Rust: What is the difference between map_err and and_then?

map_err — transform error. and_then — chain operations.

map_err:

• Transform error type.
• For error conversion.
• Preserve Ok value.

and_then:

• Chain operations.
• Flat mapping.
• For sequential operations.

1fn parse_and_double(s: &str) -> Result<i32, String> {
2    s.parse::<i32>()
3        .map_err(|e| e.to_string())
4        .and_then(|n| Ok(n * 2))
5}

Key: map_err for error conversion, and_then for chaining.