Between Option and Result in Rust

The two conversions worth memorising: ok_or adds an error, ok drops one.

fn first_word(line: &str) -> Result<&str, &'static str> {
    line.split_whitespace().next().ok_or("empty line")
}

fn main() {
    println!("{:?}", first_word("hello there"));
    println!("{:?}", first_word("   "));

    let parsed: Option<i32> = "12".parse().ok();
    println!("{:?}", parsed);

    let nested: Result<Option<i32>, std::num::ParseIntError> =
        Some("5").map(|s| s.parse()).transpose();
    println!("{:?}", nested);
}

How it works

1. ok_or turns None into an Err you choose.
2. ok throws the error away and keeps a Option.
3. transpose swaps a nested Option and Result.

Keywords and builtins used here

• Option
• Result
• Some
• first_word
• fn
• i32
• let
• main
• static
• str

The run, in numbers

Lines

15

Characters to type

407

Tokens

148

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 93 seconds.

Type this snippet

Step 3 of 3 in Option, step 3 of 15 in Error handling.

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