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Rust II: data, choices and confidence · Lesson 05 / 12
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Choices that carry data

Checkout and return are different actions, but both need a quantity. An enum expresses these alternatives.

Learning paths10 min

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  1. 1

    enum Action { Take(u16), Return(u16) } defines two variants. Each carries a quantity. Action::Take(2) is more than the number 2: it records checkout intent too.

  2. 2

    match action selects the corresponding arm. In Action::Take(n), n receives that variant’s quantity. The expression result can feed let available.

  3. 3

    match must cover every possibility. Adding a variant requires revisiting matches without a catch-all arm. This helps locate forgotten decisions.

  4. 4

    These examples use previously valid quantities. An enum alone does not guarantee stock or a legitimate return. The final project applies these rules with Result before changing state.

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enum Action { Take(u16), Return(u16) }
fn change(stock: u16, action: Action) -> u16 {
    match action { Action::Take(n) => stock - n, Action::Return(n) => stock + n }
}
fn main() { println!("{}", change(5, Action::Take(2))); }

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✦
YOUR MISSION

Represent alternatives with enum and handle them all with match.

Now it's your turn

Discovery workshop

Read, predict, investigate

Predict before revealing. This panel shows prepared results; it does not compile your code. Compiler errors are summarised; exact wording can vary with the compiler version.
Mini experiment

Learning simulation: compare cards and check your predictions. The browser is not compiling Rust or starting a server.

enum Action { Take(u16), Return(u16) }
fn change(stock: u16, action: Action) -> u16 {
    match action { Action::Take(n) => stock - n, Action::Return(n) => stock + n }
}
fn main() { println!("{}", change(5, Action::Take(2))); }
Build your answers and check when you are ready.
A moment to understand

What does an exhaustive match guarantee here?

I need a hint

Covering variants and validating quantities are different responsibilities.

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Take it with you

Add an Inspect variant without a quantity. Predict the diagnostic before compiling; then add an arm that preserves stock.

RUST II · .rs

Your Rust II workbench

Read, predict and answer here. To actually compile, extract the activity ZIP into a new folder, open a terminal beside Cargo.toml and use the commands below. Rust and your operating system tools must be installed.

cargo check --offline
cargo test --offline
cargo run --offline
My files and GitHub

The editor below saves a draft in this browser and lets you download or publish your .rs. To run it in the project, save the edited file as src/main.rs. The ZIP contains the starter version; prepared results do not change with your edits. Online grading does not verify local execution.

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