feat: add advanced_errs1

New section and exercise to demonstrate the `From` trait for errors
and its usefulness with the `?` operator.
This commit is contained in:
Taylor Yu 2021-06-09 23:26:10 -05:00 committed by ana
parent 2dc93cadda
commit 882d535ba8
2 changed files with 118 additions and 0 deletions

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@ -0,0 +1,98 @@
// advanced_errs1.rs
// Remember back in errors6, we had multiple mapping functions so that we
// could translate lower-level errors into our custom error type using
// `map_err()`? What if we could use the `?` operator directly instead?
// Make this code compile! Execute `rustlings hint advanced_errs1` for
// hints :)
// I AM NOT DONE
use std::num::ParseIntError;
use std::str::FromStr;
// This is a custom error type that we will be using in the `FromStr`
// implementation.
#[derive(PartialEq, Debug)]
enum ParsePosNonzeroError {
Creation(CreationError),
ParseInt(ParseIntError),
}
impl From<CreationError> for ParsePosNonzeroError {
fn from(e: CreationError) -> Self {
// TODO: complete this implementation so that the `?` operator will
// work for `CreationError`
}
}
// TODO: implement another instance of the `From` trait here so that the
// `?` operator will work in the other place in the `FromStr`
// implementation below.
// Don't change anything below this line.
impl FromStr for PositiveNonzeroInteger {
type Err = ParsePosNonzeroError;
fn from_str(s: &str) -> Result<PositiveNonzeroInteger, Self::Err> {
let x: i64 = s.parse()?;
Ok(PositiveNonzeroInteger::new(x)?)
}
}
#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);
#[derive(PartialEq, Debug)]
enum CreationError {
Negative,
Zero,
}
impl PositiveNonzeroInteger {
fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
match value {
x if x < 0 => Err(CreationError::Negative),
x if x == 0 => Err(CreationError::Zero),
x => Ok(PositiveNonzeroInteger(x as u64)),
}
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_parse_error() {
// We can't construct a ParseIntError, so we have to pattern match.
assert!(matches!(
PositiveNonzeroInteger::from_str("not a number"),
Err(ParsePosNonzeroError::ParseInt(_))
));
}
#[test]
fn test_negative() {
assert_eq!(
PositiveNonzeroInteger::from_str("-555"),
Err(ParsePosNonzeroError::Creation(CreationError::Negative))
);
}
#[test]
fn test_zero() {
assert_eq!(
PositiveNonzeroInteger::from_str("0"),
Err(ParsePosNonzeroError::Creation(CreationError::Zero))
);
}
#[test]
fn test_positive() {
let x = PositiveNonzeroInteger::new(42);
assert!(x.is_ok());
assert_eq!(PositiveNonzeroInteger::from_str("42"), Ok(x.unwrap()));
}
}

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@ -974,3 +974,23 @@ path = "exercises/conversions/as_ref_mut.rs"
mode = "test"
hint = """
Add AsRef<str> as a trait bound to the functions."""
# ADVANCED ERRORS
[[exercises]]
name = "advanced_errs1"
path = "exercises/advanced_errors/advanced_errs1.rs"
mode = "test"
hint = """
This exercise uses an updated version of the code in errors6. The parsing
code is now in an implementation of the `FromStr` trait. Note that the
parsing code uses `?` directly, without any calls to `map_err()`. There is
one partial implementation of the `From` trait example that you should
complete.
Details: The `?` operator calls `From::from()` on the error type to convert
it to the error type of the return type of the surrounding function.
Hint: You will need to write another implementation of `From` that has a
different input type.
"""