Best way to map Result<T, E> to Result<T, F> where F is a literal

help
1

I'm aware of the following methods

let a: u32 = "1".parse().map_err(|_| "Invalid arg")?;
let b: u32 = "2".parse().ok().ok_or("Invalid arg")?;
let c: u32 = "3".parse().or(Err("Invalid arg"))?;

However, I've noticed the Option type already has the straightforward .ok_or(literal), is there a reason why Result doesn't implement this method? And what would be another direct approach than the ones mentioned above?

1 Like
2

If you want to map the error part of the result, definitely use .map_err(), the other variants don't convey that intent.

The Option::ok_or method is meant for converting an Option into a Result. Thus, there's no logical direct counterpart to it for Result since you already have one.

3 Likes
3

Thanks, would be great if there was anything like .map_err() that accepts the error directly instead of a callback (like many other similar methods), but seems like .map_err() will have to do it for now.

4

If you want to keep the underlying error but wrap it in your own custom error, a common pattern is this

enum MyError {
    ParseError(std::num::ParseIntError)
}
“1”.parse().map_err(MyError::ParseError)

This is less noisy and works because the constructor for MyError::ParseError is effectively a single argument function that takes the underlying error as an argument and returns a MyError. This also has the benefit of keeping the underlying error which may have useful additional information.

5

The difference is that here we (may) already have some error value, and discarding and replacing it with another should probably stand out in code. I guess there could be something like replace_err(othererror), but map_err(|_| othererror) works just as well, and the idiomatic |_| makes it clear that the previous value is being discarded.

6

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