chay.dev
1// This exercise is an altered version of the `errors4` exercise. It uses some
2// concepts that we won't get to until later in the course, like `Box` and the
3// `From` trait. It's not important to understand them in detail right now, but
4// you can read ahead if you like. For now, think of the `Box<dyn ???>` type as
5// an "I want anything that does ???" type.
6//
7// In short, this particular use case for boxes is for when you want to own a
8// value and you care only that it is a type which implements a particular
9// trait. To do so, The `Box` is declared as of type `Box<dyn Trait>` where
10// `Trait` is the trait the compiler looks for on any value used in that
11// context. For this exercise, that context is the potential errors which
12// can be returned in a `Result`.
13
14use std::error::Error;
15use std::fmt;
16
17#[derive(PartialEq, Debug)]
18enum CreationError {
19 Negative,
20 Zero,
21}
22
23// This is required so that `CreationError` can implement `Error`.
24impl fmt::Display for CreationError {
25 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
26 let description = match *self {
27 CreationError::Negative => "number is negative",
28 CreationError::Zero => "number is zero",
29 };
30 f.write_str(description)
31 }
32}
33
34impl Error for CreationError {}
35
36#[derive(PartialEq, Debug)]
37struct PositiveNonzeroInteger(u64);
38
39impl PositiveNonzeroInteger {
40 fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
41 match value {
42 x if x < 0 => Err(CreationError::Negative),
43 0 => Err(CreationError::Zero),
44 x => Ok(PositiveNonzeroInteger(x as u64)),
45 }
46 }
47}
48
49fn main() -> Result<(), Box<dyn Error>> {
50 let pretend_user_input = "42";
51 let x: i64 = pretend_user_input.parse()?;
52 println!("output={:?}", PositiveNonzeroInteger::new(x)?);
53 Ok(())
54}