errors5 solution

exercises/13_error_handling/errors5.rs

@@ -1,38 +1,18 @@
-// 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. For now, think
-// of the `Box<dyn ???>` type as an "I want anything that does ???" type, which,
-// given Rust's usual standards for runtime safety, should strike you as
-// somewhat lenient!
+// This exercise is an altered version of the `errors4` exercise. It 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. For now, think of the `Box<dyn ???>` type as
+// an "I want anything that does ???" type.
 //
 // 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?
+// 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`.
 
-use std::error;
+use std::error::Error;
 use std::fmt;
-use std::num::ParseIntError;
-
-// TODO: update the return type of `main()` to make this compile.
-fn main() -> Result<(), Box<dyn ???>> {
-    let pretend_user_input = "42";
-    let x: i64 = pretend_user_input.parse()?;
-    println!("output={:?}", PositiveNonzeroInteger::new(x)?);
-    Ok(())
-}
-
-// Don't change anything below this line.
-
-#[derive(PartialEq, Debug)]
-struct PositiveNonzeroInteger(u64);
 
 #[derive(PartialEq, Debug)]
 enum CreationError {
@@ -40,17 +20,7 @@ enum CreationError {
     Zero,
 }
 
-impl PositiveNonzeroInteger {
-    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
-        match value {
-            x if x < 0 => Err(CreationError::Negative),
-            x if x == 0 => Err(CreationError::Zero),
-            x => Ok(PositiveNonzeroInteger(x as u64)),
-        }
-    }
-}
-
-// This is required so that `CreationError` can implement `error::Error`.
+// This is required so that `CreationError` can implement `Error`.
 impl fmt::Display for CreationError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let description = match *self {
@@ -61,4 +31,26 @@ impl fmt::Display for CreationError {
     }
 }
 
-impl error::Error for CreationError {}
+impl Error for CreationError {}
+
+#[derive(PartialEq, Debug)]
+struct PositiveNonzeroInteger(u64);
+
+impl PositiveNonzeroInteger {
+    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
+        match value {
+            0 => Err(CreationError::Zero),
+            x if x < 0 => Err(CreationError::Negative),
+            x => Ok(PositiveNonzeroInteger(x as u64)),
+        }
+    }
+}
+
+// TODO: Add the correct return type `Result<(), Box<dyn ???>>`. What can we
+// use to describe both errors? Is there a trait which both errors implement?
+fn main() {
+    let pretend_user_input = "42";
+    let x: i64 = pretend_user_input.parse()?;
+    println!("output={:?}", PositiveNonzeroInteger::new(x)?);
+    Ok(())
+}