The Detroit Post
Tuesday, 19 October, 2021

Rust Are Optional

author
Danielle Fletcher
• Sunday, 03 January, 2021
• 15 min read

Option s are commonly paired with pattern matching to query the presence of a value and take action, always accounting for the None case. Rust's pointer types must always point to a valid location; there are no “null” references.

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Contents

(where x is an Option) to be converted into your error type, you can implement imply From for YourErrorType. Intercut An iterator over a mutable reference to the Some variant of an Option.

GO BACK Analyzing different approaches for optional parameters in Rust Optional or default parameters are a very interesting feature of some languages that Rust specifically doesn’t cover (and looks like it won’t anytime soon).

Other languages like Python have named arguments, which make optional parameters natural and easier to read: endpoint(mandatory, opt1=val1, opt2=val2), while also allowing them to be written in any order. Even without official support, there are lots of different ways to approach them in Rust, which is what this blog post tries to analyze.

My goal is not to show which one is the “best” option, but to exhaustively showcase the different ways they can be approached, and the ups and downs of each of them. Let’s start with a typical web API wrapper library.

Sometimes the best solution is the simplest, and your project just might not need more complex approaches. Parameter names unknown when reading the code, which is specially annoying with None values, since these don’t have context to know what they are for.

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Some isn’t required, which makes it slightly easier to read. Requires generics, 2^N copies of this function may be generated, where N is the number of optional parameters.

Some APIs might feel more natural this way, if the combination of these parameters as a group makes sense. Optional parameters are now associated to their name, which makes it easier to read.

Starting from the previous approach, we could make the call inside the endpoint struct itself instead of in the API’s client by adding it to Approach, or by overriding the build method from derive_builder for a slightly less verbose version: Note: this is assuming the client contains necessary information to make the requests, like a request::Client or authentication details.

Simple to use and implement, since it doesn’t need declaring both a function in the client and a parameters' struct, only the latter. Slight builder pattern overhead, and can also fail to construct the value.

We can remove some disadvantages of the builder pattern by using a different approach and possibly a custom implementation instead of just using derive_builder. The client will now have a method that calls ApproachBuilder::default() and whatever is necessary to start building the endpoint.

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Mandatory parameters are added to that function’s signature, so that the build can never fail. This also avoids passing the client in call(API), since we can do that inside the method.

Derive_builder doesn’t know that build() can never technically fail because the mandatory parameters are provided inside the wrapper, so it’s kind of a waste to use this macro. Currently, a bit hack to implement, which makes it much more complex, specially because there’s no existing macro that can simplify this specific variation of the builder pattern (that I know of).

The call method could be removed in place of approach_f, and the optional parameters would go first. As the approach_h endpoint indicates, the shared optional parameters don’t actually have to be strictly the same.

As long as they are related, they can share the same group, and some extra verifications can be added in the final call to make sure the user is using it properly. Basically the same as the hybrid builder pattern, but with an easier implementation, and it might fit perfectly for some APIs that have clearly established groups of endpoints.

And as it’s based on the hybrid builder pattern, it may still be hack to implement and require compilation-time overhead. I wanted to at least try how existing crates approached this, and only found named and Huang, which haven’t been updated in years, and probably for good.

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I tried Huang with Rust 1.47 but got some unexpected errors, so we can assume there are no crates that support this yet. Some of these endpoints might be unnecessarily complicated or straight up weird, but I hope this was as a good showcase of the different ways optional parameters can be approached in Rust, and that reading this served as a learning experience.

I look forward to seeing new crates in the future that help make some approaches easier to implement. Bear in mind that there are a lot of different ways to implement the approaches, as I explained in this post.

Initial values Return values for functions that are not defined over their entire input range (partial functions) Return value for otherwise reporting simple errors, where None is returned on error Optional struct fields that can be loaned or “taken” Optional function arguments Nullable pointers Swapping things out of difficult situations Option s are commonly paired with pattern matching to query the presence of a value and take action, always accounting for the None case.

Rust's pointer types must always point to a valid location; there are no “null” references. It is further guaranteed that, for the cases above, one can me::transmute from all valid values of T to Option and from Some::(_) to T (but transmuting None:: to T is undefined behavior).

