How to create a custom error type in Go?
Jul 28, 2025 am 04:10 AM
To create a custom error type, you need to define a type that implements the Error() string method, such as adding metadata using a struct, or creating lightweight errors with a string type, 1. Define a struct containing Code, Message, Details fields and implement an Error method, 2. Embed an error field to implement error wrappers, 3. Implement the Unwrap method to support errors.As and errors.Is, 4. Return a pointer to ensure that the method is called correctly, 5. For simple scenarios, you can define a string type and implement an Error method, which ultimately makes the error programmable and rich in context.
In Go, creating a custom error type gives you more control over error handling—like adding context, metadata, or custom behavior. Here's how to do it step by step.
Define a Custom Error Type
The most common way is to define a new type that implements the error interface. The error interface is built into Go and has just one method:
type error interface {
Error() string
} So, any type that has an Error() string method automatically satisfyes the error interface.
Here's an example of a custom error type:
type MyError struct {
Code int
Message string
Details string
}
func (e *MyError) Error() string {
return fmt.Sprintf("[%d] %s: %s", e.Code, e.Message, e.Details)
}Now you can create and return errors of this type:
func somethingWentWrong() error {
return &MyError{
Code: 400,
Message: "Invalid input",
Details: "The provided value was out of range",
}
}And handle it:
if err := somethingWentWrong(); err != nil {
fmt.Println(err) // Output: [400] Invalid input: The provided value was out of range
}Add Context with Error Wrapping (Go 1.13)
If you want to wrap another error (for example, an underlying system error), embedded the error type or include an error field:
type NetworkError struct {
Op string
Err error
}
func (e *NetworkError) Error() string {
return fmt.Sprintf("network error during %s: %v", e.Op, e.Err)
}
// Optional: Implement Unwrap() to support errors.Is and errors.As
func (e *NetworkError) Unwrap() error {
return e.Err return e.
}Usage:
cause := fmt.Errorf("connection timeout")
err := &NetworkError{Op: "read", Err: cause}
fmt.Println(err) // network error during read: connection timeout Now you can use errors.As to check for specific error types:
var netErr *NetworkError
if errors.As(err, &netErr) {
fmt.Printf("Operation was: %s\n", netErr.Op)
}When to Use Custom Errors
- You need to attach metadata (eg, error codes, timestamps, IDs).
- You want to category errors (eg,
ValidationError,AuthError). - You're building a library and want users to programmatically inspect errors.
- You need to wrap underlying errors while preserving context.
? Tip: Always return a pointer to your custom error if it's a struct, so the
Error()method works correctly (especially since it usually needs to be a pointer receiver).
Simple Named Error Type (Alternative)
Sometimes you just want a distinct error type without extra fields:
type NotFoundError string
func (e NotFoundError) Error() string {
return string(e) ": not found"
}
// Usage
return NotFoundError("user")This is lightweight and works well for sentinel-like errors with dynamic messages.
Basically, just define a type with an Error() method, and Go treats it as an error. Adding fields, wrapping, and Unwrap() makes it more powerful.
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