How to create a custom error in Go that can be passed through the error interface?

In Go, to create a custom error, you need to define a type (usually a struct) that implements the error interface — that is, contains the Error() string method. For example:

package main

import "fmt"

type MyError struct {
    Code    int
    Message string
}

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

func doSomething() error {
    return &MyError{Code: 404, Message: "Not Found"}
}

func main() {
    err := doSomething()
    if err != nil {
        fmt.Println(err) // Error 404: Not Found
    }
}

Thus, a custom error can contain additional fields and logic, and thanks to the implementation of the Error() method, it satisfies the error interface and can be passed and handled as a regular error.