# Defining an error type

Sometimes it simplifies the code to mask all of the different errors with a single type of error. We'll show this with a custom error.

Rust allows us to define our own error types. In general, a "good" error type:

- Represents different errors with the same type
- Presents nice error messages to the user
- Is easy to compare with other types
  - Good: `Err(EmptyVec)`
  - Bad: `Err("Please use a vector with at least one element".to_owned())`
- Can hold information about the error
  - Good: `Err(BadChar(c, position))`
  - Bad: `Err("+ cannot be used here".to_owned())`
- Composes well with other errors

    use std::error;
    use std::fmt;
    use std::num::ParseIntError;

    type Result<T> = std::result::Result<T, DoubleError>;

    #[derive(Debug, Clone)]
    // Define our error types. These may be customized for our error handling cases.
    // Now we will be able to write our own errors, defer to an underlying error
    // implementation, or do something in between.
    struct DoubleError;

    // Generation of an error is completely separate from how it is displayed.
    // There's no need to be concerned about cluttering complex logic with the display style.
    //
    // Note that we don't store any extra info about the errors. This means we can't state
    // which string failed to parse without modifying our types to carry that information.
    impl fmt::Display for DoubleError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "invalid first item to double")
        }
    }

    // This is important for other errors to wrap this one.
    impl error::Error for DoubleError {
        fn description(&self) -> &str {
            "invalid first item to double"
        }

        fn cause(&self) -> Option<&error::Error> {
            // Generic error, underlying cause isn't tracked.
            None
        }
    }

    fn double_first(vec: Vec<&str>) -> Result<i32> {
        vec.first()
           // Change the error to our new type.
           .ok_or(DoubleError)
           .and_then(|s| s.parse::<i32>()
                // Update to the new error type here also.
                .map_err(|_| DoubleError)
                .map(|i| 2 * i))
    }

    fn print(result: Result<i32>) {
        match result {
            Ok(n)  => println!("The first doubled is {}", n),
            Err(e) => println!("Error: {}", e),
        }
    }

    fn main() {
        let numbers = vec!["42", "93", "18"];
        let empty = vec![];
        let strings = vec!["tofu", "93", "18"];

        print(double_first(numbers));
        print(double_first(empty));
        print(double_first(strings));
    }