Nonzero Experimentalize error type that results from applying the try operator (? ) (where x is an Option) to be converted into your error type, you can implement imply From for YourErrorType.

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Intercut An iterator over a mutable reference to the Some variant of an Option. Rust avoids the billion dollar mistake of including null s in the language.

There is plenty of material out there detailing why an Option type is better than null, so I won’t go too much into that. So, unwrap() panics and exits the program when the Option is empty i.e. None.

Since Option is actually just an ENIM, we can use pattern matching to print the middle name if it is present, or a default message if it is not. Turns out we can conveniently use ref in a pattern match to borrow a reference.

And_then() is another method that allows you to compose Options (equivalent to flat map in other languages). Suppose we have a function that returns a nickname for a real name, if it knows one.

Now, to figure out a person’s middle name’s nickname (slightly nonsensical, but bear with me here), we could do: As such, in the case of Jon, since the middle name is None, the get_nickname() function will not be called at all, and the above will print “(none found)”.

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The purpose of Null Tracking in general (of which Nullable Types are only one of many forms), is to somehow regain a modicum of safety (and sanity) in languages that have null references. If you have the chance to eliminate null references altogether, that is a much better solution since the problems that null references cause simply will not exist in the first place.

In other words, for each acts exactly like a NULL check! The only reason, really, for you to explicitly check for the existence of an Option is if you want to do something completely different in case the value is missing.

Languages like C#, Scala, and Haskell have built in syntax sugar for working with monads, and they have powerful libraries for working with monads. I will not go into details about what it means to be a monad, but e.g. one of the advantages is that there are some specific mathematical laws and properties associated with monads, and one can exploit those properties.

The fact that Java's Optional is not implemented as a monad, not even as a collection, is a significant design flaw, and I think is partially to blame for people not understanding the advantages of Option s, simply because some of those advantages cannot be realized with Java's Optional. There is also a more philosophical reason for choosing an Option type over NULL references.

There is a major difference between those two: NULL references are a language feature whereas Option is a library type. That means that if for my code, I need to handle the absence of values in a slightly different manner, I can write a Motion.

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But I cannot write a Manual reference without changing the language semantics and thus the compiler (or, for a language like C, C++, Java, Go, ECMAScript, Python, Ruby, PHP with multiple implementations, every single compiler and interpreter that exists, has existed, and will ever exist). Also, the more the language designer moves out of the language into libraries, the more the compiler writers are forced to make library code fast.

If a compiler writer figures out some clever trick to make NULL references fast, that doesn't help our hypothetical programmer who has written their own abstraction. But if a compiler writer figures out some clever trick to make Option fast, it is highly likely the same trick will also apply to Motion (and Try, Either, Result, and possibly even every collection).

Unfortunately, because it is designed to interoperate and integrate deeply with the host environment (the Java platform, the ECMAScript platform, there is also an abandoned CLI implementation), it has null references and exceptions. And Try first appeared in a library of helpers released by Twitter.

I can write my own Scala Option type, and I don't need to change the compiler for it: A NullObject works in that it conforms to an interface a can be used anywhere the normal object can be while doing some sort of default “bullish” behavior.

At this level it boils down to how the type system is formulated. Usually these primitives are reflections of either an historical implementation, or of some underlying platform constraint.

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In the case of C# the language, the type system is sufficiently complex to support an Optional type allowing value types to gain a nullable state, but there isn't a trivial way to remove null ability from the object references. Some systems prefer to have very simple primitive types, and rely on a powerful type composition system to create the desired behaviors.

In these languages a C# nullable Reference might look like def cs_reference(T) NULL | T. Using a sequence/array concept leverages our understanding of empty, and has one element.

Much of the angst over nulls are due to languages where every reference type is nullable by default. In Typescript, it would be declared as a (actually | null) type union.

On the face of it they seem similar, but a significant difference is that containers nest but type unions don't. The lookup function could return an option: Nothing if the key does not exist, otherwise Some value.

Dictionaries are just an example, the problem arise anywhere you have a data-structure with multiple levels of optional elements. Option types solve this elegantly, and in a way that is consistent with the rest of the language.

