impl error::Error for EmptyVec {}
// The same structure as before but rather than chain all `Results`
// and `Options` along, we `?` to get the inner value out immediately.
fn double_first(vec: Vec<&str>) -> Result
let first = vec.first().ok_or(EmptyVec)?;
let parsed = first.parse::
Ok(2 * parsed)
}
fn print(result: Result
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));
}
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This is actually fairly clean now. Compared with the original panic, it is very similar to replacing the unwrap calls with ? except that the return types are Result. As a result, they must be destructured at the top level.
From::from and ?
An alternative to boxing errors is to wrap them in your own error type.
use std::error;
use std::error::Error as _;
use std::num::ParseIntError;
use std::fmt;
type Result
#[derive(Debug)]
enum DoubleError {
EmptyVec,
// We will defer to the parse error implementation for their error.
// Supplying extra info requires adding more data to the type.
Parse(ParseIntError),
}
impl fmt::Display for DoubleError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
match *self {
DoubleError::EmptyVec =>
write!(f, "please use a vector with at least one element"),
// The wrapped error contains additional information and is available
// via the source() method.
DoubleError::Parse(..) =>
write!(f, "the provided string could not be parsed as int"),
}
}
}
impl error::Error for DoubleError {
fn source(&self) -> Option<&(dyn error::Error + 'static)> {
match *self {
DoubleError::EmptyVec => None,
// The cause is the underlying implementation error type. Is implicitly
// cast to the trait object `&error::Error`. This works because the
// underlying type already implements the `Error` trait.
DoubleError::Parse(ref e) => Some(e),
}
}
}
// Implement the conversion from `ParseIntError` to `DoubleError`.
// This will be automatically called by `?` if a `ParseIntError`
