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
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
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.