Creating custom errors with Golang

package shoe

import "errors"

// ------------------------------------------------

var ErrTooSmall = errors.New("that shoe is too small")
var ErrTooBig = errors.New("that shoe is too big")

// ------------------------------------------------

type NarrowError struct {
	Message string
}

func NewNarrowError(m string) NarrowError {
	return NarrowError{Message:m}
}

func (n NarrowError) Error() string {
	return n.Message
}

// ------------------------------------------------

type SmallError struct {
	Message string
}

func NewSmallError(m string) SmallError {
	return SmallError{Message:m}
}

func (s SmallError) Error() string {
	return s.Message
}

package main

import (
	"fmt"
	"log"
	"os"

	"cmd/shoe"
)

func main() {
	// Change these values to see the result.
	var (
		width  = 5
		length = 10
	)

	err := checkShoeSize("inanzzz", width, length)
	if err != nil {
		log.Println(err.Error())

		switch err.(type) {
		case shoe.NarrowError:
			fmt.Println(fmt.Sprintf("Go and get wider shoe then %d width.", width))
		case shoe.SmallError:
			fmt.Println(fmt.Sprintf("Go and get bigger shoe then %d length.", length))
		default:
			if err == shoe.ErrTooSmall {
				fmt.Println(
					fmt.Sprintf("Go and get bigger shoe then %d width and %d length.", width, length),
				)
			}
			if err == shoe.ErrTooBig {
				fmt.Println(
					fmt.Sprintf("Go and get smaller shoe then %d width and %d length.", width, length),
				)
			}
		}

		os.Exit(0)
	}
	
	fmt.Println(fmt.Sprintf("Good fit shoe, %d width %d length", width, length))
}

func checkShoeSize(customer string, width, length int) error {
	if width < 5 && length < 10 {
		return shoe.ErrTooSmall
	}

	if width > 20 && length > 30 {
		return shoe.ErrTooBig
	}

	if width < 10 {
		return shoe.NewNarrowError(
			fmt.Sprintf("Customer %s's shoe is too narrow, %d width.", customer, width),
		)
	}

	if length < 20 {
		return shoe.NewSmallError(
			fmt.Sprintf("Customer %s's shoe is too small, %d length.", customer, length),
		)
	}

	return nil
}

$ go run -race main.go 
2020/02/06 20:19:28 Customer inanzzz's shoe is too narrow, 5 width.
Go and get wider shoe then 5 width.

var ErrClosed = &Error{code: 100, message: "connection is not open"}
var ErrMemory = &Error{code: 200, message: "out of memory"}

type Error struct {
    code    int
    message string
}

func (e Error) Code() int {
    return e.code
}

func (e Error) Error() string {
    return e.message
}