Update expression-evaluation exercise: more patterns, more enums (#1582)

This modifies the exercise to lean more into interesting `match`
statements. It also uses the standard `Result` type, based on feedback
that students could understand it sufficiently at this point in the
course.

Addresses #1565.

src/pattern-matching/exercise.md

@@ -4,31 +4,18 @@ minutes: 30
 
 # Exercise: Expression Evaluation
 
-Let's write a simple recursive evaluator for arithmetic expressions. Start with
+Let's write a simple recursive evaluator for arithmetic expressions.
-an enum defining the binary operations:
-
-```rust
-{{#include exercise.rs:Operation}}
-
-{{#include exercise.rs:Expression}}
-
-{{#include exercise.rs:Res}}
-
-{{#include exercise.rs:eval}}
-    todo!()
-}
-
-{{#include exercise.rs:tests}}
-```
 
 The `Box` type here is a smart pointer, and will be covered in detail later in
 the course. An expression can be "boxed" with `Box::new` as seen in the tests.
-To evaluate a boxed expression, use the deref operator to "unbox" it:
+To evaluate a boxed expression, use the deref operator (`*`) to "unbox" it:
 `eval(*boxed_expr)`.
 
-Some expressions cannot be evaluated and will return an error. The `Res`
+Some expressions cannot be evaluated and will return an error. The standard
-type represents either a successful value or an error with a message. This is
+[`Result<Value,
-very similar to the standard-library `Result` which we will see later.
+String>`](https://doc.rust-lang.org/std/result/enum.Result.html) type is an
+enum that represents either a successful value (`Ok(Value)`) or an error
+(`Err(String)`). We will cover this type in detail later.
 
 Copy and paste the code into the Rust playground, and begin implementing
 `eval`. The final product should pass the tests. It may be helpful to use
@@ -42,5 +29,18 @@ temporarily with
 fn test_value() { .. }
 ```
 
-If you finish early, try writing a test that results in an integer overflow.
+If you finish early, try writing a test that results in division by zero or
-How could you handle this with `Res::Err` instead of a panic?
+integer overflow. How could you handle this with `Result` instead of a panic?
+
+```rust
+{{#include exercise.rs:Operation}}
+
+{{#include exercise.rs:Expression}}
+
+{{#include exercise.rs:eval}}
+    todo!()
+}
+
+{{#include exercise.rs:tests}}
+```
+

src/pattern-matching/exercise.rs

@@ -41,31 +41,18 @@ enum Expression {
 }
 // ANCHOR_END: Expression
 
-// ANCHOR: Res
-/// The result of evaluating an expression.
-#[derive(Debug, PartialEq, Eq)]
-enum Res {
-    /// Evaluation was successful, with the given result.
-    Ok(i64),
-    /// Evaluation failed, with the given error message.
-    Err(String),
-}
-// Allow `Ok` and `Err` as shorthands for `Res::Ok` and `Res::Err`.
-use Res::{Err, Ok};
-// ANCHOR_END: Res
-
 // ANCHOR: eval
-fn eval(e: Expression) -> Res {
+fn eval(e: Expression) -> Result<i64, String> {
     // ANCHOR_END: eval
     match e {
         Expression::Op { op, left, right } => {
             let left = match eval(*left) {
                 Ok(v) => v,
-                Err(msg) => return Err(msg),
+                e @ Err(_) => return e,
             };
             let right = match eval(*right) {
                 Ok(v) => v,
-                Err(msg) => return Err(msg),
+                e @ Err(_) => return e,
             };
             Ok(match op {
                 Operation::Add => left + right,