Replace Result with map_err and "?

Programming
1

I’m trying to use the ? operator with the check_approval call, as shown in the commented-out code in this code block:

pub fn authorize<LoadClient, CheckApproval>(load_client: LoadClient, check_approval: CheckApproval) -> Result<String, MyErr> 
where
   LoadClient: FnOnce(String) -> Result<Client, String>,
   CheckApproval: for<'a> FnOnce(&'a Client, &str) -> Result<&'a str, ()>,
{
    let client = load_client("hello".to_string()).map_err(|_| MyErr::LoadFailed)?;
    let permission = "something";

    // This doesn't compile
    // let authorized = check_approval(&client, permission).map_err(|_| MyErr::RequiresApproval(client, permission.to_string()))?;
    // Ok(authorized.to_string())

    // This version does
    match check_approval(&client, permission) {
        Err(_) => Err(MyErr::RequiresApproval(client, permission.to_string())),
        Ok(authorized) => Ok(authorized.to_string()),
    }
}

However, I get the following error:

error[E0505]: cannot move out of `client` because it is borrowed
  --> src/lib.rs:19:66
   |
19 |     let authorized = check_approval(&client, permission).map_err(|_| MyErr::RequiresApproval(client, permission.to_string()))?;
   |                                     -------              ------- ^^^                         ------ move occurs due to use in closure
   |                                     |                    |       |
   |                                     |                    |       move out of `client` occurs here
   |                                     |                    borrow later used by call
   |                                     borrow of `client` occurs here

Is there a way to use the ? operator without getting this error?

I’m trying to use the ? operator to simplify my code, but I’m getting the following error when I try to do so:

error[E0505]: cannot move out of `client` because it is borrowed
  --> src/lib.rs:19:66
   |
19 |     let authorized = check_approval(&client, permission).map_err(|_| MyErr::RequiresApproval(client, permission.to_string()))?;
   |                                     -------              ------- ^^^                         ------ move occurs due to use in closure
   |                                     |                    |       |
   |                                     |                    |       move out of `client` occurs here
   |                                     |                    borrow later used by call
   |                                     borrow of `client` occurs here

Is there a way to use ? to simplify my code without getting this error?

I’m trying to use the ? operator to simplify the following code block:

pub fn authorize<LoadClient, CheckApproval>(load_client: LoadClient, check_approval: CheckApproval) -> Result<String, MyErr> 
where
   LoadClient: FnOnce(String) -> Result<Client, String>,
   CheckApproval: for<'a> FnOnce(&'a Client, &str) -> Result<&'a str, ()>,
{
    let client = load_client("hello".to_string()).map_err(|_| MyErr::LoadFailed)?;
    let permission = "something";

    // This doesn't compile
    // let authorized = check_approval(&client, permission).map_err(|_| MyErr::RequiresApproval(client, permission.to_string()))?;
    // Ok(authorized.to_string())

    // This version does
    match check_approval(&client, permission) {
        Err(_) => Err(MyErr::RequiresApproval(client, permission.to_string())),
        Ok(authorized) => Ok(authorized.to_string()),
    }
}

However, I get the following error when I try to do so:

error[E0505]: cannot move out of `client` because it is borrowed
  --> src/lib.rs:19:66
   |
19 |     let authorized = check_approval(&client, permission).map_err(|_| MyErr::RequiresApproval(client, permission.to_string()))?;
   |                                     -------              ------- ^^^                         ------ move occurs due to use in closure
   |                                     |                    |       |
   |                                     |                    |       move out of `client` occurs here
   |                                     |                    borrow later used by call
   |                                     borrow of `client` occurs here

Is there a way to use the ? operator to simplify this code without getting this error?

2

No, you cannot use the ? operator in this code block without getting the error mentioned. This is because client is already borrowed by the check_approval closure and cannot be moved out of it. You can use the match statement instead of the ? operator to simplify the code and avoid the error.