So many rust articles are focused on people doing dark sorcery with "unsafe", and this is just normal every day api design, which is far more practical for most people.
Actually the From trait documentation is now extremely clear about when to implement it (https://doc.rust-lang.org/std/convert/trait.From.html#when-t...)
One question about avoiding boolean parameters, I’ve just been using structs wrapping bools. But you can’t treat them like bools… you have to index into them like wrapper.0.
Is there a way to treat the enum style replacement for bools like normal bools, or is just done with matches! Or match statements?
It’s probably not too important but if we could treat them like normal bools it’d feel nicer.
enum MyType{
...
}
impl MyType{
pub fn is_useable_in_this_way(&self) -> bool{
// possibly ...
match self {...}
}
}
pub fn use_in_that_way(e: MyType) {
if e.is_useable_in_this_way() {...}
}
if let MyType::Member(x) = e {
...
} struct PizzaOrder {
size: PizzaSize,
toppings: Vec<Topping>,
crust_type: CrustType,
ordered_at: SystemTime,
}
#[derive(PartialEq, Eq)]
struct PizzaDetails {
size: PizzaSize,
toppings: Vec<Topping>,
crust_type: CrustType,
… // additional fields
}
#[derive(Eq)]
struct PizzaOrder {
details: PizzaDetails,
ordered_at: SystemTime,
}
impl PartialEq for PizzaOrder {
fn eq(&self, rhs: &Self) -> bool {
self.details == rhs.details
}
}
Like whenever I read posts like this, they're always fairly anecdotal. Sometimes there will even be posts about how large refactor x unlocked new capability y. But the rationale always reads somewhat retconned (or again, anecdotal*). It seems to me that maybe such continuous meta-analysis of one's own codebases would have great potential utility?
I'd imagine automated code smell checking tools can only cover so much at least.
* I hammer on about anecdotes, but I do recognize that sentiment matters. For example, if you're planning work, if something just sounds like a lot of work, that's already going to be impactful, even if that judgement is incorrect (since that misjudgment may never come to light).
It's not too uncommon in other languages (sometimes under the name "immediately invoked function expression"), though depending on the language you may see lambdas involved. For example, here's one of the examples from the article ported to C++:
auto data = []() {
auto data = get_vector();
auto temp = compute_something();
data.insert_range(data.end(), temp);
std::ranges::sort(data);
return data;
}();The same day Cloudflare had its unwrap fiasco, I found a bug in my code because of a slice that in certain cases went past the end of a vector. Switched it to use iterators and will definitely be more careful with slices and array indexes in the future.
"Defensive programming" has multiple meanings. To the extent it means "avoid using _ as a catch-all pattern so that the compiler nags you if someone adds an enum arm you need to care about", "defensive" programming is good.
That said, I wouldn't use the word "defensive" to describe it. The term lacks precision. The above good practice ends up getting mixed up with the bad "defensive" practices of converting contract violations to runtime errors or just ignoring them entirely --- the infamous pattern in Java codebases of scrawling the following like of graffiti all over the clean lines of your codebase:
if (someArgument == null) {
throw new NullPointerException("someArgument cannot be null");
}
Needed file not found? Just return ""; instead.
Negative number where input must be contractually not negative? Clamp to zero.
Program crashing because a method doesn't exist? if not: hasattr(self, "blah") return None
People use the term "defensive" to refer to code like the above. They programs that "defend" against crashes by misbehaving. These programs end up being flakier and harder to debug than programs that are "defensive" in that they continually validate their assumptions and crash if they detect a situation that should be impossible.
The term "defensive programming" has been buzzing around social media the past few weeks and it's essential that we be precise that
1) constraint verification (preferably at compile time) is good; and
2) avoidance of crashes at runtime at all costs after an error has occurred is harmful.
For ints you can implement the deref trait on structs. So you can treat YourType(u64) as a u64 without destructing. I couldn’t figure out a way to do that with YouType(bool).
PartialEq and Eq for PizzaDetails is good. If there is a business function that computes whether or not someone orders the same thing, then that should start by projecting the details.
https://www.moderndescartes.com/essays/readability/
(I have not read this article closely, but it is about the right concept, so I provide it as a starting point since "readability" writ large can be an ambiguous term.)
Yes. Defensively handle all the failure modes you know how to handle, but nothing else. If you're writing a service daemon and the user passes in a config filename that doesn't exist, crash and say why. Don't try to guess, or offer up a default config, or otherwise try to paper over the idea that the user asked you to do something impossible. Pretty much anything you try other than just crashing is guaranteed to be wrong.
And for the love of Knuth, don't freaking clamp to zero or otherwise convert inputs into semantically different value than specified. (Like, it's fine to load a string representation of a float into an IEEE754 datatype if you're not working with money or other exact values. But don't parse 256 as 255 and call it good enough. It isn't.)
