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

Posted by ibobev 1 day ago

Why isn't mutable a subtype of immutable, or vice versa?(crumbles.blog)

22 points | 33 commentspage 2

comrade1234 4 hours ago|

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

bartvk 3 hours ago||

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).

brabel 1 hour ago|||

You just misinterpreted what NSArray is. It’s not an immutable type, it’s a read-only type. As others already mentioned that’s different things. If you have a read-only variable and exclusive access to it, then you can rely on immutability as well. This is what Rust has, for example. Many languages use this definition as well as it’s much more useful than having some sort of pure immutable type guarantee (which some languages do have with const). Another language that uses this is Kotlin: List is a read-only type. MutableList is a subtype of List. You can get a “const” List by only keeping a reference to the value via a List binding. This can be worked around via casting and reflection, but even in Haskell you can do unsafe things that mutate a immutable value, so I don’t think that undermines the idea.

nicky0 3 hours ago|||

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

catoc 3 hours ago||

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

cubefox 2 hours ago||

I would phrase it in terms of subsets. All cats are mammals, therefore "cat" is a subtype of "mammal". So the possible values of the type "cat" are a subset of the possible values of the type "mammal".

In contrast, neither are the mutable things a subset of the immutable things nor the other way round. It's not the case that everything mutable is immutable nor that everything immutable is mutable. The two types are disjoint.

ris 19 minutes ago|

One of the problems with this comes if you're able to dynamically promote something that was passed to you as an "immutable" type to its actual implementation type, you can break this "contract".

I'm currently being annoyed by python's type hinting system, which has exactly this sort of hierarchy for containers, but there's nothing stopping a caller/callee from using type-narrowing to "discover" that the underlying type is actually e.g. a (mutable) list, and then modifying it without any complaints from the type checker. The only way to enforce this would be to actually convert to an immutable implementation type, involving unnecessary copying.

zkmon 3 hours ago|

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 3 hours ago|

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

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

zkmon 2 hours ago|||

If the article is written for human consumption, then it fails the primary goal (or logic?) of being useful. If a human has to digest a flood of this logic slop just to get convinced about this simple concept, they are not going to be able to do anything useful.

applfanboysbgon 3 hours ago|||

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 3 hours ago||

intelligence is overrated