Rework the error handling exercise to be based on the expression evaluator exercise

src/error-handling/exercise.md

@@ -1,23 +1,19 @@
 ---
-minutes: 30
+minutes: 20
 ---
 
 # Exercise: Rewriting with Result
 
-The following implements a very simple parser for an expression language.
-However, it handles errors by panicking. Rewrite it to instead use idiomatic
-error handling and propagate errors to a return from `main`. Feel free to use
-[`thiserror`] and [`anyhow`].
-
-[`thiserror`]: https://docs.rs/thiserror
-[`anyhow`]: https://docs.rs/anyhow
-
-> **Hint:** start by fixing error handling in the `parse` function. Once that is
-> working correctly, update `Tokenizer` to implement
-> `Iterator<Item=Result<Token, TokenizerError>>` and handle that in the parser.
+In this exercise we're revisiting the expression evaluator exercise that we did
+in day 2. Our initial solution ignores a possible error case: Dividing by zero!
+Rewrite `eval` to instead use idiomatic error handling to handle this error case
+and return an error when it occurs. We provide a simple `DivideByZeroError` type
+to use as the error type for `eval`.
 
 ```rust,editable
 {{#include exercise.rs:types}}
 
-{{#include exercise.rs:panics}}
+{{#include exercise.rs:eval}}
+
+{{#include exercise.rs:tests}}
 ```

src/error-handling/exercise.rs

@@ -12,212 +12,101 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// ANCHOR: solution
-use thiserror::Error;
+#![allow(dead_code)]
 // ANCHOR: types
-use std::iter::Peekable;
-use std::str::Chars;
-
-/// An arithmetic operator.
-#[derive(Debug, PartialEq, Clone, Copy)]
-enum Op {
+/// An operation to perform on two subexpressions.
+#[derive(Debug)]
+enum Operation {
     Add,
     Sub,
+    Mul,
+    Div,
 }
 
-/// A token in the expression language.
-#[derive(Debug, PartialEq)]
-enum Token {
-    Number(String),
-    Identifier(String),
-    Operator(Op),
-}
-
-/// An expression in the expression language.
-#[derive(Debug, PartialEq)]
+/// An expression, in tree form.
+#[derive(Debug)]
 enum Expression {
-    /// A reference to a variable.
-    Var(String),
-    /// A literal number.
-    Number(u32),
-    /// A binary operation.
-    Operation(Box<Expression>, Op, Box<Expression>),
-}
-// ANCHOR_END: types
+    /// An operation on two subexpressions.
+    Op { op: Operation, left: Box<Expression>, right: Box<Expression> },
 
-fn tokenize(input: &str) -> Tokenizer {
-    return Tokenizer(input.chars().peekable());
+    /// A literal value
+    Value(i64),
 }
 
-#[derive(Debug, Error)]
-enum TokenizerError {
-    #[error("Unexpected character '{0}' in input")]
-    UnexpectedCharacter(char),
-}
-
-struct Tokenizer<'a>(Peekable<Chars<'a>>);
-
-impl<'a> Tokenizer<'a> {
-    fn collect_number(&mut self, first_char: char) -> Token {
-        let mut num = String::from(first_char);
-        while let Some(&c @ '0'..='9') = self.0.peek() {
-            num.push(c);
-            self.0.next();
-        }
-        Token::Number(num)
-    }
-
-    fn collect_identifier(&mut self, first_char: char) -> Token {
-        let mut ident = String::from(first_char);
-        while let Some(&c @ ('a'..='z' | '_' | '0'..='9')) = self.0.peek() {
-            ident.push(c);
-            self.0.next();
-        }
-        Token::Identifier(ident)
-    }
-}
-
-impl<'a> Iterator for Tokenizer<'a> {
-    type Item = Result<Token, TokenizerError>;
-
-    fn next(&mut self) -> Option<Result<Token, TokenizerError>> {
-        let c = self.0.next()?;
-        match c {
-            '0'..='9' => Some(Ok(self.collect_number(c))),
-            'a'..='z' | '_' => Some(Ok(self.collect_identifier(c))),
-            '+' => Some(Ok(Token::Operator(Op::Add))),
-            '-' => Some(Ok(Token::Operator(Op::Sub))),
-            _ => Some(Err(TokenizerError::UnexpectedCharacter(c))),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
-enum ParserError {
-    #[error("Tokenizer error: {0}")]
-    TokenizerError(#[from] TokenizerError),
-    #[error("Unexpected end of input")]
-    UnexpectedEOF,
-    #[error("Unexpected token {0:?}")]
-    UnexpectedToken(Token),
-    #[error("Invalid number")]
-    InvalidNumber(#[from] std::num::ParseIntError),
-}
-
-fn parse(input: &str) -> Result<Expression, ParserError> {
-    let mut tokens = tokenize(input);
-
-    fn parse_expr<'a>(
-        tokens: &mut Tokenizer<'a>,
-    ) -> Result<Expression, ParserError> {
-        let tok = tokens.next().ok_or(ParserError::UnexpectedEOF)??;
-        let expr = match tok {
-            Token::Number(num) => {
-                let v = num.parse()?;
-                Expression::Number(v)
-            }
-            Token::Identifier(ident) => Expression::Var(ident),
-            Token::Operator(_) => return Err(ParserError::UnexpectedToken(tok)),
-        };
-        // Look ahead to parse a binary operation if present.
-        Ok(match tokens.next() {
-            None => expr,
-            Some(Ok(Token::Operator(op))) => Expression::Operation(
-                Box::new(expr),
-                op,
-                Box::new(parse_expr(tokens)?),
-            ),
-            Some(Err(e)) => return Err(e.into()),
-            Some(Ok(tok)) => return Err(ParserError::UnexpectedToken(tok)),
-        })
-    }
-
-    parse_expr(&mut tokens)
-}
-
-fn main() -> anyhow::Result<()> {
-    let expr = parse("10+foo+20-30")?;
-    println!("{expr:?}");
-    Ok(())
-}
-// ANCHOR_END: solution
+#[derive(PartialEq, Eq, Debug)]
+struct DivideByZeroError;
+// ANCHOR_END: types
 
