diff options
| -rw-r--r-- | exercises/error_handling/errors5.rs | 16 | ||||
| -rw-r--r-- | info.toml | 21 |
2 files changed, 21 insertions, 16 deletions
diff --git a/exercises/error_handling/errors5.rs b/exercises/error_handling/errors5.rs index 365a869..9d5ee4b 100644 --- a/exercises/error_handling/errors5.rs +++ b/exercises/error_handling/errors5.rs @@ -1,7 +1,17 @@ // errors5.rs -// This program uses a completed version of the code from errors4. -// It won't compile right now! Why? +// This program uses an altered version of the code from errors4. + +// This exercise uses some concepts that we won't get to until later in the course, like `Box` and the +// `From` trait. It's not important to understand them in detail right now, but you can read ahead if you like. + +// In short, this particular use case for boxes is for when you want to own a value and you care only that it is a +// type which implements a particular trait. To do so, The Box is declared as of type Box<dyn Trait> where Trait is the trait +// the compiler looks for on any value used in that context. For this exercise, that context is the potential errors +// which can be returned in a Result. + +// What can we use to describe both errors? In other words, is there a trait which both errors implement? + // Execute `rustlings hint errors5` for hints! // I AM NOT DONE @@ -11,7 +21,7 @@ use std::fmt; use std::num::ParseIntError; // TODO: update the return type of `main()` to make this compile. -fn main() -> Result<(), ParseIntError> { +fn main() -> Result<(), Box<dyn ???>> { let pretend_user_input = "42"; let x: i64 = pretend_user_input.parse()?; println!("output={:?}", PositiveNonzeroInteger::new(x)?); @@ -619,22 +619,17 @@ name = "errors5" path = "exercises/error_handling/errors5.rs" mode = "compile" hint = """ -There are two different possible `Result` types produced within -`main()`, which are propagated using `?` operators. How do we declare a -return type from `main()` that allows both? - -Another hint: under the hood, the `?` operator calls `From::from` -on the error value to convert it to a boxed trait object, a -`Box<dyn error::Error>`, which is polymorphic-- that means that lots of -different kinds of errors can be returned from the same function because -all errors act the same since they all implement the `error::Error` trait. +There are two different possible `Result` types produced within `main()`, which are +propagated using `?` operators. How do we declare a return type from `main()` that allows both? + +Under the hood, the `?` operator calls `From::from` on the error value to convert it to a boxed +trait object, a `Box<dyn error::Error>`. This boxed trait object is polymorphic, and since all +errors implement the `error:Error` trait, we can capture lots of different errors in one "Box" +object. + Check out this section of the book: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator -This exercise uses some concepts that we won't get to until later in the -course, like `Box` and the `From` trait. It's not important to understand -them in detail right now, but you can read ahead if you like. - Read more about boxing errors: https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/boxing_errors.html |