Code:

package main

import (
    "errors"
    "fmt"
    "net/http"

    "zgo.at/guru"
)

// Error constants.
const (
    CodeFruitOverflow = iota + 1
    CodeBoozeUnderrun
    CodeExpired
)

func main() {
    // Construct a new error.
    err := guru.New(CodeFruitOverflow, "too many bananas")
    fmt.Println(err) // error 1: too many bananas

    // Retrieve the error code
    code := guru.Code(err)
    fmt.Println(code) // 1

    // Add error code to existing error.
    err = errors.New("not enough beer")
    err = guru.WithCode(CodeBoozeUnderrun, err)
    fmt.Println(err) // error 2: not enough beer

    // Add error code to existing error with context.
    err = errors.New("Dennis Ritchie")
    err = guru.Wrap(CodeExpired, err, "no longer with us")
    fmt.Println(err) // error 3: Dennis Ritchie: no longer with us

    // For HTTP applications, it may be useful to directly the HTTP status codes:
    err = guru.New(http.StatusNotAcceptable, "Justin Bieber")
    fmt.Println(err) // error 406: Justin Bieber

    // Error codes can be overriden:
    err = guru.New(1, "oh noes")
    err = guru.WithCode(2, err)
    fmt.Println(guru.Code(err)) // 2
}