It's not difficult to write the predicate same_details_as() and then it's obvious to reviewers if that's what we meant and discourages weird ad-hoc code which might stop working when the PizzaDetails is redefined.
These roles don’t really have standard titles in the industry, as far as I’m aware. At Google we were part of the larger language/library/toolchain infrastructure org.
Much of what we did was quasi-political … basically coaxing and convincing people to adopt best practices, after first deciding what those practices are. Half of the tips above were probably written by interested people from the engineering org at large and we provided the platform and helped them get it published.
Speaking to the original question, no, there were no teams just manually reading code and looking for mistakes. If buggy code could be detected in an automated way, then we’d do that and attempt to fix it everywhere. Otherwise we’d attempt to educate and get everyone to level up their code review skills.
> Half of the tips above were probably written by interested people from the engineering org at large and we provided the platform and helped them get it published.
Are you aware how those engineers established their recommendations? Did they maybe perform case studies? Or was it more just a distillation of lived experience type of deal?
How would you decompose a character string so that you could have a case-insensitive versus sensitive comparison?
:)
So much end user software tries to be "friendly" by just saying "An error occurred" regardless of what's wrong or whether you can do anything about it. Rust does better and it's a reminder that you can too.
Using ..Default::default() means “whatever additional fields are added later, I don’t care”. Which is great until someone needs to add a field to the struct, and they rely on the compiler to tell them all the places that don’t have a value for the field (so they can pass the right value depending on the situation.) Then the callers with Default are missed, and bugs can result.
Any time you say “I don’t care what happens in the future here”, you better have a good reason for that to be the case, IMO.
If your function gets ownership of, or an exclusive reference to an object, then you know for sure that this reference, for as long as it exists, is the only one in the entire program that can access this object (across all threads, 3rd party libraries, recursion, async, whatever).
References can't be null. Smart pointers can't be null. Not merely "can't" meaning not allowed and may throw or have a dummy value, but just can't. Wherever such type exists, it's already checked (often by construction) that it's valid and can't be null.
If your object's getter lends an immutable reference to its field, then you know the field won't be mutated by the caller (unless you've intentionally allowed mutable "holes" in specific places by explicitly wrapping them in a type that grants such access in a controlled way).
If your object's getter lends a reference, then you know the caller won't keep the reference for longer than the object's lifetime. If the type is not copyable/cloneable, then you know it won't even get copied.
If you make a method that takes ownership of `self`, then you know for sure that the caller won't be able to call any more methods on this object (e.g. `connection.close(); connection.send()` won't compile, `future.then(next)` only needs to support one listener, not an arbitrary number).
If you have a type marked as non-thread safe, then its instances won't be allowed in any thread-spawning functions, and won't be possible to send through channels that cross threads, etc. This is verified globally, across all code including 3rd party libraries and dynamic callbacks, at compile time.
We do the work that’s too large in scope for other teams to handle, and clearly documenting and enforcing best practices is one component of that. Part of that is maintaining a comprehensive linting suite, and the other part is writing documentation and educating developers. We also maintain core libraries and APIs, so if we notice many teams are doing the same thing in different ways, we’ll sit down and figure out what we can build that’ll accommodate most use cases.
I would have guessed linters would have complained about what's being suggested there. Is the something special about var: _ thing that avoids it?
In PHP a pattern I often employ is:
match ($value) {
static::VALUE_1 => ...,
static::VALUE_2 => ...,
default => static::unreachable()
}
static function unreachable() {
throw new Exception('Unreachable');
}
But what I've found is that if I do that, then other programmers go in later and just add in the case to the match statement so it runs. Which, of course, breaks other stuff down stream, because it's not a valid value we can actually use. Or worse: they add a default match arm that doesn't work! Just so the PHP interpreter doesn't complain.
But with this, now the reader knows "the person who wrote this considered what happens when something bad is passed in, and decided we cant handle it. There's probably a good reason for that". So they don't touch it.
Now, PHP has unique challenges because it's so dynamic. If someone passes in the wrong thing we might end up coercing null to zero and messing up calculations, or we might end up truncating a float or something. Ideally we prevent this with enums, but enums are a pain in the ass to write because of autoloading semantics (I don't want to write a whole new file for just a few cases)
The 'defensive' nature refers to the mindset of the programmer (like when guilty people are defensive when being asked a simple question), that he isn't sure of anything in the code at any point, so he needs to constantly check every invariant.
Enterprise code is full of it, and it can quickly lead to the program becoming like 50% error handling by volume, many of the errors being impossible to trigger because the app logic is validating a condition already checked in the validation layer.
Its presence usually betrays a lack of understanding of the code structure, or even worse, a faulty or often bypassed validation layer, which makes error checking in multiple places actually necessary.
