How to handle exception (Err) in unwrap_or_else? - rust

Question

struct OverflowError {}
fn test_unwrap() -> Result<String, OverflowError> {
let a: Result<String, u8> = Err(100);
let a: String = a.unwrap_or_else(|err| {
if err < 100 {
String::from("Ok")
} else {
// I want to return from the function (not just the closure)
// This is compile error with error:
// "the ? operator can only be used in a closure that returns Result or Option"
Err(OverflowError {})?
}
});
Ok(a)
}
error[E0277]: the `?` operator can only be used in a closure that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
--> src/lib.rs:13:13
|
6 | let a: String = a.unwrap_or_else(|err| {
| ______________________________________-
7 | | if err < 100 {
8 | | String::from("Ok")
9 | | } else {
... |
13 | | Err(OverflowError {})?
| | ^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a closure that returns `std::string::String`
14 | | }
15 | | });
| |_____- this function should return `Result` or `Option` to accept `?`
|
= help: the trait `std::ops::Try` is not implemented for `std::string::String`
= note: required by `std::ops::Try::from_error`
It's the simplified version of my code. Basically inside the unwrap_or_else closure, there might be a conditional error (e.g. IOError). In such cases, I'd like to terminate the function early (using ?). But obviously it doesn't work since it is currently in a closure and the closure doesn't expect a Result type.
What's the best practice to handle this?

Answer 1

What you want is or_else():
struct OverflowError {}
fn test_unwrap() -> Result<String, OverflowError> {
let a: Result<String, u8> = Err(100);
let a: String = a.or_else(|err| {
if err < 100 {
Ok(String::from("Ok"))
} else {
Err(OverflowError {})
}
})?;
Ok(a)
}
Simplified:
struct OverflowError {}
fn test_unwrap() -> Result<String, OverflowError> {
Err(100).or_else(|err| {
if err < 100 {
Ok(String::from("Ok"))
} else {
Err(OverflowError {})
}
})
}