How to Create Custom Error Types in Go

Define a custom error type by creating a struct that implements the error interface with an Error() string method. This allows you to attach specific context or data to the error, which you can then check using errors.Is or errors.As.

package main

import (
	"errors"
	"fmt"
)

type MyError struct {
	Code int
	Msg  string
}

func (e *MyError) Error() string {
	return fmt.Sprintf("error %d: %s", e.Code, e.Msg)
}

func main() {
	err := &MyError{Code: 404, Msg: "not found"}
	if errors.Is(err, &MyError{}) {
		fmt.Println("Handled")
	}
	fmt.Println(err)
}

A custom error type is a specific variable you create to hold extra details about what went wrong, like an error code or a specific message. Instead of just returning a generic text string, you return this custom object so your program can check exactly what kind of failure occurred and react differently. Think of it like a specific error ticket with a barcode, rather than just a sticky note that says "something broke."