Cannot return a value from within an if in a match branch

help
1

I have a result which I would like unpack using match and then set the new value of by returning a value from match.

I can do this in a simple example:

let mut res = Ok(5);

res = match res {
    Err(e) => Err(format!("Error in match: {}", e)),
    Ok(v) => Ok(v - 1),
};

println!("res={:?}", res);

(Playground)

However if I want to add an if statement into the match branch, and then return a value, I cannot. I get an error saying the expected return type is () not Result<foo, bar>.

As seen in here:

let mut res = Ok(5);

// Increment if even, decrement if odd
res = match res {
    Err(e) => Err(format!("Error in match: {}", e)),
    Ok(v) => {
        if v % 2 == 0 {
            Ok(v + 1) // <<< Error here
        }

        Ok(v - 1)
    }
};

println!("res={:?}", res);

(Playground)

Why is this? How can I return a value from the second example in the way I want? I suspect there is a way to do this, but I am not going about it correctly.

2

You need an explicit else:

if v % 2 == 0 {
    Ok(v + 1)
} else {
    Ok(v - 1)
}
3

Oh okay. Thank you very much.

4

You can only omit the return for the last line in an expression.

5

I think the missing else branch is the problem, adding return statements still has an error: Playground.

6

This seems like a nice place for a match guard:

    res = match res {
        Err(e) => Err(format!("Error in match: {}", e)),
        Ok(v) if v % 2 == 0 => Ok(v + 1),
        Ok(v) => Ok(v - 1),
    };
3 Likes
7

The playground link you provided doesn't work because the return type is implicitly (), while the Ok values returned are of type Result<{number}, String>.

This means that an else wouldn't work either, because that wasn't the only issue with the code.

8

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