Step 5 of 7

main can return a Result

? needs a function that returns Result, and main is a function. It can return Result<(), E>:

use std::num::ParseIntError;

fn main() -> Result<(), ParseIntError> {
    let width: u32 = "1920".parse()?;
    let height: u32 = "1080".parse()?;
    println!("{} pixels", width * height);
    Ok(())
}

2073600 pixels

• Result<(), ParseIntError>: on success there's nothing to give back, so the Ok type is (), the unit type.
• Ok(()) at the end says "finished successfully". The double parentheses are Ok wrapped around the unit value ().
• If any ? hits an error, main returns that Err.

What happens when main returns Err

Rust prints Error: followed by the error's {:?} form to stderr, and the program ends with exit code 1 instead of 0. The exit code is how a program tells whoever started it (a shell script, a CI pipeline, another program) whether it worked: 0 means success, anything else means failure.

$ cargo run
Error: ParseIntError { kind: InvalidDigit }
$ echo $?
1

This makes main() -> Result great for tools and scripts: you write the happy path with ? and every failure stops the program with a non-zero exit code. The message is the developer-oriented {:?} form, though. Polished command-line tools print a friendly message themselves (module 40 covers that).

Any error type works, as long as it can be printed with {:?}. With String errors:

fn main() -> Result<(), String> {
    Err(String::from("config file missing"))
}

// prints to stderr:  Error: "config file missing"

Common mistakes

• Forgetting Ok(()) at the end. Without it, main's body ends with a statement, so it "returns" (), not a Result:

use std::num::ParseIntError;

fn main() -> Result<(), ParseIntError> {
    let width: u32 = "1920".parse()?;
    println!("{width}");
}

error[E0308]: mismatched types
  | fn main() -> Result<(), ParseIntError> {
  |    ----      ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Result<(), ParseIntError>`, found `()`

• Mixing error types in main. read_line fails with an std::io::Error, parse with a ParseIntError. A main that returns Result<(), ParseIntError> can't use ? on both. The next module introduces Box<dyn Error>, a type that holds any error, which solves this. Until then, expect on read_line is reasonable: if reading stdin fails, there's nothing better to do.

Your task

A scoreboard reads points from input (several per line, separated by spaces) and prints the running total after each one. Make main return Result<(), ParseIntError> and replace the unwrap() with ?, so a bad value stops the program with exit code 1 (after the totals printed so far). For input 10 20 30 the output is:

running total: 10
running total: 30
running total: 60

Challenge 2

A room planner reads one line with a width and a height. Make main return Result<(), ParseIntError> and use ? for both parses. Print width: <w> right after parsing the width, then height: <h>, then area: <w*h>. For input 3 4:

width: 3
height: 4
area: 12

If a value is missing or not a number, the program should stop with exit code 1.

Challenge 3

An ATM processes three withdrawals, 30, 50 and 40, from a balance of 100. Make main return Result<(), String> and apply each withdrawal with withdraw(...)?, printing withdrew <amount>, balance <balance> after each success and all done at the end.

The third withdrawal fails, so your program should print two lines and then stop with exit code 1 (Rust prints the Error: ... line to stderr).

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