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Saturday, 16 October, 2021

Rust Enum

James Lee
• Friday, 20 November, 2020
• 18 min read

The ENIM keyword allows the creation of a type which may be one of a few different variants. The most common place you'll see this is in imply blocks using the Self alias.



Let’s look at a situation we might want to express in code and see why ends are useful and more appropriate than structs in this case. We can express this concept in code by defining an IpAddrKind enumeration and listing the possible kinds an IP address can be, V4 and V6.

Note that the variants of the ENIM are name spaced under its identifier, and we use a double colon to separate the two. Given that you just learned about structs in Chapter 5, you might tackle this problem as shown in Listing 6-1.

Listing 6-1: Storing the data and IpAddrKind variant of an IP address using a struct The second instance, loop back, has the other variant of IpAddrKind as its kind value, V6, and has address ::1 associated with it.

This new definition of the Paddy ENIM says that both V4 and V6 variants will have associated String values: We attach data to each variant of the ENIM directly, so there is no need for an extra struct.

There’s another advantage to using an ENIM rather than a struct: each variant can have different types and amounts of associated data. Version four type IP addresses will always have four numeric components that will have values between 0 and 255.


However, as it turns out, wanting to store IP addresses and encode which kind they are is so common that the standard library has a definition we can use! This code illustrates that you can put any kind of data inside an ENIM variant: strings, numeric types, or structs, for example.

Defining an ENIM with variants such as the ones in Listing 6-2 is similar to defining different kinds of struct definitions, except the ENIM doesn’t use the struct keyword and all the variants are grouped together under the Message type. But if we used the different structs, which each have their own type, we couldn’t as easily define a function to take any of these kinds of messages as we could with the Message ENIM defined in Listing 6-2, which is a single type.

In this example, we’ve created a variable m that has the value Message::Write(String::from(“hello”)), and that is what self will be in the body of the call method when runs. Let’s look at another ENIM in the standard library that is very common and useful: Option.

In the previous section, we looked at how the Paddy ENIM let us use Rust ’s type system to encode more information than just the data into our program. This section explores a case study of Option, which is another ENIM defined by the standard library.

The Option type is used in many places because it encodes the very common scenario in which a value could be something or it could be nothing. Expressing this concept in terms of the type system means the compiler can check whether you’ve handled all the cases you should be handling; this functionality can prevent bugs that are extremely common in other programming languages.

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At that time, I was designing the first comprehensive type system for references in an object-oriented language. My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler.

But I couldn’t resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

Because this null or not-null property is pervasive, it’s extremely easy to make this kind of error. As such, Rust does not have nulls, but it does have an ENIM that can encode the concept of a value being present or absent.

The Option ENIM is so useful that it’s even included in the prelude; you don’t need to bring it into scope explicitly. For now, all you need to know is that means the Some variant of the Option ENIM can hold one piece of data of any type.

In effect, this error message means that Rust doesn’t understand how to add an i8 and an Option, because they’re different types. Not having to worry about incorrectly assuming a not-null value helps you to be more confident in your code.

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The Option ENIM has many methods that are useful in a variety of situations; you can check them out in its documentation. In general, in order to use an Option value, you want to have code that will handle each variant.

The match expression is a control flow construct that does just this when used with ends: it will run different code depending on which variant of the ENIM it has, and that code can use the data inside the matching value. Ends in Rust are similar to those of other compiled languages like C, but have important differences that make them considerably more powerful.

What Rust calls ends are more commonly known as Algebraic Data Types if you're coming from a functional programming background. The second shows off a hypothetical example of something storing location data, with Chord being any other type that's needed, for example a struct.

The third example demonstrates the kind of data a variant can store, ranging from nothing, to a tuple, to an anonymous struct. In that they cannot be instantiated at all, and are used mainly to mess with the type system in interesting ways.

Here's a table of contents of what you'll learn in this lesson:(click on a link to skip to its section) An ENIM is a set of named integer constants and allow us to create a set of symbolic names that map to known numerical values.

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Simply put, an ENIM is a word that represents a number. Boolean true and false values are actually just words representing the numbers 1 and 0 respectively.

Ends allow us to create our own words that map to numbers like Boolean. To declare an enumeration, we write the ENIM keyword, followed by a unique name and a code block.

Inside the code block we declare our actual words that will map to numbers, separated by commas. Ends in Rust are conventionally written with pascal case.

In pascal case we write the first letter of the first word in uppercase. Any subsequent words have an uppercase first letter and are not separated with an underscore.

In the example above, we declare an ENIM called Auth, with two integer values. Once again we use the # statement above the ENIM and struct to allow us to print it to the console.

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Because of that, the option ENIM is a generic, which means it has a placeholder for a type. The between the angle brackets is where we specify the type that the Option ENIM may return.

Because we specify built as the generic type, Some can only return true or false. Finally, we’ll cover how the if let construct is another convenient and concise idiom available to you to handle ends in your code.

