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Why isn't mutable a subtype of immutable, or vice versa?

12 points ibobev 1 day ago 11 comments

gus_massa 12 hours ago | parent

Yep, I wondered that for a few optimizations in Racket. It's harder than it looks, probably something about covariant or contravariant types, I gave up.

comrade1234 1 hour ago | parent

Isn't NSMutableArray a subclass of NSArray in a few languages?

catoc 1 hour ago | parent

Yes, that’s ObjC, any NSMutableArray is an NSArray (and an NSObject) - classes passed by reference.

Swift is completely different. Standard arrays are structures, always mutable - value types passed by value

bartvk 35 minutes ago | parent

Good one, yes. It was like that in Objective-C, and in the early versions of Swift. I know you know this, but it's useful to summarize for myself:

Basically inheritance is the wrong tool for this kind of stuff. NSMutableArray inherits from NSArray, so it can be passed to anywhere NSArray is expected (upcasting).

So you design your classes and expect them to be immutable, but you can't use NSArray anywhere. Because otherwise, it'll be mutable after all. You can do a runtime check as a workaround.

(I truly believe OOP should only be taught in computer science as a relic).

nicky0 10 minutes ago | parent

"Was" like that? .... Some of us still use Objective-C!

zkmon 55 minutes ago | parent

The fact that it requires so much explanation, indicates the level of degradation in the reasoning ability of the audience. A value of a subtype shall deliver all of the expectations of its super type, because it is wearing both the hats of super type and sub type.

Shorel 39 minutes ago | parent

Not all all. It merely indicates that this is an strict logic approach to the topic.

https://commonplacefacts.com/2022/07/27/principia-mathematic...

applfanboysbgon 28 minutes ago | parent

What part of GP's one-sentence explanation is not strictly logical? Does obfuscating a simple logical concept by describing it in academia-wanky-terms like Liskov's Substitution Principle make it More Logical? Or does it just make the author and their in-crowd feel more intelligent?

Note, also, that the article isn't even objective. It asserts that the definition of a subtype is Liskov's principle. However, Liskov's principle is only one of multiple possible definitions. In other words, the article is really only invoking Liskov's name as an appeal to authority. So much for strict logic.

pestatije 24 minutes ago | parent

intelligence is overrated

raincole 52 minutes ago | parent

In other words, immutable != read only.

In C# ReadOnlyCollection<T> and ImmutableArray<T> are two completely different things for this exact reason.

BlackFly 12 minutes ago | parent

If you pair mutability with exclusive access then you have handled the objection raised by the (some may say overly) strict definition of subtyping here and also the attempt to argue over the objection. No code which asks for an immutable instance will ever observe the mutability because the ask for an immutable reference is exclusive. Therefore, you could pass the mutable reference but so long as something holds onto that reference the mutability is no longer available to other code.

So mutability xor aliasing provides this strict subtyping relation. Of course, you also then need ways of loosening this by providing objects without such a contract and you enter the land of interior mutability, where again the mutable methods can be understood as a part of a subtype because a holder of the reference without mutable methods was explicitly told that there was no the guarantee that the object wouldn't change.