with (nearly) all of our intuitions about reference and value types and only rarely (if ever?) sends us down dead ends.
I think this is the real dispute behind the whole disagreement. What, specifically, comprises the contract? What does that mean?
In the original example, we had this:
We have already agreed that conforming to this protocol with a mutating class is a reasonable thing to do. Since we agree on that, any consumer that uses this protocol must assume mutation is at least possible:
Because that is true, I do not think there is any such thing as "break[ing] the mutability contract of a protocol". A protocol does not guarantee to callers that functions are non-mutating.
I think what you want is
but Swift does not have that feature. Maybe it should; although I think there are edge cases that are non-trivial to solve. But it doesn't, and so there is no 'contract of non-mutability' being provided to clients that we could break even if we wanted to.
Not bad at all. For example
NSLocale
is implemented by a class for historical reasons but its animal spirit is very much a value type. In 2015 I think a struct with inner mutability is clearly the right way to re-implement this API, and the other ways are all obviously wrong. If you disagree I would be fascinated to hear competing proposals on this.That is a really good point, thanks.
Well here we come back to: "what is the contract?" Because in my mind, there is no contract of non-mutability in a protocol. Swift does not even allow such a thing to exist.
But even if we try to infer one, I think the specific example in the post is surprisingly resistant to "naive" definitions of immutable contracts. Looking back at the example:
MockSender
mutatessentMessages
, but that is not part of theCanSend
contract. So considering the client caseThe client cannot know–and in fact has no way to find out–that the function actually mutated anything. Only certain privileged callers, who know the details of
MockSend
in particular, could even find out that any mutation has taken place.If a contract breaks in the forest, and nobody's around to see it, is it really broken?