Rust ’s ends are most similar to algebraic data types in functional languages, such as F#, Ocaml, and Haskell. It doesn't see strings as copyable (“does not implement the Copy trait”).

We would not see this with 'primitive' types like numbers, since they are just values; they are allowed to be copyable because they are cheap to copy. It does not refer to any other memory, so it's just as cheap to copy as to move.

Here a reference rs1 is made to a value tmp which only lives for the duration of its block: Gone, dead, gone back to the Big Heap in the Sky: dropped.

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Tuples are convenient, but saying t.1 and keeping track of the meaning of each part is tedious for anything that isn't straightforward. Initializing this struct is a bit clumsy, so we want to move the construction of a Person into its own function.

To understand the complaint, you have to see the problem from the point of view of Rust. Rust cannot allow a situation where that reference could suddenly become invalid.

String literals exist for the duration of the whole program, which is called the 'static' lifetime. It is not the most pretty notation, but sometimes ugliness is the necessary price of being precise.

This can also be used to specify a string slice that is returned from a function: That works for the special case of static strings, but this is very restrictive.

You can usefully think of lifetime parameters as being part of the type of value. It's basically impossible because structs must be moveable, and any move will invalidate the reference.

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Getting comfortable with Rust involves learning the basic traits of the standard library (they tend to hunt in packs.) Display controls how values are printed out with “{}” and is implemented just like Debug.

Recall the informal definition of an iterator; it is a struct with a next method which may return Some -thing or None. This is because 0.1 is not precisely representable as a float, so a little formatting help is needed.

And we get cleaner output (this format means 'one decimal after dot'.) All the default iterator methods are available, so we can collect these values into a vector, map them, and so forth.

We want a function which will dump out any value that implements Debug. Here is a first attempt at a generic function, where we can pass a reference to any type of value.

C++ template errors are famously bad, because all the compiler knows (ultimately) is that some operator or method is not defined. The C++ committee knows this is a problem, and so they are working toward concepts, which are pretty much like trait-constrained type parameters in Rust.

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Rust generic functions may look a bit overwhelming at first, but being explicit means you will know exactly what kind of values you can safely feed it, just by looking at the definition. The body of the function is compiled separately for each unique type.

With polymorphic functions, the same machine code works with each matching type, dynamically dispatching the correct method. Monomorphic produces faster code, specialized for the particular type, and can often be inclined.

As always, there are trade-offs; an experienced person learns to make the right choice for the job. The match expression is the basic way to handle ENIM values.

So this will cycle endlessly through the various directions in this particular, arbitrary, order. The solution is to say # in front of ENIM Direction.

If this isn't so, or you want to redefine equality, then you are free to define Partial explicitly. Rust ends in their full form are like C unions on steroids, like a Ferrari compared to a Fiat UNO.

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But they also know how what kind of value they contain, and that is the superpower of match : Rust is not letting you extract the string contained in the original value.

I mentioned earlier that match is picky about exact types; here we follow the hint and things will work; now we are just borrowing a reference to that contained string. Before we move on, filled with the euphoria of a successful Rust compilation, let's pause a little.

The issue is a combination of the exactness of matching, with the determination of the borrow checker to foil any attempt to break the Rules. One of those Rules is that you cannot yank out a value which belongs to some owning type.

You will get exactly the same error if you try to pull out a string from a vector, say with *v.get(0).unwrap() (* because indexing returns references.) Often that inferred type is cool, when you eat up a value and extract its contents.

This is a special case of restructuring ; we have some data and wish to either pull it apart (like here) or just borrow its values. It's a slightly obscure gotcha (look up the E00008 error) where if you have an if guard you need to borrow, since the if guard happens in a different context, a move will take place otherwise.


If you are in a function returning Result, then the question-mark operator provides a much more elegant solution: A great deal of Rust's power comes from closures.

This is the necessary cost of Rust's promise to not sneakily make any allocations. In JavaScript, the equivalent mutate(function() {s = “hello”;}) will always result in a dynamically allocated closure.

Because we might need to call them at a point where the original context no longer exists. A major use of closures is within iterator methods.

The three kinds correspond (again) to the three basic argument types. Personally I prefer being explicit, but it's important to understand both forms, and their implications.

It's a definite gotcha for Pythonistas used to saying for s in DEC ! So the implicit form for s in over is usually the one you want, just as at is a good default in passing arguments to functions.

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When calling methods, Rust will deference automatically, so the problem isn't obvious. But |x: string| x == “one”| will not work, because operators are more strict about type matching.

(Strictly speaking, this is lexical order, since human languages are very diverse and have strange rules.) Here is a method which inserts nodes in lexical order of the strings.

The type parameter must be declared in the imply block with its constraints: So generic structs need their type parameter(s) specified in angle brackets, like C++.

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