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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