 /*
-// ANCHOR: panics
-fn tokenize(input: &str) -> Tokenizer {
-    return Tokenizer(input.chars().peekable());
-}
-
-struct Tokenizer<'a>(Peekable<Chars<'a>>);
-
-impl<'a> Tokenizer<'a> {
-    fn collect_number(&mut self, first_char: char) -> Token {
-        let mut num = String::from(first_char);
-        while let Some(&c @ '0'..='9') = self.0.peek() {
-            num.push(c);
-            self.0.next();
-        }
-        Token::Number(num)
-    }
-
-    fn collect_identifier(&mut self, first_char: char) -> Token {
-        let mut ident = String::from(first_char);
-        while let Some(&c @ ('a'..='z' | '_' | '0'..='9')) = self.0.peek() {
-            ident.push(c);
-            self.0.next();
+// ANCHOR: eval
+// The original implementation of the expression evaluator. Update this to
+// return a `Result` and produce an error when dividing by 0.
+fn eval(e: Expression) -> i64 {
+    match e {
+        Expression::Op { op, left, right } => {
+            let left = eval(*left);
+            let right = eval(*right);
+            match op {
+                Operation::Add => left + right,
+                Operation::Sub => left - right,
+                Operation::Mul => left * right,
+                Operation::Div => if right != 0 {
+                    left / right
+                } else {
+                    panic!("Cannot divide by zero!");
+                },
+            }
         }
-        Token::Identifier(ident)
+        Expression::Value(v) => v,
     }
 }
+// ANCHOR_END: eval
+*/
 
-impl<'a> Iterator for Tokenizer<'a> {
-    type Item = Token;
-
-    fn next(&mut self) -> Option<Token> {
-        let c = self.0.next()?;
-        match c {
-            '0'..='9' => Some(self.collect_number(c)),
-            'a'..='z' => Some(self.collect_identifier(c)),
-            '+' => Some(Token::Operator(Op::Add)),
-            '-' => Some(Token::Operator(Op::Sub)),
-            _ => panic!("Unexpected character {c}"),
+// ANCHOR: solution
+fn eval(e: Expression) -> Result<i64, DivideByZeroError> {
+    match e {
+        Expression::Op { op, left, right } => {
+            let left = eval(*left)?;
+            let right = eval(*right)?;
+            Ok(match op {
+                Operation::Add => left + right,
+                Operation::Sub => left - right,
+                Operation::Mul => left * right,
+                Operation::Div => {
+                    if right == 0 {
+                        return Err(DivideByZeroError);
+                    } else {
+                        left / right
+                    }
+                }
+            })
         }
+        Expression::Value(v) => Ok(v),
     }
 }
+// ANCHOR_END: solution
 
-fn parse(input: &str) -> Expression {
-    let mut tokens = tokenize(input);
-
-    fn parse_expr<'a>(tokens: &mut Tokenizer<'a>) -> Expression {
-        let Some(tok) = tokens.next() else {
-            panic!("Unexpected end of input");
-        };
-        let expr = match tok {
-            Token::Number(num) => {
-                let v = num.parse().expect("Invalid 32-bit integer");
-                Expression::Number(v)
-            }
-            Token::Identifier(ident) => Expression::Var(ident),
-            Token::Operator(_) => panic!("Unexpected token {tok:?}"),
-        };
-        // Look ahead to parse a binary operation if present.
-        match tokens.next() {
-            None => expr,
-            Some(Token::Operator(op)) => Expression::Operation(
-                Box::new(expr),
-                op,
-                Box::new(parse_expr(tokens)),
-            ),
-            Some(tok) => panic!("Unexpected token {tok:?}"),
-        }
-    }
-
-    parse_expr(&mut tokens)
+// ANCHOR: tests
+#[test]
+fn test_error() {
+    assert_eq!(
+        eval(Expression::Op {
+            op: Operation::Div,
+            left: Box::new(Expression::Value(99)),
+            right: Box::new(Expression::Value(0)),
+        }),
+        Err(DivideByZeroError)
+    );
 }
 
 fn main() {
-    let expr = parse("10+foo+20-30");
-    println!("{expr:?}");
+    let expr = Expression::Op {
+        op: Operation::Sub,
+        left: Box::new(Expression::Value(20)),
+        right: Box::new(Expression::Value(10)),
+    };
+    println!("expr: {expr:?}");
+    println!("result: {:?}", eval(expr));
 }
-// ANCHOR_END: panics
-*/
+// ANCHOR_END: tests

src/error-handling/solution.md

@@ -1,7 +1,9 @@
 # Solution
 
-<!-- compile_fail because `mdbook test` does not allow use of `thiserror` -->
+```rust,editable
+{{#include exercise.rs:types}}
 
-```rust,editable,compile_fail
 {{#include exercise.rs:solution}}
+
+{{#include exercise.rs:tests}}
 ```