Rust contains a very useful function called expect. This method is used with any form of call that has an Option or a Result type (for example, Result in the file-writing example has the options of File or Error). It works by moving the value out of the option and returning it. If the option/result type contains an error, the expect call stops your program and prints out the error message.

For example, the following statement will return either File or Error into file:

let file: Result<File, Error> = options.open("my_file.txt").expect("Opening the file failed"); 

A shorter form is available in the unwrap method. This is the same as the expect method, but it doesn't print out anything in case of a failure. In general, Some(a).unwrap() will return a.

Expect is usually favored instead of unwrap, since the full error message makes it easier to find where the error came from.