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Rust II: data, choices and confidence · Lesson 08 / 12
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Errors we can handle

Input may be text, zero or a quantity above our limit. Result lets us respond without pretending everything succeeded.

Learning paths12 min

Let's discover together

  1. 1

    Result<T, E> has two variants: Ok with the result and Err with the error. Here T is u16 and E is a fixed &'static str message. That text lives in the program for its entire execution.

  2. 2

    trim removes edge whitespace. parse::<u16>() attempts conversion to a 16-bit unsigned integer. Text, negative values and numbers outside the type can fail before the business rule.

  3. 3

    map_err(|_| "number") maps the conversion error to a short message. The small function between bars ignores the original error. ? returns that Err to the caller; next lesson follows this passage.

  4. 4

    Even a valid u16 may be invalid for our workshop: we accept only 1 through 20, inclusive. Avoid unwrap on fallible input; choose one response for Ok and another for Err.

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 main() { println!("{:?}", quantity("3")); }

Official links and next step

✦
YOUR MISSION

Separate conversion from validation and return Ok or Err.

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 main() { println!("{:?}", quantity("3")); }
Build your answers and check when you are ready.
A moment to understand

Does successful conversion make input valid for every rule?

I need a hint

Zero fits u16, but the workshop requires a positive quantity.

Loading your journey…

Take it with you

Test 1, 20, 21, -1, empty text and " 3 ". Group outputs into success, number error and range error. Explain why validation is more than conversion.

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

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