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65 lines
2.2 KiB
Rust
65 lines
2.2 KiB
Rust
// This program uses an altered version of the code from errors4.
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//
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// This exercise uses some concepts that we won't get to until later in the
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// course, like `Box` and the `From` trait. It's not important to understand
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// them in detail right now, but you can read ahead if you like. For now, think
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// of the `Box<dyn ???>` type as an "I want anything that does ???" type, which,
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// given Rust's usual standards for runtime safety, should strike you as
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// somewhat lenient!
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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 Trait is
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// the trait the compiler looks for on any value used in that context. For this
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// exercise, that context is the potential errors which can be returned in a
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// Result.
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//
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// What can we use to describe both errors? In other words, is there a trait
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// which both errors implement?
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use std::error;
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use std::fmt;
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use std::num::ParseIntError;
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// TODO: update the return type of `main()` to make this compile.
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fn main() -> Result<(), Box<dyn ???>> {
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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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// Don't change anything below this line.
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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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match value {
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x if x < 0 => Err(CreationError::Negative),
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x if x == 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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// This is required so that `CreationError` can implement `error::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::Error for CreationError {}
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