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Why isn't mutable a subtype of immutable, or vice versa?
- gus_massa 9d agoYep, 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 8d agoIsn't NSMutableArray a subclass of NSArray in a few languages?
- catoc 8d agoYes, 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 8d agoGood 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 8d ago"Was" like that? .... Some of us still use Objective-C!
- bartvk 7d agoOoops sorry :-) Keep at it!
- brabel 8d agoYou 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.
- zkmon 8d agoThe 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 8d agoNot all all. It merely indicates that this is an strict logic approach to the topic. https://commonplacefacts.com/2022/07/27/principia-mathematica-300-page-proof-one-plus-one-equals-two/ https://commonplacefacts.com/2022/07/27/principia-mathematic...
- applfanboysbgon 8d agoWhat 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 8d agointelligence is overrated
- zkmon 8d agoIf 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.
- raincole 8d agoIn other words, immutable != read only. In C# ReadOnlyCollection<T> and ImmutableArray<T> are two completely different things for this exact reason.
- BlackFly 8d agoIf 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.
- cubefox 8d agoI 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 8d agoOne 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.
- js8 8d agoI feel like the explanation is overcomplicated. Types are properties of values, not variables. Mutability is a property of variable, not of a value. So it's kind of a categorical error. (I want to joke here that all categorical errors are just type errors in category theory.) When we speak of "type of a variable", we mean this variable can only be assigned (bound to) values of certain type. This has nothing to do with whether it can be reassigned (i.e. mutability). So you don't even need the notion of subtyping to explain this. Also, one could probably define variable as a monad over its type.
- noelwelsh 8d agoA nitpick: Types are properties of expressions, not values. Type errors happen at compile time, before code runs. Values only exist at run time.
- js8 8d agoFair enough. Although I don't fully subscribe to the dichotomy of compile vs run time, we can say that.
- pxeger1 8d agoIn semantics, types are properties of values and expressions. Type safety is about whether the type of an expression always matches the type of the value it evaluates to.
- jounker 8d agoDoesn’t this break down with dependent type systems?
- rtpg 8d agoExperimentally, at least one major programming language (Rust) places mutability into the type system. Whether or not something belongs into a type system is ultimately determined by the type system. We can choose whether or not mutability is considered a part of a type. > When we speak of "type of a variable", we mean this variable can only be assigned (bound to) values of certain type. This has nothing to do with whether it can be reassigned (i.e. mutability). This is a bit too simplistic IMO. You're talking about name bindings, the article is talking more about things like interior mutability. Rebinding a name is ... generally not a type system concern by my understanding.
- Mikhail_Edoshin 8d agoSo the problem is that the semantic of methods (car, cdr) is not well defined: it is either "give me that member" or "give me that member and guarantee next time the result will be same". And the solution is to clearly separate those by adding more method names.
- anankaie 8d agoThe cons example is conflating implementation details (return the value passed in to cons) with the semantics (return the left-side of this pair that was instantiated by cons). In the first case it is a category error to think about mutability. In the second mutability makes perfect sense. Moreover, I suspect it is possible to construct an interface such that to prove statically that you can Liskov Substitute a type into it would be equivalent to deciding Halt: All you need are extensional semantics in your type system.
- bruce343434 8d agoSo really there are 2 orthogonal axes: - A: can I change the value - B: can something else change the value (can I depend on a predictable stable value) Because the axes are orthogonal, hierarchical based subtyping (inheritance) breaks, but type classes (interfaces), ad hoc polymorphism, would work. In C, const answers A In rust, due to pointer aliasing restrictions (either one mut pointer xor any amount of read only pointers), (lack of) mut answers both A and B
- PhilipRoman 8d agoIn C, const can also answer B, but only in non-pointer contexts.
- Sharlin 8d agoAs a total aside, the names `car` and `cdr` for "head" and "tail" are honestly some of the most baffling historical relics in all of computing.
- MiroslavPokorny 8d agoWhat i dont understand is why does everything have to be so brief. Take car and cdr there is only one character difference, more character differences is a good thing not bad.
- thaumasiotes 8d ago> What i dont understand is why does everything have to be so brief. The concept is to make things easier for people who are familiar with the language. Making things harder for that group is always counterproductive, because they are the only people who can do any useful work.
- MiroslavPokorny 8d agoIf a person cant handle typing a 8 character var name and can only handle short names that are 3 chars, then you have a much bigger problem.
- gumby 8d agoMore short names fit in your fovea than long names so for the most common operations they are the easiest to read. It’s the same reason the most common words in human languages are the shortest (and programming languages are for humans to use so subject to the same pressure. Also, Back In The Old Days memory was very expensive and precious so short identifiers were important. Often labels were tightly constrained, for example being limited to six upper-case characters so they would fit in a single word.
- Sharlin 8d agoThe usual convention (which also occurs in natural languages) is that often-used words should be short, rarely used should (and can afford to be) more descriptive.
- MiroslavPokorny 8d agoBecause if mutable is a sub type of immutable, it wouldnt be right to give a mutable value when the receiver expected immutable values. The same is also true if one reverses the relationship, the mutable operations should not be defined on the immutable, and if they were that would be dumb because the immutable implementations should fail, which is silly.
- raffael_de 8d agoas far as my set theoretical intuition goes, immutable has to be a subtype of mutable, if anything.