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`let else` Statements

src/pattern-matching/let-control-flow/let-else.md

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<!-- Copyright 2025 Google LLC SPDX-License-Identifier: CC-BY-4.0 -->

let else Statements

For the common case of matching a pattern and returning from the function, use let else. The "else" case must diverge (return, break, or panic - anything but falling off the end of the block).

rust,editable
# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
    let s = if let Some(s) = maybe_string {
        s
    } else {
        return Err(String::from("got None"));
    };

    let first_byte_char = if let Some(first) = s.chars().next() {
        first
    } else {
        return Err(String::from("got empty string"));
    };

    let digit = if let Some(digit) = first_byte_char.to_digit(16) {
        digit
    } else {
        return Err(String::from("not a hex digit"));
    };

    Ok(digit)
}

fn main() {
    println!("result: {:?}", hex_or_die_trying(Some(String::from("foo"))));
}
<details> The rewritten version is:
rust
# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
    let Some(s) = maybe_string else {
        return Err(String::from("got None"));
    };

    let Some(first_byte_char) = s.chars().next() else {
        return Err(String::from("got empty string"));
    };

    let Some(digit) = first_byte_char.to_digit(16) else {
        return Err(String::from("not a hex digit"));
    };

    Ok(digit)
}

More to Explore

  • This early return-based control flow is common in Rust error handling code, where you try to get a value out of a Result, returning an error if the Result was Err.
  • If students ask, you can also demonstrate how real error handling code would be written with ?.
</details>