coding-horror/basic-computer-games · error

Failed to read the line

Error message

Failed to read the line

What it means

This panic is triggered by `.expect("Failed to read the line")` on `io::stdin().read_line()` inside `Question::ask()` in the Literature Quiz game. `read_line` returns `Err` only when the underlying stdin stream encounters an I/O error or is closed/redirected to an invalid source. Because `.expect` is used instead of graceful error handling, any such failure aborts the entire process with this message.

Source

Thrown at 57_Literature_Quiz/rust/src/main.rs:63

        question: &'a str,
        choices: Vec<&'a str>,
        answer: u8,
        correct_response: &'a str,
        wrong_response: &'a str,
    }

    impl Question<'_>{
        fn ask(&self) -> bool {
            println!("{}", self.question);
            for i in 0..4 {
                print!("{}){}", i+1, self.choices[i]);
                if i != 3 { print!(", ")};
            }
            println!("");
            let mut user_input: String = String::new();
            io::stdin()
                .read_line(&mut user_input)
                .expect("Failed to read the line");

            if user_input.starts_with(&self.answer.to_string()) {
                println!("{}", self.correct_response);
                true
            } else {
                println!("{}", self.wrong_response);
                false
            }
        }
    }

    let questions: Vec<Question> = vec![
        Question{
            question: "IN PINOCCHIO, WHAT WAS THE NAME OF THE CAT?",
            choices: vec!["TIGGER", "CICERO", "FIGARO", "GUIPETTO"],
            answer: 3,
            wrong_response: "SORRY...FIGARO WAS HIS NAME.",
            correct_response: "VERY GOOD!  HERE'S ANOTHER.",

View on GitHub (pinned to 5301155192)

Solutions

1. Replace `.expect("Failed to read the line")` with `match` on the `Result`, printing a friendly prompt and retrying on `Err` rather than panicking.
2. Detect EOF specifically (0 bytes read) and exit the quiz cleanly with a goodbye message instead of crashing.
3. If running in CI or a non-interactive context, provide input via a piped file or `expect`-style automation so stdin never closes mid-question.

Example fix

// before
io::stdin()
    .read_line(&mut user_input)
    .expect("Failed to read the line");

// after
match io::stdin().read_line(&mut user_input) {
    Ok(0) => {
        println!("\nInput closed. Goodbye!");
        std::process::exit(0);
    }
    Ok(_) => {}
    Err(e) => {
        eprintln!("Could not read input: {}", e);
        std::process::exit(1);
    }
}

// Check stdin availability before reading
use std::io::IsTerminal;
if !io::stdin().is_terminal() && std::env::var("FORCE_NONINTERACTIVE").is_err() {
    eprintln!("Warning: stdin is not a terminal; input may fail.");
}

// Rust does not use type guards; instead use Result matching
fn safe_read_line() -> Option<String> {
    let mut buf = String::new();
    match io::stdin().read_line(&mut buf) {
        Ok(0) | Err(_) => None,
        Ok(_) => Some(buf),
    }
}

match io::stdin().read_line(&mut user_input) {
    Ok(0) => { println!("Input closed."); std::process::exit(0); }
    Ok(_) => { /* proceed */ }
    Err(e) => { eprintln!("Read error: {}", e); std::process::exit(1); }
}

When it happens

Trigger: The program's stdin is not a readable TTY — e.g., it is launched in an environment without an attached terminal, stdin is redirected from a closed file descriptor, or an automated test harness pipes EOF (Ctrl+D) into the process. In `Question::ask()`, the call is on the hot path of every quiz question, so the first such stdin disruption panics immediately.

Common situations: Running the binary under CI or a service manager (systemd, Docker with no `-it`) that does not wire up stdin. Piping `/dev/null` or an empty file as input. Pressing Ctrl+D on the terminal instead of typing an answer. Embedding the quiz in a GUI wrapper that forgets to provide stdin.

Related errors

• No valid input
• CANNOT READ INPUT!
• CANNOT READ INPUT!
• Failed to read line.
• Failed to read line.

AI-assisted analysis of coding-horror/basic-computer-games@5301155192 (2026-08-13). Data as JSON: /api/errors/da9eb88a482f4608. Report an issue: GitHub.