One example is validating every parameter in every call layer, as if the act of passing things around has the ability to degrade information.
With a capitalization bit mask of course!
And you can speed up full equality comparisons with a quick cap equality check first.
(That is the how. The when is probably "never". :)
Was it a fiasco? Really? The rust unwrap call is the equivalent to C code like this:
int result = foo(…);
assert(result >= 0);
You can write buggy code in rust just like you can in any other language.
"The fromJust function extracts the element out of a Just and throws an error if its argument is Nothing."
You might think that the Haskell behavior is “safer” in some sense, but there’s a huge gotcha: exceptions in pure code are the mortal enemy of lazy evaluation. Lazy evaluation means that an exception can occur after the catch block that surrounded the code in question has exited, so the exception isn’t guaranteed to get caught.
Exceptions can be ok in a monad like IO, which is what they’re intended for - the monad enforces an evaluation order. But if you use a partial function like fromJust in pure code, you have to be very careful about forcing evaluation if you want to be able to catch the exception it might generate. That’s antithetical to the goal of using exceptions - now you have to write to code carefully to make sure exceptions are catchable.
The bottom line is that for reliable code, you need to avoid fromJust and friends in Haskell as much you do in Rust.
The solution in both languages is to use a linter to warn about the use of partial functions: HLint for Haskell, Clippy for Rust. If Cloudflare had done that - and paid attention to the warning! - they would have caught that unwrap error of theirs at linting time. This is basically a learning curve issue.
https://docs.rs/itertools/latest/itertools/trait.Itertools.h...
If you read the postmortem, they talk in depth about what the issue really was - which from memory is that their software statically allocated room for 20 rules or something. And their database query unexpected returned more than 20 items. Oops!
I can see the argument for renaming unwrap to unwrap_or_panic. But no alternate spelling of .unwrap() would have saved cloudflare from their buggy database code.
Nope. Rust never makes any guarantees that code is panic-free. Quite the opposite. Rust crashes in more circumstances than C code does. For example, indexing past the end of an array is undefined behaviour in C. But if you try that in rust, your program will detect it and crash immediately.
More broadly, safe rust exists to prevent undefined behaviour. Most of the work goes to stopping you from making common memory related bugs, like use-after-free, misaligned reads and data races. The full list of guarantees is pretty interesting[1]. In debug mode, rust programs also crash on integer overflow and underflow. (Thanks for the correction!). But panic is well defined behaviour, so that's allowed. Surprisingly, you're also allowed to leak memory in safe rust if you want to. Why not? Leaks don't cause UB.
You can tell at a glance that unwrap doesn't violate safe rust's rules because you can call it from safe rust without an unsafe block.
[1] https://doc.rust-lang.org/reference/behavior-considered-unde...
A function must check its arguments. It cannot assume that the arguments are already checked (against its own requirements). This is regardless of what called it, or where the values came from.
> Underscore expressions, denoted with the symbol _, are used to signify a placeholder in a destructuring assignment.
[0]: https://doc.rust-lang.org/reference/expressions/underscore-e...
All integer overflow, not just unsigned. Similarly, in release mode (by default) all integer overflow is fully defined as two's complement wrap.
Question: how to encourage such patterns within a team? I often find it difficult to do it during code reviews and leading to unproductive arguments about "code style" and "preferences".
Funnily, these arguments do not happen when a linter pops a warning instead...
You can go ahead and grep your codebase for this today, instead of waiting for an incident.
I'm a fairly new migrant from Java to C#, and when I do some kind of collection lookup, I still need to check whether the method will return a null, throw an exception, expect an out+variable, or worst of all, make up some kind of default. C#'s equivalent to unwrap seems to be '!' (or maybe .Val() or something?)
Also, when I say safety guarantees, I'm not talking about safe rust. I'm talking about Rust features that prevent bugs, like the borrow checker, types like Result and many others.
In other languages I get most of the benefits by sticking to functional programming practices and not mutating stuff all over the place. Rust's type system sort of encodes that, and maybe a little more, by making safe mutation a known non-interfering thing.
The argument to the contrary is that reading the error out-load showed “the config initializer failing to return a valid configuration”. A panic trace saying “config init failed” is a minor improvement.
If we’re gonna guess and point fingers, I think the configuration init should be doing its own panicking and logging when it blows up.
1. It tells you which variable is null. While I think modern Java will include that detail in the exception, that's fairly new. So if you had `a.foo(b.getBar(), c.getBaz())`, was a, b, or c null? Who knows!
2. Putting it in the constructor meant you'd get a stack trace telling you where the null value came from, while waiting until it was used made it a lot harder to track down the source.
Not applicable to all situations, but it could be genuinely helpful, and has been to me.
I probably don't have enough context but whatever identity makes up "your order" goes in the PizzaOrder and not the PizzaDetails. The delivery address, for example, goes in the PizzaOrder.
