Better Ways of Handling Errors

Several good ways of dealing with errors in Rust

Git repo: https://github.com/caveofprogramming/

Many Rust functions return a Result to indicate success or failure.

• In this case of success, the Ok constant of Result can contain the desired return value.

• In the case of failure, the Err constant may contain more information about the error.

We’re forced to handle these two possibilities when we try to get whatever value Ok might be wrapping.

Here are some ways of doing that, without resorting to match or if let.

Example Function

This function either returns Ok or Err, depending on which one is uncommented.

We’ll pretend that when calling this function, if everything goes fine, we want to get the String that Ok wraps.

Unwrap and Expect

These both directly return the value Ok wraps, if Ok is returned from the function.

If Err is returned, they “panic” and the program terminates.

expect() lets us supply our own additional error message.

unwrap_or_else

This lets us use a “closure” to supply our own value if an Err is returned, so the program doesn’t crash out with a panic and can continue.

In this case text is either set to the value wrapped by the Ok return result of run(), or, if run() returns Err, it gets set to “Didn’t work out”.

The Question Mark Operator

Within a function that returns a Result, the question mark operator can be used.

This returns whatever Result::Ok wraps if Ok is returned from whatever is attempted inside the function, else it returns the Result::Err from the function, terminating the function’s execution via a normal return.

This function attempts to create a file. The File::create function returns a Result enum. If create returns Ok, the question mark operator returns whatever Ok wraps, which in this case is a file handle.

Otherwise it returns from create_file() with Result::Err.

In other words, one of two things can happen:

• File::create(filename)? evaluates to a file handle

• The create_file function returns with a Result::Err

We can chain these two function calls together with the ? operator:

If we execute the following function and notfound.txt does not exist, the function returns with a “not found” error in the returned Err.

The line with Ok on it is not reached.