I am pretty skeptical about the whole idea about adding such exceptions to the single mutable reference rule. It may be safe to share references to simple structs in terms of raw bytes access, but as long as some non-trivial invariants are involved, it can be a source of nasty bugs.
In my programming language I generally don't allow having more than one mutable reference to a variable. The only exception is when two references point to different elements of structs/tuples. This gives some flexibility without sacrificing correctness.
> Because of this, Ante code can safely have multiple mutable borrow references to the same struct at the same time.
I doubt it can work in multithreaded code. Allowing sharing mutable references (even to simple structs) means race conditions, temporal inconsistency between different struct fields and even incorrect read results for basic integer types (if the target CPU can't atomically read/write values of types like u64).
Creator of Ante here, Ante inherits Rust's Send/Sync for thread-safety. `mut` refs and `Rc` which provides shared mutability don't implement either and thus can't be shared across threads. So shared mutability is only within a single thread.
Given their "shape stability" design, not necessarily. The three ways that multithreaded access can cause UB are:
* changing the type of the underlying memory (e.g. because it's part of an enum variant and you changed the tag, or because you changed the length of a vector)
* data races (can be defined away by making [effectively] every access Relaxed, as Java does
* use after free (resolved here by reference counting)
In Rust, any type specified like this (all accesses are Relaxed, "shape stable", and reference counted) can already be used in safe code using & references. In theory. But the first property (forcing all accesses to be Relaxed) is very annoying to achieve for arbitrary user data types--even if those types are Copy or other kinds of plain old data--which is a problem e.g. for specifying stuff like sequence locking. The example they give here with bare unions is also very annoying to use in Rust even though this mode of use is safe, because the type system doesn't track which variant is active. So I definitely think there's room to innovate ergonomically here.
(It does seem from the text like this is intended for a single-threaded context, where I think the arguments against Cell are a little less persuasive, but it's still true that it's very awkward to try to figure out how to safely project a Cell down to the exact fields you need to mutate, even though something like LambdaRust will tell you it's safe to do so).
By accepting semantics like this you are, of course, opting out of a lot of potential optimizations around both shared and unique accesses, but you are already doing this in most langauges anyway, so if you're willing to eat the performance cost this can be quite acceptable. The bigger problem (briefly noted in the post) is that the kind of recursive analysis they're proposing doesn't necessarily compose well. Rust explicitly opted out of most types of analysis that can't be efficiently summarized at the function signature level to improve compilation speed. Historically, not being able to efficiently summarize functions that do this kind of stuff has been a big thing that killed attempts to automatically add borrow checking like facilities onto existing C++ code, too. But maybe a language designed for it from the ground up will avoid this problem.
Not if you define the behavior. Many programming languages have robust memory models ranging from "happens before" style systems like those in Java or Modern C++ to the Software Transactional Memory system in GHC/Haskell.
Correct, shared mutation is an extremely well-understood problem in the sense that there are points in the programming language design space that admit it with present technology.
I'm really looking forward to the next generation of languages. Not only are the old 90's languages like Java, PHP, and JavaScript now actually doing things correctly. The new languages like Zig, Go, Swift and Rust continue to figure out new ways of trying things like generics. Spinoffs like V, Ante, and others are perfect testing grounds.
Creator of Ante here. It's just a term I made up for mutation which does not change the "shape" of data in a way that can invalidate any of that data. Mutating a struct or tuple field, even if nested, is stable for example, but mutating an optional string to None is not because you may be dropping a string which there may be a reference to somewhere.
I see, so would it would only be relevant to unions, or would adding a new node to a linked list or pushing another element in a dynamic array or setting a pointer to null also count?
What would your ideal version look like? I recommend reading the C++ or Rust equivalent code in the article. I like the example because it is copied 1:1 with example code from a textbook with only some keywords changed and a Copy constraint. Any other language without a GC and with unboxed types today represents it far more verbosely to the point where the meaning becomes obscured and a typo becomes more likely to survive code review.
I am pretty skeptical about the whole idea about adding such exceptions to the single mutable reference rule. It may be safe to share references to simple structs in terms of raw bytes access, but as long as some non-trivial invariants are involved, it can be a source of nasty bugs.
In my programming language I generally don't allow having more than one mutable reference to a variable. The only exception is when two references point to different elements of structs/tuples. This gives some flexibility without sacrificing correctness.
> Because of this, Ante code can safely have multiple mutable borrow references to the same struct at the same time.
