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right let's try this one again
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60 changed files with 130 additions and 939 deletions
exercises/error_handling
73
exercises/error_handling/errors1.rs
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73
exercises/error_handling/errors1.rs
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// errors1.rs
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// This function refuses to generate text to be printed on a nametag if
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// you pass it an empty string. It'd be nicer if it explained what the problem
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// was, instead of just sometimes returning `None`. The 2nd test currently
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// does not compile or pass, but it illustrates the behavior we would like
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// this function to have.
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// Scroll down for hints!!!
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pub fn generate_nametag_text(name: String) -> Option<String> {
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if name.len() > 0 {
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Some(format!("Hi! My name is {}", name))
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} else {
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// Empty names aren't allowed.
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None
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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// This test passes initially if you comment out the 2nd test.
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// You'll need to update what this test expects when you change
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// the function under test!
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#[test]
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fn generates_nametag_text_for_a_nonempty_name() {
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assert_eq!(
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generate_nametag_text("Beyoncé".into()),
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Some("Hi! My name is Beyoncé".into())
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);
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}
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#[test]
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fn explains_why_generating_nametag_text_fails() {
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assert_eq!(
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generate_nametag_text("".into()),
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Err("`name` was empty; it must be nonempty.".into())
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);
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}
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}
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// `Err` is one of the variants of `Result`, so what the 2nd test is saying
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// is that `generate_nametag_text` should return a `Result` instead of an
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// `Option`.
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// To make this change, you'll need to:
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// - update the return type in the function signature to be a Result that
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// could be the variants `Ok(String)` and `Err(String)`
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// - change the body of the function to return `Ok(stuff)` where it currently
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// returns `Some(stuff)`
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// - change the body of the function to return `Err(error message)` where it
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// currently returns `None`
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// - change the first test to expect `Ok(stuff)` where it currently expects
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// `Some(stuff)`.
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72
exercises/error_handling/errors2.rs
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72
exercises/error_handling/errors2.rs
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// errors2.rs
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// Say we're writing a game where you can buy items with tokens. All items cost
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// 5 tokens, and whenever you purchase items there is a processing fee of 1
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// token. A player of the game will type in how many items they want to buy,
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// and the `total_cost` function will calculate the total number of tokens.
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// Since the player typed in the quantity, though, we get it as a string-- and
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// they might have typed anything, not just numbers!
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// Right now, this function isn't handling the error case at all (and isn't
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// handling the success case properly either). What we want to do is:
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// if we call the `parse` function on a string that is not a number, that
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// function will return a `ParseIntError`, and in that case, we want to
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// immediately return that error from our function and not try to multiply
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// and add.
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// There are at least two ways to implement this that are both correct-- but
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// one is a lot shorter! Scroll down for hints to both ways.
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use std::num::ParseIntError;
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pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
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let processing_fee = 1;
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let cost_per_item = 5;
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let qty = item_quantity.parse::<i32>();
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Ok(qty * cost_per_item + processing_fee)
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn item_quantity_is_a_valid_number() {
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assert_eq!(
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total_cost("34"),
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Ok(171)
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);
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}
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#[test]
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fn item_quantity_is_an_invalid_number() {
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assert_eq!(
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total_cost("beep boop").unwrap_err().to_string(),
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"invalid digit found in string"
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);
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}
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}
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// One way to handle this is using a `match` statement on
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// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
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// `Err(something)`. This pattern is very common in Rust, though, so there's
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// a `?` operator that does pretty much what you would make that match statement
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// do for you! Take a look at this section of the Error Handling chapter:
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// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
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// and give it a try!
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62
exercises/error_handling/errors3.rs
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62
exercises/error_handling/errors3.rs
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// errors3.rs
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// This is a program that is trying to use a completed version of the
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// `total_cost` function from the previous exercise. It's not working though--
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// we can't use the `?` operator in the `main()` function! Why not?
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// What should we do instead? Scroll for hints!
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use std::num::ParseIntError;
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fn main() {
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let mut tokens = 100;
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let pretend_user_input = "8";
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let cost = total_cost(pretend_user_input)?;
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if cost > tokens {
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println!("You can't afford that many!");
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} else {
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tokens -= cost;
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println!("You now have {} tokens.", tokens);
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}
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}
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pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
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let processing_fee = 1;
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let cost_per_item = 5;
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let qty = item_quantity.parse::<i32>()?;
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Ok(qty * cost_per_item + processing_fee)
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}
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// Since the `?` operator returns an `Err` early if the thing it's trying to
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// do fails, you can only use the `?` operator in functions that have a
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// `Result` as their return type.
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// Hence the error that you get if you run this code is:
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// ```
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// error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
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// ```
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// So we have to use another way of handling a `Result` within `main`.
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// Decide what we should do if `pretend_user_input` has a string value that does
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// not parse to an integer, and implement that instead of using the `?`
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// operator.
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138
exercises/error_handling/errorsn.rs
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138
exercises/error_handling/errorsn.rs
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// errorsn.rs
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// This is a bigger error exercise than the previous ones!
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// You can do it! :)
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//
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// Edit the `read_and_validate` function so that it compiles and
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// passes the tests... so many things could go wrong!
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//
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// - Reading from stdin could produce an io::Error
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// - Parsing the input could produce a num::ParseIntError
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// - Validating the input could produce a CreationError (defined below)
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//
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// How can we lump these errors into one general error? That is, what
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// type goes where the question marks are, and how do we return
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// that type from the body of read_and_validate?
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//
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// Scroll down for hints :)
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use std::error;
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use std::fmt;
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use std::io;
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// PositiveNonzeroInteger is a struct defined below the tests.
