In this lesson, you will learn how to flatten deeply nested Result<Result<T, E1>, E2> structures to keep your error handling logic clean and readable.
As your systems grow in complexity, you may encounter functions that return nested results. This often happens when you combine an operation that might fail due to "infrastructure" reasons (like a database connection error) with an operation that might fail due to "business logic" reasons (like a validation failure).
When you see a return type like Result<Result<User, ValidationError>, DatabaseError>, you have entered a maintenance nightmare. To extract the User, you would have to write nested match statements or use cumbersome double-question-mark syntax, which makes the code difficult to follow and even harder to debug.
Flattening the Nest
The goal is to unify these errors into a single, cohesive type. Instead of letting the nested structure dictate your control flow, you should map the inner error into your top-level error domain.
Consider this example of a typical "nested" mess:
fn fetch_user() -> Result<Result<User, ValidationError>, DatabaseError> {
// ... logic that might fail at the DB level or validation level
}
// How we usually fix this:
fn fetch_user_flattened() -> Result<User, AppError> {
fetch_user()
.map_err(AppError::from) // Convert DatabaseError
.and_then(|res| res.map_err(AppError::from)) // Convert ValidationError
}
By using .and_then() combined with .map_err(), you collapse the inner Result into the outer one. This technique treats the business-logic error and the infrastructure error as different variants of a single AppError enum.
Once flattened, your call sites become significantly cleaner. You no longer need to check for a "result inside a result"; you simply check if the operation succeeded or failed, regardless of the source of the failure. This aligns your code with the "one true error path" philosophy, making it easier to propagate failures using the ? operator without losing the context of why an operation failed.
Key Takeaway: Always unify nested results into a flat, cohesive error type as early as possible to prevent "Result-pyramid" code and keep your error propagation logic simple and maintainable.