The Detroit Post
Tuesday, 19 October, 2021

Rust Do Nothing In Match

author
Elaine Sutton
• Tuesday, 20 October, 2020
• 8 min read

Making statements based on opinion; back them up with references or personal experience. Rust has an extremely powerful control flow operator called match that allows you to compare a value against a series of patterns and then execute code based on which pattern matches.

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Contents

Patterns can be made up of literal values, variable names, wildcards, and many other things; Chapter 18 covers all the different kinds of patterns and what they do. The power of match comes from the expressiveness of the patterns and the fact that the compiler confirms that all possible cases are handled.

Think of a match expression as being like a coin-sorting machine: coins slide down a track with variously sized holes along it, and each coin falls through the first hole it encounters that it fits into. We can write a function that can take an unknown United States coin and, similarly as the counting machine, determine which coin it is and return its value in cents, as shown here in Listing 6-3.

First, we list the match keyword followed by an expression, which in this case is the value coin. When the match expression executes, it compares the resulting value against the pattern of each arm, in order.

If that pattern doesn’t match the value, execution continues to the next arm, much as in a coin-sorting machine. If you want to run multiple lines of code in a match arm, you can use curly brackets.

For example, the following code would print “Lucky penny!” every time the method was called with a Coin::Penny but would still return the last value of the block, 1 : As an example, let’s change one of our ENIM variants to hold data inside it.

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No other coins got state designs, so only quarters have this extra value. We can add this information to our ENIM by changing the Quarter variant to include a Upstate value stored inside it, which we’ve done here in Listing 6-4.

Listing 6-4: A Coin ENIM in which the Quarter variant also holds a Upstate value While we sort our loose change by coin type, we’ll also call out the name of the state associated with each quarter so if it’s one our friend doesn’t have, they can add it to their collection.

Expression, thus getting the inner state value out of the Coin ENIM variant for Quarter. This function is very easy to write, thanks to match, and will look like Listing 6-5.

Listing 6-5: A function that uses a match expression on an Option The Some(5) value doesn’t match the pattern None, so we continue to the next arm.

Especially in the case of Option, when Rust prevents us from forgetting to explicitly handle the None case, it protects us from assuming that we have a value when we might have null, thus making the billion-dollar mistake discussed earlier impossible. Rust also has a pattern we can use when we don’t want to list all possible values.

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By putting it after our other arms, the _ will match all the possible cases that aren’t specified before it. As a result, we can say that we want to do nothing for all the possible values that we don’t list before the _ placeholder.

However, the match expression can be a bit wordy in a situation in which we care about only one of the cases. This week's WWE Not episode saw Tyler Rust make his in-ring debut for the company.

There was an angle mid- match where Rust was tossed to the floor in front of Thatcher. Rust returned to the ring while Camp and Thatcher argued until referees broke it up.

Triple H took to Twitter after the show and gave major praise to Rust. Rust had made his first Not TV appearance the week before that, during the “Thatch-as-Thatch-Can” in-ring segment where he and Thatcher brawled with Camp as the go-home build for the “Takeover: Wakames 2020” match, which saw Camp defeat Thatcher.

Givens later joked about the post- match fist-bump with Rust after responding to a fan tweet. They just announced his signing back on December 2, in the same WWE Performance Center Class as Devon Aiken (Desmond Xavier) and Zachary Green (Zachary Went) of The Rascal.

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At the time of the signing, WWE touted how Rust has competed for wow in Germany, as well as promotions in Japan and across the United States. They filed to trademark the “Tyler Rust name on December 2, the same day his signing was announced.

Reload to refresh your session. You signed out in another tab or window. Hi, when I log in a server I have a few seconds to gather wood, stone, then I can't open the loot box, I can't get wood, I can't get stone, I can't kill players, my torch isn't consumed.

Yey Please fix that bug. Every pattern must be handled exhaustively either explicitly or by using wildcards like _ in the match.

Match can be used to gain access to the inner members of an ENIM and use them directly. One of the primary goals of the Rust project is to enable safe systems programming.

At the same time, to provide safety, Rust programs and data types must be structured in a way that allows static checking to ensure soundness. Rust has features and restrictions that operate in tandem to ease writing programs that can pass these checks and thus ensure safety.

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For example, Rust incorporates the notion of ownership deeply into the language. Rust's match expression is a construct that offers an interesting combination of such features and restrictions.

A match expression takes an input value, classifies it, and then jumps to code written to handle the identified class of data. Structural pattern matching: case analysis with ergonomics vastly improved over a C or Java style switch statement.

(Incidentally, nearly all the code in this post is directly executable; you can cut-and-paste the code snippets into a file demo.rs, compile the file with --test, and run the resulting binary to see the tests run.) This ability to simultaneously perform case analysis and bind input substructure leads to powerful, clear, and concise code, focusing the reader's attention directly on the data relevant to the case at hand.

For this method of problem-solving to work, the breakdown must be collectively exhaustive ; all the cases you identified must actually cover all possible scenarios. This helps catch bugs in program logic and ensures that the value of a match expression is well-defined.

Unlike many languages that offer pattern matching, Rust embraces both statement- and expression-oriented programming. Many functional languages that offer pattern matching encourage one to write in an “expression-oriented style”, where the focus is always on the values returned by evaluating combinations of expressions, and side effects are discouraged.

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(The ability to return from one match arm in the suggest_guess_fixed function earlier was an example of this.) An important case where this arises is when one wants to initialize some state and then borrow from it, but only on some control-flow branches.

In short, for soundness, the Rust language ensures that data is always initialized before it is referenced, but the designers have strived to avoid requiring artificial coding patterns adopted solely to placate Rust's static analyses (such as requiring one to initialize string above with some dummy data, or requiring an expression-oriented style). That version of tree_weight has one big downside, however: it takes its input tree by value.

In fact, in Rust, match is designed to work quite well without taking ownership. Match works by doing this evaluation and then inspecting the data at that memory location.

So, if we want a version of tree_weight that merely borrows a tree rather than taking ownership of it, then we will need to make use of this feature of Rust's match. The only differences are: we take t as a borrowed reference (the & in its type), we added a reference *t, and, importantly, we use ref -bindings for left and right in the Node case.

The *t here is not making a copy of the tree, nor moving it to a new temporary location, because match is treating it as an L-value. The only piece left is the ref -binding, which is a crucial part of how restructuring bind of L-values works.

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Either way, such pattern bindings do mean that the variable I has ownership of a value of type T. We can pass these borrowed references to trees into the recursive calls to tree_weight_v2, as the code demonstrates.

This allows mutation and ensures there are no other active references to that data at the same time. A restructuring binding form like match allows one to take mutable references to disjoint parts of the data simultaneously.

Note that the code above now binds payload by a ref but -pattern; if it did not use a ref pattern, then payload would be bound to a local copy of the integer, while we want to modify the actual integer in the tree itself. Note also that the code is able to bind left and right simultaneously in the Node arm.

The compiler knows that the two values cannot alias, and thus it allows both smut -references to live simultaneously. Rust takes the ideas of algebraic data types and pattern matching pioneered by the functional programming languages, and adapts them to imperative programming styles and Rust's own ownership and borrowing systems.

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