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.
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).
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.
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.
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?
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.
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.
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.
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.
I 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.
Experimentally, 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.
You have a point; I am looking at it from quite functional programming perspective, because that's how type systems are typically understood. So from that perspective, interior mutability is a form of rebinding.
When you say "we can choose mutability as a part of a type", the question is, what kind of errors are we trying to prevent? What is the semantics we want to give? From that it should be obvious whether it can be subtype or not.
> So from that perspective, interior mutability is a form of rebinding
really? i honestly dont have _too_ much experience with functional languages, basically only elixir and consequently some amount of erlang in that vain... but i'd feel like thats not the same? but it may be that my point of view is too narrow.
from my experience with that functional language, the equivalent to this scenario would be a struct - and wherever i can mutate properties within it -- or need to reconstruct the struct from scratch.
both have technical consequences, eg if i passed the struct into a consumer somewhere which keeps it, it would get the "modified" version automatically when the property was changed
but on reconstruction, it'd have to introduce some kind of event listener to handle the reconstruction.
simple example for such a scenario would be eg a session within a SSE api.
the mutated struct would trivially allow for an uninterrupted stream no matter how long the session is extended, the latter needs to pay attention so its not opening a memory leak to support that feature.
A system programming language needs to match hardware where mutability is associated with memory addresses, something functional languages can afford to abstract away.
In computer science, specifically programming language theory, types are properties of terms (syntactic expressions). They’re not properties of values. It wouldn’t be possible in general to typecheck a program before running it if you had to have a value before you could discover its type.
> Mutability is a property of variable, not of a value. [...] This has nothing to do with whether it can be reassigned (i.e. mutability).
I feel like you're using "mutable" wrong, as it's used colloquially. E.g. in JS, MDN itself talks about "reassignment"[0] and not "mutability" even though people often use "mutability" as a word to refer to the distinction between `const` and `let`.
Note that you used "reassignment" which is the specific word for "mutating a binding", but the article is not referring to bindings at all.
In statically typed languages, both variables and values have a type.
> Mutability is a property of variable, not of a value.
In which languages?
In D, mutability is a property of a type. And since it's a statically typed language, types are properties of variables so mutability is also a property of variables as well as values.
> 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 seems confused. "Mutable" ot "immutable" are types like any other, like "String" or "Integer".
Mutability vs immutability describe the relationship between identity and state. Mutable values retain identity when their state changes. For immutable values, getting a new state requires a new identity, and importantly, existing identities never have their states changed.
Variables can be described by types. That is, a mutable slot referencing a value can be a value itself (a reference to a reference) and we can use subtype relationships to describe it. Variables are covariant when used as inputs (i.e code reads from the variable) and contravariant when used as outputs, and invariant when used as both. You can logically supply a Box<Cat> to a vet(in Box<Animal>) routine, and supply Box<Animal> to catchAndStore(out Box<Cat>). Substitute `ref X` for `Box<X>` when using a language that can pass variables by reference.
So 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.
The cons example is conflating implementation details (return the value passed in to cons) with the semantics (return the left-side of this pair). 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.
Because 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.
40 comments
[ 3.7 ms ] story [ 49.3 ms ] threadSwift is completely different. Standard arrays are structures, always mutable - value types passed by value
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).
The problem is the "exclusive access to it", most language aren't Rust.. Some Ada-implementation has the problem ( https://news.ycombinator.com/item?id=43001394 )
https://commonplacefacts.com/2022/07/27/principia-mathematic...
In C# ReadOnlyCollection<T> and ImmutableArray<T> are two completely different things for this exact reason.
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.
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.
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.
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.
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.
When you say "we can choose mutability as a part of a type", the question is, what kind of errors are we trying to prevent? What is the semantics we want to give? From that it should be obvious whether it can be subtype or not.
really? i honestly dont have _too_ much experience with functional languages, basically only elixir and consequently some amount of erlang in that vain... but i'd feel like thats not the same? but it may be that my point of view is too narrow.
from my experience with that functional language, the equivalent to this scenario would be a struct - and wherever i can mutate properties within it -- or need to reconstruct the struct from scratch.
both have technical consequences, eg if i passed the struct into a consumer somewhere which keeps it, it would get the "modified" version automatically when the property was changed
but on reconstruction, it'd have to introduce some kind of event listener to handle the reconstruction.
simple example for such a scenario would be eg a session within a SSE api. the mutated struct would trivially allow for an uninterrupted stream no matter how long the session is extended, the latter needs to pay attention so its not opening a memory leak to support that feature.
In computer science, specifically programming language theory, types are properties of terms (syntactic expressions). They’re not properties of values. It wouldn’t be possible in general to typecheck a program before running it if you had to have a value before you could discover its type.
I feel like you're using "mutable" wrong, as it's used colloquially. E.g. in JS, MDN itself talks about "reassignment"[0] and not "mutability" even though people often use "mutability" as a word to refer to the distinction between `const` and `let`.
Note that you used "reassignment" which is the specific word for "mutating a binding", but the article is not referring to bindings at all.
[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
In statically typed languages, both variables and values have a type.
> Mutability is a property of variable, not of a value.
In which languages?
In D, mutability is a property of a type. And since it's a statically typed language, types are properties of variables so mutability is also a property of variables as well as values.
This seems confused. "Mutable" ot "immutable" are types like any other, like "String" or "Integer".
Variables can be described by types. That is, a mutable slot referencing a value can be a value itself (a reference to a reference) and we can use subtype relationships to describe it. Variables are covariant when used as inputs (i.e code reads from the variable) and contravariant when used as outputs, and invariant when used as both. You can logically supply a Box<Cat> to a vet(in Box<Animal>) routine, and supply Box<Animal> to catchAndStore(out Box<Cat>). Substitute `ref X` for `Box<X>` when using a language that can pass variables by reference.
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.
- 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