Error type
An error type is a data type whose values represent failure conditions and may carry information or behavior for reporting and handling those failures.
Refactoring to Rust
A custom error type is a programmer-defined type that represents the distinct failure conditions an operation can return.
- How it is used
- Represents distinct operation failures so callers can handle them separately or deliberately propagate them.
- How deep it goes
- Works from a single enum variant through calculator parsing and stack failures to propagation and public API visibility; defers error formatting implementation.
- Examples
- Uses negative FizzBuzz input, invalid calculator numbers, an empty stack, and a compiler error for a private return type.
- What you can do after
- Build it from scratch: define error variants, return them in `Result`, propagate them with `?`, and expose the type when required.
Rust Servers, Services, and Apps
A custom error type is a programmer-defined Rust type that represents application-specific failures and can unify multiple underlying error types.
- How it is used
- Unifies failures across database access and web handlers so Actix can return meaningful HTTP errors.
- How deep it goes
- Works through enum variants, From conversions, ResponseError status and body methods, and handler return types.
- Examples
- Uses a parsing-and-file-I/O mismatch, a course-insertion handler, and a registration error enum.
- What you can do after
- Build it from scratch for an Actix service, propagate database failures, and return category-appropriate HTTP responses.
Read how Rust Servers, Services, and Apps explains error type →
Go in Action, Second Edition
An error type is Go's interface type for representing failed or abnormal operation results through an Error method.
- How it is used
- Makes failures from key/value store operations explicit in function return values.
- How deep it goes
- Defines the one-method interface and works through a slice-backed custom error, without descending below interface behavior.
- Examples
- Uses key/value store operations and a bulk lookup that collects missing keys in a custom error.
- What you can do after
- Build it from scratch by implementing Error() string on a custom type and returning it when a bulk lookup finds missing keys.
Learn Go with Pocket-Sized Projects
A custom error is an application-defined error value or type representing a specific failure condition.
- How it is used
- Uses typed and custom errors to carry domain failures across business, repository, and gRPC API layers.
- How deep it goes
- Defines error fields and Error methods, demonstrates validation and status conversion, and tests ErrorAs without exploring broader error hierarchies.
- Examples
- Uses InvalidInputError for blank habit names and a not-found repository error when ticking an unknown habit.
- What you can do after
- Apply it via Go error types, errors.Is, ErrorAs, and gRPC status conversion to validate inputs and expose stable failures.
Read how Learn Go with Pocket-Sized Projects explains error type →
Go in Practice, Second Edition
- How it is used
- Uses custom error types to preserve failure details and let callers choose handling based on the specific error.
- How deep it goes
- Explains the error interface, implements structured parser and HTTP errors, and stops short of broader type-system internals.
- Examples
- Demonstrates ParseError diagnostics and conversion of non-success HTTP responses into structured errors.
- What you can do after
- Build custom Go errors that carry structured diagnostics and handle unexpected HTTP responses explicitly.
Read how Go in Practice, Second Edition explains error type →
Related concepts
- failureimplements · A failure is an unsuccessful outcome or condition in which an operation, process, or system does not achieve an expected result.
- ResponseErrorimplements · ResponseError is an Actix Web trait that defines how an application error is converted into an HTTP response and status code.
- From traitused by · From is Rust’s standard-library trait for infallibly converting a value of one type into another type.
- Errorimplements · std::error::Error is Rust’s standard-library trait for types that represent errors and can provide descriptions and source-error relationships.
- responseproduces · An HTTP response is a protocol-formatted message a server sends to a client in reply to a request.
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