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Rust II: data, choices and confidence · Lesson 09 / 12
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Propagate without hiding

A record combines a name and quantity. Each step can fail; ? stops the path with the relevant error.

Learning paths12 min

Let's discover together

  1. 1

    Our teaching format is name;quantity, such as sensor;3. split_once finds the first separator. ok_or("format") converts its Option to Result: a missing separator is a format error.

  2. 2

    quantity(raw)? continues with the number on Ok. On Err, it immediately returns that error from the current function. It neither fixes the text nor retries the call.

  3. 3

    Here both functions return the same error type, &'static str. ? can also convert compatible errors in other programs; its enclosing function needs a compatible return type.

  4. 4

    This is not a general CSV reader: it has no quoting, escaping or separators inside names. Documenting limits avoids overstating the parser. In the project, every record is validated before the collection is used.

FROM LESSON TO PRACTICE

A guide to follow at your pace

Read the steps before using the reference below. Commands run in the indicated system’s terminal; signup happens on the official site. Rullst Academy exercises are practice and do not verify your installation or external account.

fn quantity(text: &str) -> Result<u16, &'static str> {
    let n = text.trim().parse::<u16>().map_err(|_| "number")?;
    if !(1..=20).contains(&n) { return Err("range"); }
    Ok(n)
}
fn parse_line(line: &str) -> Result<u16, &'static str> {
    let (name, raw) = line.split_once(';').ok_or("format")?;
    if name.trim().is_empty() { return Err("name"); }
    let amount = quantity(raw)?;
    Ok(amount)
}
fn main() { println!("{:?}", parse_line("sensor;3")); }

Official links and next step

✦
YOUR MISSION

Read simple records and explain how ? propagates errors.

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.

fn quantity(text: &str) -> Result<u16, &'static str> {
    let n = text.trim().parse::<u16>().map_err(|_| "number")?;
    if !(1..=20).contains(&n) { return Err("range"); }
    Ok(n)
}
fn parse_line(line: &str) -> Result<u16, &'static str> {
    let (name, raw) = line.split_once(';').ok_or("format")?;
    if name.trim().is_empty() { return Err("name"); }
    let amount = quantity(raw)?;
    Ok(amount)
}
fn main() { println!("{:?}", parse_line("sensor;3")); }
Build your answers and check when you are ready.
A moment to understand

What happens when quantity returns Err?

I need a hint

Notice the early return, not only the final Ok.

Loading your journey…

Take it with you

Add tests for sensor, ;3 and sensor;21. Draw the stages and mark where each input stops. Decide what an extra line should mean before changing the parser.

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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At your own pace

My files and GitHub

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