A guide to closures in Rust (opens in new tab)

I would further elaborate on your point by saying that as someone who likes to hang out on programming fora and help people as I was helped when I was young, this terminology confusion causes real confusion in the field as well.

Many programmers-in-training find it confusing as to why whether or not a function has a name seems to be tied to whether or not it has an environment (not that they phrase it that way but that's what it boils down to), and my usual answer is to explain something much like your post; the resolution is, there is no such connection. I have fielded this question in one form or another many times.

"Anonymous function" should not be used as a synonym for "closure". Generally I will follow along with the dominant language terminology if I am clearly in the context of a very particular language, but may include why I don't like the language community's terminology if it helps my point.

In some language all functions capture their environment, making that original statement more true than you make it out to be.

Personally I think the term closure should be banned, because it only adds confusion.

There are just functions, some named some anonymous, some capturing the environment, some not, some being disallowed by some constraints from the host language. The term closure cuts through this space in a useless and rather ad-hoc way.

If you want to do anything with the closure it gets a little tricker, for example if you did something naive like

    fn fn_that_takes_closure (f: fn (i32, 3i2) -> i32) { ... }

    let x = |a, b| a + b;
    fn_that_takes_closure(x);

That would not compile, because `x` does not have the type `fn (i32, i32) -> i32` (that's reserved for "normal" functions). Every closure in Rust has a unique anonymous type, which means if you want to do anything with them besides calling them, you need to understand `Fn`, `FnOnce`, and `FnMut` traits and/or `dyn` trait objects. To write that above you'd need to do something like this:

    fn fn_that_takes_closure<F> (f: F)
    where
        F: Fn(i32, i32) -> i32

It becomes more painful when the closure is actually closing over something (especially if that thing is mutable), since the borrow checker gets involved.

Welcome to functional programming, where functions are values.

For a long time, people thought closures were only practical in garbage collected languages. I've used closures in C. (Against my will, basically. It was a C library that was best-of-breed and there was no other choice.) Getting the memory management right on them was effectively impossible. You know where the values are created for sure, but creating clean specifications of when the closures were destroyed quickly becomes insane in any real program. Very simple in a 20-line sample, very complicated when you got a long-lived callback interacting with multiple other resources that may or may not have their own "interesting" lifespans.

That Rust can do closures at all, do them in a sufficiently useful way that they are practical, and still maintain its safety guarantees without garbage collection is, in my opinion, in the top five accomplishments of the language. Prior to it actually happening I think it's fair to say most programming language developers would have said it's not possible, that you'll either need GC or the requisite type system will be impractically complex. At least their quirkiness generally fits the rest of the language and isn't much "new" quirkiness.

True, but isn't the complexity of Rust closures essential rather than accidental?

Fundamentally closures are easy in e.g. Go because you don't have to think about lifetimes, at all. As soon as you capture a variable, the GC guarantees it won't be dropped from underneath your feet. With non-GC'd languages that responsibility moves from the GC to the programmer. The trickyness of using closures in Rust seems largely to be the trickyness of managing the lifetimes of captured variables.

That's because Rust wants you to use the stack almost exclusively for memory allocation because that's the obvious way to do automatic memory management w/o a GC. So the moment you want to return closures you have to allocate them on the heap (Box them).

Rust is a modern, functional programming language where the programmer works in a straightjacket, and this is most painfully evident the moment you want to use closures like you're programming in Haskell.

In Haskell you don't have to think about stack vs. heap allocation. But w/o a way to make it easier for the compiler to choose stack allocation where that would be safe, Haskell ends up being heap-heavy. Rust takes the opposite tack and is stack-heavy, but unlike Haskell Rust forces you to be explicit about the alternative.

It's not like Rust doesn't have a GC. Arc is a GC after all. It's just that Rust makes it hard to write natural code using the heap and GC.

I know I keep repeating this, but...

Rust does not have a massive threshold. Memory management has a massive threshold. Rust just has a lot of safety rails you might keep bumping into if you struggle with memory management. The fact that other languages let you drive off the cliff doesn't mean they have a smaller threshold.

It has a massive threshold for beginners for sure.

Personally I have a background in C/C++ and moved towards C#, Go, JS etc. With an interest in many more, but the above were my main languages for a decennia.

9 years ago I picked up Go, and have used it the most as my main language since then. 7 Years ago I picked up Rust, but only since 6 months ago really intensive (including publishing the guide that came out of my experience at https://rust-lang.guide/).

Over those 7 years I've only done a production project around the start of that journey and now since 6 months ago I'm writing production code in it again and haven't stopped since. But I've learned the language like 3 different times in that journey.

It's not an easy one to get into. A lot of things to wrap your head around, especially if your goal is to really master it, rather then just "woohoo I can compile my program". I still not master it, but I do feel now fluent in it and can express my ideas well. There are also no more fights with the language or its tooling.

Compared to that, Go(lang) is super easy to get into. Ridiculously easy. Perhaps too easy. You can give it to a programmer of any level, and they probably can ship their first feature that same day. The difference is however, that. First of all with Golang it's super easy to ship code with nil pointer exceptions in it or data races. I've seen it in the best code bases. And sure, plenty of people will tell if you do it "right" you'll have no issues. I've hear similar comments from ex-colleagues still in the C++ world as well. Secondly, Golang is very opinionated and if you derive in ideas or needs a bit from what they want you to do, you are in a bit of a problem. This has been slightly improved, but it's still very limited and still a lot of magic that only built-in features can perform. And thirdly, the language is very minmal (Golang). So codebases look very verbose very quickly. Also harder to express ideas (as error support is limited, and sum types are not a thing, neither are other things like pattern matchings). It's also a lot less "functional".

