212 lines
5.1 KiB
Markdown
212 lines
5.1 KiB
Markdown
# result and ?
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`Result<T>` is an enum to describe possible errors. It has two variants:
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- `Ok(T)`: a value T was found
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- `Err(e)`: An error was found with a value `e`
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In short words, the expected outcome is `Ok`, while the unexpected outcome is `Err`.
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1. ππ
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```rust,editable
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// FILL in the blanks and FIX the errors
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use std::num::ParseIntError;
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fn multiply(n1_str: &str, n2_str: &str) -> __ {
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let n1 = n1_str.parse::<i32>();
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let n2 = n2_str.parse::<i32>();
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Ok(n1.unwrap() * n2.unwrap())
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}
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fn main() {
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let result = multiply("10", "2");
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assert_eq!(result, __);
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let result = multiply("t", "2");
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assert_eq!(result.__, 8);
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println!("Success!")
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}
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```
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### ?
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`?` is almost exactly equivalent to `unwrap`, but `?` returns instead of panic on `Err`.
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2. ππ
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```rust,editable
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use std::num::ParseIntError;
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// IMPLEMENT multiply with ?
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// DON'T use unwrap here
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fn multiply(n1_str: &str, n2_str: &str) -> __ {
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}
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fn main() {
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assert_eq!(multiply("3", "4").unwrap(), 12);
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println!("Success!")
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}
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```
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3. ππ
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```rust,editable
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use std::fs::File;
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use std::io::{self, Read};
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fn read_file1() -> Result<String, io::Error> {
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let f = File::open("hello.txt");
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let mut f = match f {
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Ok(file) => file,
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Err(e) => return Err(e),
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};
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let mut s = String::new();
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match f.read_to_string(&mut s) {
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Ok(_) => Ok(s),
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Err(e) => Err(e),
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}
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}
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// FILL in the blanks with one code line
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// DON'T change any code else
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fn read_file2() -> Result<String, io::Error> {
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let mut s = String::new();
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__;
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Ok(s)
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}
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fn main() {
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assert_eq!(read_file1().unwrap_err().to_string(), read_file2().unwrap_err().to_string());
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println!("Success!")
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}
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```
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### map & and_then
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[map](https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.map) and [and_then](https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.and_then) are two common combinators for `Result<T, E>` (also for `Option<T>`).
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4. ππ
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```rust,editable
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use std::num::ParseIntError;
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// FILL in the blank in two ways: map, and then
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fn add_two(n_str: &str) -> Result<i32, ParseIntError> {
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n_str.parse::<i32>().__
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}
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fn main() {
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assert_eq!(add_two("4").unwrap(), 6);
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println!("Success!")
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}
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```
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5. πππ
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```rust,editable
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use std::num::ParseIntError;
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// With the return type rewritten, we use pattern matching without `unwrap()`.
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// But it's so Verbose..
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fn multiply(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
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match n1_str.parse::<i32>() {
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Ok(n1) => {
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match n2_str.parse::<i32>() {
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Ok(n2) => {
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Ok(n1 * n2)
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},
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Err(e) => Err(e),
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}
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},
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Err(e) => Err(e),
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}
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}
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// Rewriting `multiply` to make it succinct
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// You should use BOTH of `and_then` and `map` here.
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fn multiply1(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
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// IMPLEMENT...
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}
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fn print(result: Result<i32, ParseIntError>) {
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match result {
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Ok(n) => println!("n is {}", n),
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Err(e) => println!("Error: {}", e),
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}
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}
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fn main() {
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// This still presents a reasonable answer.
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let twenty = multiply1("10", "2");
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print(twenty);
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// The following now provides a much more helpful error message.
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let tt = multiply("t", "2");
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print(tt);
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println!("Success!")
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}
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```
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### Type alias
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Using `std::result::Result<T, ParseIntError>` everywhere is verbose and tedious, we can use alias for this purpose.
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At a module level, creating aliases can be particularly helpful. Errors found in the a specific module often has the same `Err` type, so a single alias can succinctly defined all associated `Results`. This is so useful even the `std` library even supplies one: [`io::Result`](https://doc.rust-lang.org/std/io/type.Result.html).
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6. π
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```rust,editable
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use std::num::ParseIntError;
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// FILL in the blank
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type __;
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// Use the above alias to refer to our specific `Result` type.
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fn multiply(first_number_str: &str, second_number_str: &str) -> Res<i32> {
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first_number_str.parse::<i32>().and_then(|first_number| {
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second_number_str.parse::<i32>().map(|second_number| first_number * second_number)
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})
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}
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// Here, the alias again allows us to save some space.
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fn print(result: Res<i32>) {
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match result {
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Ok(n) => println!("n is {}", n),
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Err(e) => println!("Error: {}", e),
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}
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}
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fn main() {
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print(multiply("10", "2"));
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print(multiply("t", "2"));
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println!("Success!")
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}
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```
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### Using Result in `fn main`
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Typically `the` main function will look like this:
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```rust
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fn main() {
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println!("Hello World!");
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}
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```
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However `main` is also able to have a return type of `Result`. If an error occurs within the `main` function it will return an error code and print a debug representation of the error( Debug trait ).
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The following example shows such a scenario:
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```rust,editable
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use std::num::ParseIntError;
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fn main() -> Result<(), ParseIntError> {
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let number_str = "10";
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let number = match number_str.parse::<i32>() {
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Ok(number) => number,
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Err(e) => return Err(e),
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};
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println!("{}", number);
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Ok(())
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}
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``` |