// 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`
// needs to be converted into a `DoubleError`.
impl From<ParseIntError> for DoubleError {
fn from(err: ParseIntError) -> DoubleError {
DoubleError::Parse(err)
}
}
fn double_first(vec: Vec<&str>) -> Result<i32> {
let first = vec.first().ok_or(DoubleError::EmptyVec)?;
// Here we implicitly use the `ParseIntError` implementation of `From` (which
// we defined above) in order to create a `DoubleError`.
let parsed = first.parse::<i32>()?;
Ok(2 * parsed)
}
fn print(result: Result<i32>) {
match result {
Ok(n) => println!("The first doubled is {}", n),
Err(e) => {
println!("Error: {}", e);
if let Some(source) = e.source() {
println!(" Caused by: {}", source);
}
},
}
}
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 adds a bit more boilerplate for handling errors and might not be needed in all applications. There are some libraries that can take care of the boilerplate for you.
From::from and Enums
An Iter::map operation might fail, for example:
fn main() {
let strings = vec!["tofu", "93", "18"];
let numbers: Vec<_> = strings
.into_iter()
.map(|s| s.parse::<i32>())
.collect();
println!("Results: {:?}", numbers);
}
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Let's step through strategies for handling this.
filter_map calls a function and filters out the results that are None.
fn main() {
let strings = vec!["tofu", "93", "18"];
let numbers: Vec<_> = strings
.into_iter()
.filter_map(|s| s.parse::<i32>().ok())
.collect();
println!("Results: {:?}", numbers);
}
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Result implements FromIter so that a vector of results (Vec<Result<T, E>>) can be turned into a result with a vector (Result<Vec<T>, E>). Once an Result::Err is found, the iteration will terminate.