This is not to compare Golang vs Rust too much, even though I did. My point was more, and I probably butcher my own comment not making that point well, is that I think it's fair to have a massive threshold for beginners to get into a programming language. As long as in return it means you get a very powerful tool in rerturn to do some of your work. And Rust is def. such a tool. Many people boast about how peformant it is. And while that is true, it is for me for most projects more of an extra benefit rather than the things I really like it for. What I like with Rust is that it gives me really high confidence in my code (comparable to projects I've written in Elm and Haskell in the past). When my project compiles I really am pretty confident it will not give me runtime surprises beyond mistakes elsewhere (e.g. infrastructure choices). It's a very expressive language and pleasant to use in that way.

And then there is of course the fact that it is truly FOSS, has a very nice community, great documentation and a lot of excellent learning resources.

I am not a fanboy of much, neither of Rust, but I do really appreciate its existence and I am happy to use it where it suits. Yes it is a massive threshold, but one worth to pay.

> it does seem like a language with a massive threshold for beginners.

Programming with manual memory management should be (comparatively) hard. It's not where beginners should start. C makes it far too easy to write code with objectively bad effects on the system, as evidenced by the countless vulnerabilities discovered in critical real-world systems over the last half-century.

> after a few rounds with that compiler.

I find this phrasing curious. Do you feel that you are fighting the compiler?

You will probably get a lot of varied responses and also some people who just defend the language of their choice.

Let me give you my opinion who just started learning 3 weeks ago. And I needed around 1 week until I could write a simple parser by hand and around 2 more until I am now being quite productive in this new language.

It is actually very simple go get into, depending on your experience. What do mean by 'threshold for beginners', what are beginners for you. Do you mean someone learning their first programming language or someone already proficient with one or multiple trying to learn Rust now? Because a lot of the features of Rust are present in other language and your mental models will be similar enough for them in Rust accelerating your learning curve. Of course someone new to programming will have problems as the features that are unique to rust (borrow, move) in addition to everything else leads to a lot of overhead sure.

My experience is with Java, Python and Javascript/Typescript as I know them sufficiently well to create programs and I also dabbled with many other languages just trying out how they feel (Scheme, Racket, Nim, Go, Haskell, C, C++, Ruby, Smalltalk). When starting with Rust it took very little time to get comfortable. The language feels very well designed and Option, Result type are very logical to use for return types. Many languages have these so depending with what you are familiar with this might be new to you but it is surely not more complicated than explaining try/catch blocks. Of course dealing with Option/Result is a bit uncomfortable in the beginning but If you just browse a bit through the standard library and learn about the `?` operator you see many patterns how to deal with them in an efficient manner (map, map_or_else, or, or_else, flatten, unwrap, unwrap_or,...). Rust has closures but now you find them in almost any language so it is not really surprising (maybe only nuances that come up with the borrow checker). Most of the standard traits and enums are quite straightforward such as `Default`, `From`, `FromStr`, `Iterator`, `Clone`, `PartialEq`, all the operator traits.

Also the borrow checker and some lifetime compiler errors are for sure a thing that can lead to thresholds and for my time spent with the language I would say there are many things I will need to learn to fully be able to grok the language. However, most of the time you can circumvent these problems by just not using lifetimes in your own code and by cloning variables. So for people that want to write really efficient code you can do this without cloning variables if you have a better understanding of them but as a beginner you have a simple escape hatch most of the time and allows you to gradually gain a better model of lifetimes/borrowing rules.

I think the only really threshold is the trait object problem. If you want to build any kind of nested structure or if you want some kind of "polymorphism" which is a natural reaction depending which programming language you have used. You will need to deal with understanding them and this might take some time.

Async programming in Rust can still be a bit awkward.

They're working hard on it though. https://blog.rust-lang.org/inside-rust/2023/05/03/stabilizin...

We'll have Async traits in stable Rust after the summer (normally) and in 2024 they'll start working on supporting async closures :)

So if all is well, you're experience should start to become more pleasant next year. For now your closures will either have to return a future (without use of `async` keyword) or you better avoid it for now. Then again, hadn't really had the need for async closures, but I did had a big need for Async traits, as implementing Traits with manual futures was so far a big pain in the ass, and sometimes still not optimal. So I for one, welcome async traits with open arms.

The lack of asynchronous closures does make it awkward, as you often need to put an `async move` block inside the closure for things to work correctly, which means the closure becomes an FnOnce and you have to work around that using clones.

It’s one of the worst corners of the ecosystem currently, and I’d very much recommend avoiding it.

I may be biased because I generally avoid closures entirely (I prefer the certainty over ownership and type signatures that traditional functions give), but I do my best to avoid closures when working with async in Rust. A lot of examples for frameworks will make use of async closures, and I typically convert those to functions as quickly as possible, which can be tricky the first time because of the elided types.

Async code is hard to reason about. Async closures is what I like most about JavaScript (JS) though. Because JS pass all variables by value they get frozen in time when passed to a function. This make async code easier to reason about, as all variables are immutable.

I'd love to see a side by side comparison on how someone would use goroutines in Go vs tokio in Rust. Including with closures

Yeah I think the ligature debate is a bit tiring frankly but one place where putting ligatures is very clearly a bad idea is in code examples. A blog is not my IDE, I can't turn them off and even worse I don't even know they are on! Can you use a unicode arrow for Rust closures? Is that what this is? I don't know!

Do you mean the arrows? As long as they are fixed-width -single or double- it's ok and even pretty, but if variable-width fonts in code is a no-no for me.

What is machine-specific about i32? It's a signed 32-bit integer, regardless of the machine you're on.