The Detroit Post
Saturday, 16 October, 2021

Rust Match When

Danielle Fletcher
• Sunday, 11 October, 2020
• 9 min read

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.

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Rust 2018, by contrast, will infer the & s and ref s, and your original code will Just Work. This affects not just match, but patterns everywhere, such as in let statements, closure arguments, and for loops.

The mental model of patterns has shifted a bit with this change, to bring it into line with other aspects of the language. In Rust 2018, the type of the value being matched informs the binding mode, so that if you match against an option with a Some variant, you are put into ref mode automatically, giving you a borrowed view of the internal data.

Here's a table of contents of what you'll learn in this lesson:(click on a link to skip to its section) Rust allows us to control the flow of our program through specific expressions that evaluate conditions.

Based on the result of these conditions, Rust will execute sections of specified code. Before a user can gain access to the member area, they need to provide the correct login credentials.

If they provide the correct credentials, they are allowed into the exclusive member area. Our actual code will not look exactly like the example above, but it serves to give you an example of what conditional control is.

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We control the flow of our program with the following statements, in combination with relational comparison and logical operators. If the condition proves true, the compiler will execute the if statement’s code block.

An if statement is written with the keyword if, followed by the condition(s) we want it to evaluate. Then, we specify a code block with open and close curly braces.

Inside the code block we write one or more expressions to be executed if the evaluation proves true. The closing curly brace will tell the compiler where the if statement ends.

The compiler then prints the message inside the if statement’s code block. When the compiler has finished executing the if statement, it will move on to any code after it.

In the example above, the first condition proves false, but the second condition proves true so the compiler is allowed to execute the code in the code block. The compiler will move to evaluate each if statement in the ladder when it’s done with previous one.

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In the example above, we add a secondary if statement at the end of the first, separated by the else keyword. The compiler will perform the evaluations hierarchically, that’s to say, it will start out the outer if statement and move inwards.

The compiler then evaluates the inner if statement and executes its code block if its condition proves true. If you’re familiar with a languages in the C family, it would be similar to a switch statement, although the syntax is different.

Note that the code block is terminated with a semicolon after the closing curly brace. The final value to match is simply an underscore.

In this case, it matched to “B” on the left and so it printed the string “Great job, you got a B!”. This is simply because we don’t need to, the execution statement of the match already prints a message.

When the compiler finds a match on the left of the operator, it will return the value on the right of the into the result variable. This time, the compiler matched to “A” on the left, so it stored the string “Excellent!” into the result variable.

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To do this, we simply write the if statement where the value should be in the variable initialization. The execution expressions must provide a value to be assigned to the variable.

The individual statements in the code blocks are not terminated with a semicolon. The if statement itself is terminated with a semicolon after the else closing curly brace.

In the example above, the if statement will prove true so the value assigned to the number variable is 1. If we try to assign different types of values to it through the if statement, the compiler will raise an error.

In many languages, such as C or Python we can write a shortened version of a simple if/else statement, known as the ternary operator. Rust, removed the ternary operator back in 2012.

The language doesn’t support a ternary operator anymore. We can control the flow of our application with conditional logic through if, else if, else and match statements.

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We can conditionally set the value of a variable with an if statement with an if let expression. An if let expression places the if statement as the value of a variable declaration.

An if let expression is terminated with a semicolon after the code block closing brace. Values inside the code blocks of an if let expression are not terminated with a semicolon.

Making statements based on opinion; back them up with references or personal experience. 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.

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.

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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, 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.

(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.

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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.

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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