I have always been critical of the Rust hype but unwrap is completely fine. Is an escape hatch has legitimate uses. Some code is fine to just fail.
It is easy to spot during code review. I have never programmed Rust professional and even I would have asked about the unwrap in the cloudfare code if I had reviewed that. You can even enforce to not use unwrap at all through automatic tooling.
Rust encourages that behavior. Sometimes rightly, but it does build a habit.
I spoke previously about how the Rust book uses the external rand create as a key example and it sets the tone for developers. I'm changing that stance somewhat since it was a decent strategic choice to have crypto packages plug-and-play. But tit still builds a habit.
Anyone who has learned how to program Rust knows that unwrap() will panic if the thing you are unwrapping is Err/None. It's not unassuming at all. When the only person who could be tripped up by a method name is a complete newbie to the language, I don't think it's actually a problem.
Similarly, assert() isn't immediately obvious to a beginner that it will cause a panic. Heck, the term "panic" itself is non obvious to a beginner as something that will crash the program. Yet I don't hear anyone arguing that the panic! macro needs to be changed to crash_this_program. The fact of the matter is that a certain amount of jargon is inevitable in programming (and in my view this is a good thing, because it enables more concise communication amongst practitioners). Unwrap is no different than those other bits of jargon - perhaps non obvious when you are new, but completely obvious once you have learned it.
Yeah, that originally turned me off from the language entirely. I also changed my mind eventually.
As you said, calling first() is a far better approach.
Ideally, imho, a struct is a dumb data holder - it is there to pass associated pieces of data together (or hold a complex unavoidable state change hidden from the user like Arc or Mutex).
All that is to say that adding a field to an existing struct and possibly populating it sparsely in some remote piece of code should not changed existing behavior.
I wonder whether there's a way to communicate to whoever makes changes to the pizza details struct that it might have unintended consequences down the line.
Should one wrap PizzaDetails with PizzaComparator? Or better even provide it as a field in PizzaOrder? Or we are running into Java-esq territory of PizzaComparatorBuilderDefaultsConstructorFactory?
Should we introduce a domain specific PizzaFlavor right under PizzaDetails that copies over relevant fields from PizzaDetails, and PizzaOrder compares two orders by constructing and comparing their flavours instead? A lot of boilerplate.. but what is being considered important to the pizza flavor is being explicitly marked.
In a prod codebase I'd annotate this code with "if change X chaange Y" pre submit hook - this constraint appears to be external to the language itself and live in the domain of "code changes over time". Protobufs successfully folded versioning into the language itself though. Protobufs also have field annotations, "{important_to_flavour=true}" field annotation would be useful here.
It's an overstatement to say that these are "shunned by practically everybody". They're commonly used in scenarios where the author is confident that the failure condition can't happen due to e.g. a prior test or guard, or that failure can be reliably caught. For example, you can catch a `read` exception reliably in IO. They're also commonly used in GHCi or other interactive environments.
I disagree that the Rust perspective on unwrap is significantly different. Perhaps for beginning programmers in the language? But the same is often true of Haskell. Anyone with some experience should understand the risks of these functions, and if they don't, they'll eventually learn the hard way.
One pattern in Rust that may mislead beginners is that unwrap is often used on things like builders in example docs. The logic here is that if you're building some critical piece of infra that the rest of the program depends on, then if it fails to build the program is toast anyway, so letting it panic can make sense. These examples are also typically scenarios where builder failure is unusual. In that case, it's the author's choice whether to handle failure or just let it panic.
You’re right that rust forces you to explicitly decide what to do with Result::Err. But that’s exactly what we see here. .unwrap() is handling the error case explicitly. It says “if this is an error, crash the program. Otherwise give me the value”. It’s a very useful function that was used correctly here. And it functioned correctly by crashing the program.
I don’t see the problem in this code, beyond it not giving a good error message as it crashed. As the old joke goes, “Task failed successfully.”
Almost every codebase I’ve ever worked in, in every programming language, could use better human readable error messages. But they’re notoriously hard to figure out ahead of time. You can only write good error messages for error cases you’ve thought through. And most error cases only become apparent when you stub your toe on them for real. Then you wonder how you missed it in the first place.
In any case, this sort of thing has nothing to do with rust.
Fun facts: Rust’s default panic handler also throws a runtime exception just like C++ and other languages. Rust also has catch blocks (std::panic::catch_unwind). But its rarely used. By convention, panicking in rust is typically used for unrecoverable errors, when the program should probably crash. And Result is used when you expect something to be fallable - like parsing user input.
You see catch_unwind in the unit test runner. (That’s why a failing test case doesn’t stop other unit tests running). And in web servers to return 50x. You can opt out of this behaviour with panic=abort in Cargo.toml, which also makes rust binaries a bit smaller.