Custom error types in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a custom error enum named MyError.

Rust

#[derive(Debug)]
enum MyError {
    [1]
}

Drag options to blanks, or click blank then click option'

AIoError(std::io::Error)

BError(String)

CMyErrorType

DCustomError

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variant name without data when a message is needed

Trying to use a type name as a variant

The enum variant Error(String) defines a custom error variant holding a string message.

2fill in blank

medium

Complete the code to implement the Display trait for MyError.

Rust

impl std::fmt::Display for MyError {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        match self {
            MyError::Error(msg) => write!(f, [1], msg),
        }
    }
}

Drag options to blanks, or click blank then click option'

A"{}"

B"MyError: {}"

C"Error: {}"

D"Error occurred: {}"

Attempts:

3 left

💡 Hint

Common Mistakes

Adding extra text that is not needed

Forgetting to include the placeholder {}

The format string "{}" directly prints the error message string.

3fill in blank

hard

Fix the error in the From trait implementation to convert std::io::Error into MyError.

Rust

impl From<std::io::Error> for MyError {
    fn from(err: std::io::Error) -> MyError {
        MyError::[1](err.to_string())
    }
}

Drag options to blanks, or click blank then click option'

AIoError

BCustomError

CError

DMyErrorType

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variant name that does not exist

Passing the error type directly instead of converting to string

The variant Error holds a String, so we convert the io::Error to string and wrap it.

4fill in blank

hard

Fill in the blank to implement the std::error::Error trait for MyError.

Rust

impl std::error::Error for MyError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        [1]
    }
}

Drag options to blanks, or click blank then click option'

ANone

B"Custom error"

CSome(self)

D"MyError occurred"

Attempts:

3 left

💡 Hint

Common Mistakes

Returning Some(self) in source which causes lifetime issues

The source method returns None because this error has no underlying cause.

5fill in blank

hard

Fill all three blanks to create a function that returns a Result with MyError and uses the ? operator.

Rust

fn read_file(path: &str) -> Result<String, MyError> {
    let content = std::fs::read_to_string(path)[1];
    Ok([2])
}

fn main() {
    match read_file("file.txt") {
        Ok(data) => println!("File content: {}", [3]),
        Err(e) => println!("Error: {}", e),
    }
}

Drag options to blanks, or click blank then click option'

A?

Bcontent

Cdata

D.unwrap()

Attempts:

3 left

💡 Hint

Common Mistakes

Using .unwrap() which panics on error

Printing the wrong variable in main

The ? operator propagates errors. The variable content holds the file data, and data is used in the print statement.

Practice

1. What is the main purpose of creating a custom error type in Rust?

easy

A. To automatically fix bugs in the code.

B. To make the program run faster by avoiding errors.

C. To replace the need for using Result types.

D. To describe specific problems in your program with clear error messages.

Solution

1. Step 1: Understand the role of custom errors

   Custom error types help programmers describe specific problems clearly in their code.

2. Step 2: Compare with other options

   Custom errors do not speed up code, fix bugs automatically, or replace Result types; they provide clear error descriptions.

3. Final Answer:

   To describe specific problems in your program with clear error messages. -> Option D

4. Quick Check:

   Custom error purpose = clear problem description [OK]

Hint: Custom errors explain problems clearly, not fix or speed up code [OK]

Common Mistakes:

• Thinking custom errors fix bugs automatically
• Confusing custom errors with performance improvements
• Believing custom errors replace Result types

2. Which of the following is the correct way to start defining a custom error enum in Rust?

easy

A. enum MyError { Io(std::io::Error), Parse(std::num::ParseIntError) }

B. struct MyError { message: String }

C. fn MyError() -> Error {}

D. class MyError { error: String }

Solution

1. Step 1: Identify Rust enum syntax

   Rust uses enum to define custom error types with variants, like Io(std::io::Error).

2. Step 2: Check other options

   struct is for data structures, fn is for functions, and class is not Rust syntax.

3. Final Answer:

   enum MyError { Io(std::io::Error), Parse(std::num::ParseIntError) } -> Option A

4. Quick Check:

   Custom error enum = enum with variants [OK]

Hint: Custom errors use enum with variants, not struct or class [OK]

Common Mistakes:

• Using struct instead of enum for error variants
• Trying to define errors as functions
• Using class keyword which is invalid in Rust

3. What will be the output of this Rust code snippet?

use std::fmt;

#[derive(Debug)]
enum MyError {
    NotFound,
    PermissionDenied,
}

impl fmt::Display for MyError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            MyError::NotFound => write!(f, "Item not found"),
            MyError::PermissionDenied => write!(f, "Permission denied"),
        }
    }
}

fn main() {
    let err = MyError::NotFound;
    println!("Error: {}", err);
}

medium

A. Error: NotFound

B. Error: Item not found

C. Error: Permission denied

D. Compilation error due to missing trait

Solution

1. Step 1: Understand Display implementation

   The fmt method matches MyError::NotFound and writes "Item not found" to the formatter.

2. Step 2: Check main output

   Printing err with {} uses Display, so output is "Error: Item not found".

3. Final Answer:

   Error: Item not found -> Option B

4. Quick Check:

   Display prints custom message = "Item not found" [OK]

Hint: Display trait controls {} output, matching variant message shown [OK]

Common Mistakes:

• Expecting Debug output instead of Display
• Confusing variant name with message
• Assuming compilation error without Display trait

4. Identify the error in this custom error implementation:

enum MyError {
    Io(std::io::Error),
}

impl std::fmt::Display for MyError {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

medium

A. Cannot access self.0 because enum variant is not destructured.

B. Missing implementation of Debug trait.

C. Display trait cannot be implemented for enums.

D. write! macro requires a string literal, not a variable.

Solution

1. Step 1: Check how self.0 is accessed

   In an enum, you cannot directly access self.0 without matching or destructuring the variant.

2. Step 2: Understand correct pattern

   You must match self to get the inner std::io::Error value before using it in write!.

3. Final Answer:

   Cannot access self.0 because enum variant is not destructured. -> Option A

4. Quick Check:

   Enum variant fields need matching before access [OK]

Hint: Match enum variants before accessing inner fields [OK]

Common Mistakes:

• Trying to access enum fields directly without match
• Thinking Display can't be implemented for enums
• Assuming Debug trait is mandatory for Display

5. You want to create a custom error type that can hold either an IO error or a parsing error, and provide user-friendly messages. Which of these is the best complete approach?

hard

A. Create separate error structs for IO and parse errors without enums, and use panic! for messages.

B. Use a struct with two optional fields for IO and parse errors, and implement Display only.

C. Define an enum with variants for each error type, implement Display with match arms for messages, and implement std::error::Error.

D. Use a type alias for Result with String errors and skip custom error types.

Solution

1. Step 1: Choose enum for multiple error types

   An enum cleanly groups different error variants like IO and parse errors.

2. Step 2: Implement Display and std::error::Error traits

   Display provides friendly messages; implementing std::error::Error integrates with Rust error handling.

3. Final Answer:

   Define an enum with variants for each error type, implement Display with match arms for messages, and implement std::error::Error. -> Option C

4. Quick Check:

   Enum + Display + Error trait = best custom error pattern [OK]

Hint: Use enum + Display + Error trait for best custom error design [OK]

Common Mistakes:

• Using struct with optional fields causing confusion
• Skipping std::error::Error trait implementation
• Using panic! instead of proper error handling