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fn read_and_validate(b: &mut io::BufRead) -> Result<PositiveNonzeroInteger, ???> {
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let mut line = String::new();
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b.read_line(&mut line);
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let num: i64 = line.trim().parse();
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let answer = PositiveNonzeroInteger::new(num);
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answer
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}
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// This is a test helper function that turns a &str into a BufReader.
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fn test_with_str(s: &str) -> Result<PositiveNonzeroInteger, Box<error::Error>> {
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let mut b = io::BufReader::new(s.as_bytes());
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read_and_validate(&mut b)
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}
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#[test]
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fn test_success() {
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let x = test_with_str("42\n");
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assert_eq!(PositiveNonzeroInteger(42), x.unwrap());
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}
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#[test]
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fn test_not_num() {
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let x = test_with_str("eleven billion\n");
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assert!(x.is_err());
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}
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#[test]
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fn test_non_positive() {
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let x = test_with_str("-40\n");
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assert!(x.is_err());
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}
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#[test]
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fn test_ioerror() {
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struct Broken;
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impl io::Read for Broken {
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fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
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Err(io::Error::new(io::ErrorKind::BrokenPipe, "uh-oh!"))
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}
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}
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let mut b = io::BufReader::new(Broken);
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assert!(read_and_validate(&mut b).is_err());
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assert_eq!("uh-oh!", read_and_validate(&mut b).unwrap_err().to_string());
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}
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#[derive(PartialEq,Debug)]
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struct PositiveNonzeroInteger(u64);
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impl PositiveNonzeroInteger {
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fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
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if value == 0 {
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Err(CreationError::Zero)
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} else if value < 0 {
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Err(CreationError::Negative)
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} else {
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Ok(PositiveNonzeroInteger(value as u64))
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}
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}
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}
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#[test]
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fn test_positive_nonzero_integer_creation() {
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assert!(PositiveNonzeroInteger::new(10).is_ok());
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assert_eq!(Err(CreationError::Negative), PositiveNonzeroInteger::new(-10));
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assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
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}
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#[derive(PartialEq,Debug)]
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enum CreationError {
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Negative,
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Zero,
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}
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impl fmt::Display for CreationError {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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f.write_str((self as &error::Error).description())
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}
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}
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impl error::Error for CreationError {
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fn description(&self) -> &str {
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match *self {
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CreationError::Negative => "Negative",
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CreationError::Zero => "Zero",
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}
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}
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}
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// First hint: To figure out what type should go where the ??? is, take a look
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// at the test helper function `test_with_str`, since it returns whatever
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// `read_and_validate` returns and`test_with_str` has its signature fully
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// specified.
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// Next hint: There are three places in `read_and_validate` that we call a
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// function that returns a `Result` (that is, the functions might fail).
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// Apply the `?` operator on those calls so that we return immediately from
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// `read_and_validate` if those function calls fail.
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// Another hint: under the hood, the `?` operator calls `From::from`
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// on the error value to convert it to a boxed trait object, a Box<error::Error>,
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// which is polymorphic-- that means that lots of different kinds of errors
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// can be returned from the same function because all errors act the same
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// since they all implement the `error::Error` trait.
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// Check out this section of the book:
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// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
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// Another another hint: Note that because the `?` operator returns
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// the *unwrapped* value in the `Ok` case, if we want to return a `Result` from
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// `read_and_validate` for *its* success case, we'll have to rewrap a value
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// that we got from the return value of a `?`ed call in an `Ok`-- this will
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// look like `Ok(something)`.
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// Another another another hint: `Result`s must be "used", that is, you'll
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// get a warning if you don't handle a `Result` that you get in your
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// function. Read more about that in the `std::result` module docs:
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// https://doc.rust-lang.org/std/result/#results-must-be-used
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45
exercises/error_handling/option1.rs
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45
exercises/error_handling/option1.rs
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// option1.rs
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// This example panics because the second time it calls `pop`, the `vec`
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// is empty, so `pop` returns `None`, and `unwrap` panics if it's called
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// on `None`. Handle this in a more graceful way than calling `unwrap`!
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// Scroll down for hints :)
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fn main() {
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let mut list = vec![3];
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let last = list.pop().unwrap();
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println!("The last item in the list is {:?}", last);
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let second_to_last = list.pop().unwrap();
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println!("The second-to-last item in the list is {:?}", second_to_last);
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}
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// Try using a `match` statement where the arms are `Some(thing)` and `None`.
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// Or set a default value to print out if you get `None` by using the
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// function `unwrap_or`.
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// Or use an `if let` statement on the result of `pop()` to both destructure
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// a `Some` value and only print out something if we have a value!
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43
exercises/error_handling/result1.rs
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43
exercises/error_handling/result1.rs
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// result1.rs
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// Make this test pass! Scroll down for hints :)
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#[derive(PartialEq,Debug)]
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struct PositiveNonzeroInteger(u64);
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#[derive(PartialEq,Debug)]
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enum CreationError {
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Negative,
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Zero,
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}
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impl PositiveNonzeroInteger {
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fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
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Ok(PositiveNonzeroInteger(value as u64))
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}
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}
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#[test]
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fn test_creation() {
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assert!(PositiveNonzeroInteger::new(10).is_ok());
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assert_eq!(Err(CreationError::Negative), PositiveNonzeroInteger::new(-10));
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assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
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}
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// `PositiveNonzeroInteger::new` is always creating a new instance and returning an `Ok` result.
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// It should be doing some checking, returning an `Err` result if those checks fail, and only
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// returning an `Ok` result if those checks determine that everything is... okay :)
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