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fdada8b3d4
In errors5.rs, there are two lines of a pattern matching block for which the order is reversed between the exercise file and the solution file. Since these lines are not changed as part of the exercise, this commit updates the exercise to make the order of the lines consistent with the solution, so that users will focus only on the lines that change between the exercise and the solution.
57 lines
1.9 KiB
Rust
57 lines
1.9 KiB
Rust
// This exercise is an altered version of the `errors4` exercise. It uses some
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// concepts that we won't get to until later in the course, like `Box` and the
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// `From` trait. It's not important to understand them in detail right now, but
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// you can read ahead if you like. For now, think of the `Box<dyn ???>` type as
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// an "I want anything that does ???" type.
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//
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// In short, this particular use case for boxes is for when you want to own a
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// value and you care only that it is a type which implements a particular
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// trait. To do so, The `Box` is declared as of type `Box<dyn Trait>` where
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// `Trait` is the trait the compiler looks for on any value used in that
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// context. For this exercise, that context is the potential errors which
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// can be returned in a `Result`.
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use std::error::Error;
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use std::fmt;
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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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// This is required so that `CreationError` can implement `Error`.
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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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let description = match *self {
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CreationError::Negative => "number is negative",
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CreationError::Zero => "number is zero",
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};
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f.write_str(description)
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}
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}
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impl Error for CreationError {}
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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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match value {
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x if x < 0 => Err(CreationError::Negative),
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0 => Err(CreationError::Zero),
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x => Ok(PositiveNonzeroInteger(x as u64)),
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}
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}
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}
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// TODO: Add the correct return type `Result<(), Box<dyn ???>>`. What can we
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// use to describe both errors? Is there a trait which both errors implement?
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fn main() {
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let pretend_user_input = "42";
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let x: i64 = pretend_user_input.parse()?;
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println!("output={:?}", PositiveNonzeroInteger::new(x)?);
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Ok(())
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}
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