A question that comes up every once in a while on the Carp chatroom and in Github issues is how memory management works. Especially people who are not familiar with Rust understandably have trouble grasping how to program with references, and why some functions take references while others don’t. Today, I will try to explain these concepts for my fellow Carpenters.
A fair note: while this blog post might be beneficial to your understanding of the Rust borrow checker, this is not my focus. I don’t know Rust very well, and it is entirely possible that it does things differently and I’m just confusing your understanding of how its machinery works. I therefore suggest that you don’t try to compare the two too much, lest you fall prey to faulty assumptions.
An unfortunate piece of syntax
I have an inkling that at least some of the confusion surrounding references stems from the obvious comparison to references in more familiar languages such as C and Go. Not only the names are similar: even the syntax is identical. Ampersands take a thing by reference, as has always been the case. But the semantics of “taking a thing by reference” are not the same at all.
In the languages mentioned above, taking a reference has direct implications on the memory level. In languages with first-class memory access—read C and C++ and other languages that allow for nasty things such as pointer arithmetic—, taking a reference means that you end up with a pointer, i.e. a number representing the point in memory where the thing lives. Languages like Go take a safer subset of that behavior, and allow people to pass things by value or by reference. Most of you will at least have a basic intuition about some of this behavior, but if not, you can read my blog post about these concepts which might or might not make sense.
This is not generally problematic—except for the class of bugs it opens—were it not for the unfortunate design decision of many borrow-checked languages—Rust and Carp, at least—to reuse that name and the syntax surrounding it. It is, albeit, not the same thing at all, and I will henceforth refrain from ever mentioning it again. Let’s look at Carp, then.
Who owns what?
The idea surrounding borrow-checked languages is that of ownership and lending. Capitalist at heart, these languages obsess over the flows of property. This serves a simple goal: if you know who owns what at any given moment, you also know when you can get rid of it.
The Carp compiler tries very hard to do as much as possible for you. It infers almost any type, the occasional annotation nonwithstanding, for once. In the same vein, it tries to make memory management as simple and automatic as possible.
References are a piece of that puzzle. They are similar to type annotations, but instead of helping the type checker figure out the types in your program, it operates on the memory level. It helps let the borrow checker know who you think should own what. You then engage in a conversation with your compiler, effectively, working together to build a program with correct memory semantics.
As becomes apparent quite quickly, however, the process is a lot more manual than type checking. As it turns out, ownership has for a long time been largely implicit in our programs, and we haven’t spent quite as much time trying to figure out the underlying theories in this domain as we have for typing.
But let’s get back to the point, shall we? By default, things are owned by the scope they are enclosed in. Anything that is introduced by a
let form, for instance, is owned by that block. If you pass it to another function as an argument, it will take over ownership.
; test : (Fn [String] ()) (defn test [x] ; x : String (IO.println &x)) (defn main  ; because string literals are of type &String (let [a @"test"] ; a : String (test a)))
In Figure 11, the function
test will take ownership over the string that is being passed to it. This means that it now controls the memory associate to it, and it wouldn’t be safe for our main function to reuse it. Thus,
main gives control over to
; test : (Fn [String] ()) (defn test [x] ; x : String (IO.println &x)) (defn main  (let-do [a @"test"] ; a : String (test a) (test a)))
In Figure 2, we try to call
test twice with the same argument. This will fail to compile, because, as I detailed above, we gave up ownership of
a. It is no longer ours, and, as such, we are not allowed to do anything with it anymore. The Carp compiler will tell us that we’re trying to use a “given-away” value.
This is not always what we want. Often, you will like to call a function on a thing without giving up ownership. The program in Figure 2 is already a good illustration: we want to be able to print a thing without it getting invalidated under our feet, because we probably also want to work with it. This is where references come in.
; test : (Fn [&String] ()) (defn test [x] ; x : &String (IO.println x)) (defn main  (let-do [a @"test"] ; a : String (test &a) (test &a)))
The program in Figure 3 will compile. By handing over references to
test—and changing the call to
println accordingly—we essentially tell the compiler that we˚re not done with
a just yet. We’ve only borrowed it to
Special cases and implications
You cannot return references in Carp. This hopefully makes sense by now: if you lend out a piece of memory and then your scope ends, noone is in charge of it anymore. By definition, the ownership of return values must be handed over to the caller in the Carp model.
Unfortunately, there are some special cases to that rule, due to design choices surrounding Carp and its view on simplicity2. If you ever work with arrays, you’ll come across the indexing function
nth. It returns a reference. How is that possible? It’s actually quite simple: the array element is still owned by the array. It is not a copy, and thus you cannot delete it, because you would invalidate the array by doing so. That rule also holds for struct accessors, as of early 2018.
A tale of debugging
Especially in the beginning, I often felt like I just threw in random copies and references until the program compiled, both in Carp and Rust. I haven’t revisited Rust in a while, but I think it would be easier now that I’ve mastered the conceptually simpler model in Carp.
What I want to tell you with this addendum is that it’s completely normal if it takes you some time to develop an intuition about the concept of borrowing. What you get in return is programs that are very likely to be correct3.
Don’t be scared of compiler errors. The Carp compiler is here to help you write better programs and guide you in the process of establishing a sound model of the information and memory flow of your applications. Sure, all that red text looks scary, but it is more an admission of failure on the compiler’s part than an indictment.
This has been a whirlwind tour of borrow checking in Carp. It has been fairly high-level, and I didn’t talk about many of the implementation details. If you want to know more about the inner workings of the Carp compiler, I suggest that you read my blog post from late last year; it might just quench your thirst.
What you should’ve gotten from this is a better intuition when it comes to references. If you haven’t, feel free to shoot me an e-mail or drop by the Gitter channel, where we will be very happy to help you sort through the mess.
1. Why are string literals references, you might rightfully ask. Carp embeds string literals in the binary, and thus they don’t need to be handled by the borrow checker; they will never need to be freed.
2. Rust solves this problem using lifetimes, which Carp decided to omit. There is a possibility that they might be added in the future, though.
3. They might not be optimal, and a few tweaks in the model might result in fewer copies and thus more performant programs. But before worrying about optimality, we should worry about safety.