I doubt it can work in multithreaded code. Allowing sharing mutable references (even to simple structs) means race conditions, temporal inconsistency between different struct fields and even incorrect read results for basic integer types (if the target CPU can't atomically read/write values of types like u64).
What if I omit the "uniq" keyword? Is it still valid code, just not protected by the compiler from union types changing underneath?
I can see how multiple mutable references is fine in a single-threaded context, but surely this would cause UB in the multi-thread context?
Creator of Ante here, Ante inherits Rust's Send/Sync for thread-safety. `mut` refs and `Rc` which provides shared mutability don't implement either and thus can't be shared across threads. So shared mutability is only within a single thread.
Given their "shape stability" design, not necessarily. The three ways that multithreaded access can cause UB are:
* changing the type of the underlying memory (e.g. because it's part of an enum variant and you changed the tag, or because you changed the length of a vector) * data races (can be defined away by making [effectively] every access Relaxed, as Java does * use after free (resolved here by reference counting)
In Rust, any type specified like this (all accesses are Relaxed, "shape stable", and reference counted) can already be used in safe code using & references. In theory. But the first property (forcing all accesses to be Relaxed) is very annoying to achieve for arbitrary user data types--even if those types are Copy or other kinds of plain old data--which is a problem e.g. for specifying stuff like sequence locking. The example they give here with bare unions is also very annoying to use in Rust even though this mode of use is safe, because the type system doesn't track which variant is active. So I definitely think there's room to innovate ergonomically here.
(It does seem from the text like this is intended for a single-threaded context, where I think the arguments against Cell are a little less persuasive, but it's still true that it's very awkward to try to figure out how to safely project a Cell down to the exact fields you need to mutate, even though something like LambdaRust will tell you it's safe to do so).
By accepting semantics like this you are, of course, opting out of a lot of potential optimizations around both shared and unique accesses, but you are already doing this in most langauges anyway, so if you're willing to eat the performance cost this can be quite acceptable. The bigger problem (briefly noted in the post) is that the kind of recursive analysis they're proposing doesn't necessarily compose well. Rust explicitly opted out of most types of analysis that can't be efficiently summarized at the function signature level to improve compilation speed. Historically, not being able to efficiently summarize functions that do this kind of stuff has been a big thing that killed attempts to automatically add borrow checking like facilities onto existing C++ code, too. But maybe a language designed for it from the ground up will avoid this problem.
I was thinking of the data race case, and I was unaware of relaxed access used by other languages. Thanks for sharing!
Not if you define the behavior. Many programming languages have robust memory models ranging from "happens before" style systems like those in Java or Modern C++ to the Software Transactional Memory system in GHC/Haskell.
Correct, shared mutation is an extremely well-understood problem in the sense that there are points in the programming language design space that admit it with present technology.
As always, there are tradeoffs.
I'm really looking forward to the next generation of languages. Not only are the old 90's languages like Java, PHP, and JavaScript now actually doing things correctly. The new languages like Zig, Go, Swift and Rust continue to figure out new ways of trying things like generics. Spinoffs like V, Ante, and others are perfect testing grounds.
What about Vale? Is this a rename of it or something new?
Hi, article's author here (Verdagon, Vale's creator), and no, I'm just writing about someone else's language (Ante) that I thought was interesting.
Ante is making some very intriguing steps forward in memory safety design and I thought others would find it interesting too.
Thank you for clarifying!
What is "stable shape"?
Creator of Ante here. It's just a term I made up for mutation which does not change the "shape" of data in a way that can invalidate any of that data. Mutating a struct or tuple field, even if nested, is stable for example, but mutating an optional string to None is not because you may be dropping a string which there may be a reference to somewhere.
I see, so would it would only be relevant to unions, or would adding a new node to a linked list or pushing another element in a dynamic array or setting a pointer to null also count?
>"I have to admit, this is beautiful"
Terse it is, beautiful it is not (well my version of beautiful that is - easy to read and understand). this is not to diminish the language.
What would your ideal version look like? I recommend reading the C++ or Rust equivalent code in the article. I like the example because it is copied 1:1 with example code from a textbook with only some keywords changed and a Copy constraint. Any other language without a GC and with unboxed types today represents it far more verbosely to the point where the meaning becomes obscured and a typo becomes more likely to survive code review.
That's what I thought too when I first saw it. But then I actually took a moment to read it and eventually changed my mind.