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|
// errorsn.rs
// This is a bigger error exercise than the previous ones!
// You can do it!
//
// Edit the `read_and_validate` function so that it compiles and
// passes the tests... so many things could go wrong!
//
// - Reading from stdin could produce an io::Error
// - Parsing the input could produce a num::ParseIntError
// - Validating the input could produce a CreationError (defined below)
//
// How can we lump these errors into one general error? That is, what
// type goes where the question marks are, and how do we return
// that type from the body of read_and_validate?
//
// Scroll down for hints :)
use std::error;
use std::fmt;
use std::io;
// PositiveNonzeroInteger is a struct defined below the tests.
fn read_and_validate(b: &mut io::BufRead) -> Result<PositiveNonzeroInteger, ???> {
let mut line = String::new();
b.read_line(&mut line);
let num: i64 = line.trim().parse();
let answer = PositiveNonzeroInteger::new(num);
answer
}
// This is a test helper function that turns a &str into a BufReader.
fn test_with_str(s: &str) -> Result<PositiveNonzeroInteger, Box<error::Error>> {
let mut b = io::BufReader::new(s.as_bytes());
read_and_validate(&mut b)
}
#[test]
fn test_success() {
let x = test_with_str("42\n");
assert_eq!(PositiveNonzeroInteger(42), x.unwrap());
}
#[test]
fn test_not_num() {
let x = test_with_str("eleven billion\n");
assert!(x.is_err());
}
#[test]
fn test_non_positive() {
let x = test_with_str("-40\n");
assert!(x.is_err());
}
#[test]
fn test_ioerror() {
struct Broken;
impl io::Read for Broken {
fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
Err(io::Error::new(io::ErrorKind::BrokenPipe, "uh-oh!"))
}
}
let mut b = io::BufReader::new(Broken);
assert!(read_and_validate(&mut b).is_err());
assert_eq!("uh-oh!", read_and_validate(&mut b).unwrap_err().to_string());
}
#[derive(PartialEq,Debug)]
struct PositiveNonzeroInteger(u64);
impl PositiveNonzeroInteger {
fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
if value == 0 {
Err(CreationError::Zero)
} else if value < 0 {
Err(CreationError::Negative)
} else {
Ok(PositiveNonzeroInteger(value as u64))
}
}
}
#[test]
fn test_positive_nonzero_integer_creation() {
assert!(PositiveNonzeroInteger::new(10).is_ok());
assert_eq!(Err(CreationError::Negative), PositiveNonzeroInteger::new(-10));
assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
}
#[derive(PartialEq,Debug)]
enum CreationError {
Negative,
Zero,
}
impl fmt::Display for CreationError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.write_str((self as &error::Error).description())
}
}
impl error::Error for CreationError {
fn description(&self) -> &str {
match *self {
CreationError::Negative => "Negative",
CreationError::Zero => "Zero",
}
}
}
// First hint: To figure out what type should go where the ??? is, take a look
// at the test helper function `test_with_str`, since it returns whatever
// `read_and_validate` returns and`test_with_str` has its signature fully
// specified.
// Next hint: There are three places in `read_and_validate` that we call a
// function that returns a `Result` (that is, the functions might fail).
// Wrap those calls in a `try!` macro call so that we return immediately from
// `read_and_validate` if those function calls fail.
// Another hint: under the hood, the `try!` macro calls `From::from`
// on the error value to convert it to a boxed trait object, a Box<error::Error>,
// which is polymorphic-- that means that lots of different kinds of errors
// can be returned from the same function because all errors act the same
// since they all implement the `error::Error` trait.
// Check out this section of the book:
// https://doc.rust-lang.org/stable/book/error-handling.html#standard-library-traits-used-for-error-handling
// Another another hint: Note that because the `try!` macro returns
// the *unwrapped* value in the `Ok` case, if we want to return a `Result` from
// `read_and_validate` for *its* success case, we'll have to rewrap a value
// that we got from the return value of a `try!` call in an `Ok`-- this will
// look like `Ok(something)`.
// Another another another hint: `Result`s must be "used", that is, you'll
// get a warning if you don't handle a `Result` that you get in your
// function. Read more about that in the `std::result` module docs:
// https://doc.rust-lang.org/std/result/#results-must-be-used
|