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Jacques JacquesB49.3k1818 gold badges105105 silver badges143143 bronze badges Pass me an option, or an empty collection, or a null object, and I can avoid needing the check.

Use this wisely and it makes code easier to read. Insist on checking for null and checks clutter code and confusion is caused because now some nulls are meant to cause exceptions and other aren’t.

Some might argue to fail early, but that could have happened before the null even got here. Pass me a null and you have to hope I know what you wanted to be done with it.

If you believe in fail early and want a process halting exception thrown just throw it. It is possible to design complex systems that don’t even permit the use of null.

The third state is determined statically at compile time. Those types of static checks are a relatively recent invention.

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So a big part of the reason why more languages don't use statically-checked nullable is because options were invented first. I think more languages will drift to the statically-checked nullable model over time, but more languages will also drift to the option model, because it has its own advantages.

There is Either, Try, IO, Future, Task, Observable, Validation, and many more, all with their own use cases. It seems very odd to me to give options special treatment in the compiler and leave all the rest in libraries, especially given how common it is to do things like change Option code to Either code when requirements change.

AFAIK, Option type will have runtime overhead, while nullable types won't, because Option time is an ENIM (consuming memory). In Rust, Option> is represented by a nullable pointer.

Which is what Option does, and it optimizes the heap case to use null pointers. Swift has a clever feature in the compiler: If “all bits zero” is not a valid value for type T, then the type optional uses all bits zero to represent nil.

Haskell never had null in the first place, with that idea being represented with the Maybe type from the very beginning. Scala in particular draws a lot of inspiration from Haskell while keeping null purely for interoperability with Java.

Although Rust has null pointers under the hood for bare metal work and foreign function interoperability, it opted to consider working with pointers unsafe, providing the Option type for that purpose in safe contexts. It's nothing more than a design choice that draws inspiration from Haskell.

It happens to fit well with Rust's model of integrating lifetimes into its type system, so it seems like a natural design choice. Whether Nullable or Options are better is a hotly debated issue among programming language enthusiasts.

There is nothing objectively superior about Option types, just certain use cases it happens to excel at. Both Options and Nullable types (with static analysis) solve the same problem, each with their own trade offs.

In the best case, the two are identical in core function and performance. Options have the benefit of nesting, whereas Nullable are less verbose, have stronger performance guarantees, and tend to come with less complex type systems (potentially saving compilation time).

Semantically, Options and nullable types are pretty similar. Rather, you seem more interested in the implementation detail of using null pointers rather than some sort of ENIM.

Thus, nullable versions of those types couldn't be implemented as null pointers. The easy case is Option, which can be implemented as a null pointer.

The compiler knows that Nonzero can never contain a null pointer, so it sets that field deep inside the String to null to indicate that the Option is None. The consequence is that nullable pointer types wouldn't work for a language like Rust.

I suspect that all languages with optional types that care about performance will ensure it gets implemented as a null pointer when that is suitable. So, there is no performance reason to shy away from Optional types in favor of nullable.

So we need the compiler to be clever enough to know that after the if-statement, p cannot be null. So either you make massive changes in the compiler, or the feature is quite useless.

First, there is a very general and very useful feature in Swift: ends with associated values. Optional are not even part of the language, they are implemented in the standard library.

Nil is translated to a “NilConvertibleLiteral” (I'm probably spelling this wrong). The other bit of syntactic sugar makes all the difference: “if let x = expression {...} else {...}.

If it is not nil, then the non- optional value is extracted and assigned to x, and the first list of statements is executed; if the expression is nil, the second list of statements is executed (optional). Like a function converting a string to an integer returns an optional Int, so you write “if let i = Int(string) {success} else {failure}, and you can't avoid the test.

You actually cannot perform nil tests for non- optional values. Converting Objective-C to Swift I found lots of paranoid nil tests just disappearing.

A reference can be null but this fact is not encoded in the type system. Anytime you receive a pointer to something you don't know whether it is pointing to an actual object or null.

However, this isn't an issue in TypeScript and other languages that understand null references. I can release an update to my library and not break any of its consumers.

The same problem with Option also exists in the reverse -- when making return types provide stronger guarantees. Earn 10 reputation in order to answer this question.

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