errors6.rs

[zzz6519003]

// errors6.rs

// Using catch-all error types like Box<dyn error::Error> isn't recommended
// for library code, where callers might want to make decisions based on the
// error content, instead of printing it out or propagating it further. Here,
// we define a custom error type to make it possible for callers to decide
// what to do next when our function returns an error.

// Execute rustlings hint errors6 or use the hint watch subcommand for a hint.

use std::num::ParseIntError;

// This is a custom error type that we will be using in parse_pos_nonzero().
#[derive(PartialEq, Debug)]
enum ParsePosNonzeroError {
Creation(CreationError),
ParseInt(ParseIntError),
}

impl ParsePosNonzeroError {
fn from_creation(err: CreationError) -> ParsePosNonzeroError {
ParsePosNonzeroError::Creation(err)
}
// TODO: add another error conversion function here.
fn from_parseint(err: ParseIntError) -> ParsePosNonzeroError {
ParsePosNonzeroError::ParseInt(err)
}
}

fn parse_pos_nonzero(s: &str) -> Result<PositiveNonzeroInteger, ParsePosNonzeroError> {
// TODO: change this to return an appropriate error instead of panicking
// when parse() returns an error.
// let x: i64 = s.parse().unwrap();
// PositiveNonzeroInteger::new(x).map_err(ParsePosNonzeroError::from_creation)
match s.parse() {
Ok(n) => {
match PositiveNonzeroInteger::new(n) {
Ok(n) => Ok(n),
Err(err) => Err(ParsePosNonzeroError::from_creation(err))
}
}
Err(err) => {
Err(ParsePosNonzeroError::from_parseint(err))
}
}
}

// Don't change anything below this line.

#[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!(
        parse_pos_nonzero("not a number"),
        Err(ParsePosNonzeroError::ParseInt(_))
    ));
}

#[test]
fn test_negative() {
    assert_eq!(
        parse_pos_nonzero("-555"),
        Err(ParsePosNonzeroError::Creation(CreationError::Negative))
    );
}

#[test]
fn test_zero() {
    assert_eq!(
        parse_pos_nonzero("0"),
        Err(ParsePosNonzeroError::Creation(CreationError::Zero))
    );
}

#[test]
fn test_positive() {
    let x = PositiveNonzeroInteger::new(42);
    assert!(x.is_ok());
    assert_eq!(parse_pos_nonzero("42"), Ok(x.unwrap()));
}

}