Interfaces in Go

An interface type is a method set. If a type contains methods of the interface, it implements the interface. 

A type can implement multiple interfaces. For instance all type implement the empty interface.

An interface may use a  interface type name  in place of a method specification. This is called embedding interface.

type ReadWriter interface {	
Read(b Buffer) bool
Write(b Buffer) bool
type File interface {
ReadWriter // same as adding the methods of ReadWriter

The empty interface

A type of empty interface can hold values of any type.  E.g. For implementing a linkedlist in Golang, we could declare the linkedlist struct as follows

type Node struct {
    Next *Node
    Data interface{}

 This allows us to use the same struct to hold data of different types.

n := linkedlist.New(0) // int Data
for m := n; m != nil; m = m.Next {
n1 := linkedlist.New("a") //string Data

Useful interfaces in Go stdlib

Error Interface

The error type is an interface that has a method Error.

type error interface {    
Error() string

The most commonly used implementation of the error interface is the errorString type in the errors package.

// errorString is a trivial implementation of error.type errorString struct {    
s string
func (e *errorString) Error() string {
return e.s

Handler Interface

The Handler Interface in the net/http package requires one method ServerHTTP

type Handler interface {	
  ServeHTTP(ResponseWriter, *Request)

Within that same package you will find HandlerFunc implements the Handler interface. 

type HandlerFunc func(ResponseWriter, *Request)     

// ServeHTTP calls f(w, r).  
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {  
f(w, r)  

The HandlerFunc makes it possible for us to pass in any function to make it a Handler. All we would have to do is wrap it in  HandlerFunc. 

http.Handle("/", http.HandlerFunc(indexHandlerHelloWorld))null

We can have also have our own struct that has fields and methods and implements the Handler Interface by defining the ServeHTTP method as a member of the struct. 

Stringer Interface

The fmt package has a stringer interface. It can be implemented by a type that declares the String method. 

type Stringer interface {        
String() string

If a type implements the stringer interface, a call to fmt.Println or fmt.Printf of the variable of type will use that method. 

E.g. If you want  to print a struct in a formatted fashion with key names and values, the struct needs to implement the Stringer interface

type Family struct {
    Name string
    Age int

func (f *Family) String() string {
    return fmt.Sprintf("Name: %s\t Age: %d", f.Name, f.Age)
func main() {
family := []Family{
        {"Alice", 23},
        {"David", 6},
        {"Erik", 2},
        {"Mary", 32},

    for _, i := range family {


The fmt package has other interfaces like Scanner, Formatter and State.



Auto-generate code using Go templates

The Golang template package is very useful in generating custom code especially if the code is very similar but needs multiple tweaks to work for different platforms or products.

I have used Golang templates for code generation in the following scenarios:

  1. Generating multiple Chromium and Firefox extensions for varied products.
  2. Interfacing with different API’s to get data into our main systems

I’m also looking into using it to auto generate custom mobile apps.

Here, I will explain in detail how I build new chrome extensions in seconds using Golang templates.

There is a detailed description on how to build a chrome extension. Once you have a skeletal chrome extension, it is easy to duplicate and create multiple custom extensions if needed or even allow to build Firefox extension using the same set of extension files.

All you need is a set of template files and your config json values.

E.g. The chrome extension manifest.json.template file will look like this with template fields:

 "name": "{{.Name}}",
 "version": "{{.Version}}",
 "manifest_version": 2,
 "default_locale": "en",
 "description": "{{.Description}}",  
 "background": {  "page": "background.html"    },  
 "browser_action": {      "default_title": "{{.Tooltip}}",   
 "default_icon": "icon.png"          },
"icons": { "16": "icon16.png",
            "48": "icon48.png",        
            "128": "icon128.png"     
"homepage_url": "{{.Protocol}}://{{.Domain}}",  
"permissions": [    "tabs",      

Similarly we write templates file for all the extension files like popup.js etc.

To build a basic chrome extension, I define the following in a global.json file

 "production": "true",
 "author": "Maria De Souza",
 "author_home": "https://mariadesouza.com/",
 "protocol": "https",
 "version" : "",
 "domain" : "www.mysite.com",
 "name" : "testExtension",
 "description" : "This is a test extension",
 "tooltip":"Click here to view settings",
 "title" : "Test extension",

These settings can be overridden by a product specific json file:

"title" : "My cool new extension",
"version" : "",

The product.json can be a subset of the original config.

Now we can get to fun part, building a script to generate the extensions. We first define a struct to unmarshal our config json and use it in our build script.

type Config struct { 
Production  string `json:"production,omitempty"` 
Author      string `json:"author,omitempty"` 
AuthorHome  string `json:"author-home,omitempty"` 
Version     string `json:"version,omitempty"` 
Domain      string `json:"domain,omitempty"` 
Protocol    string `json:"protocol,omitempty"` 
Name        string `json:"name,omitempty"` 
Description string `json:"description,omitempty"` 
Tooltip     string `json:"tooltip,omitempty"` 
Title       string `json:"title,omitempty"` 
Browser     string `json:"browser,omitempty"` 
ProductDir  string `json:"product_dir,omitempty"` 
HomePage    string `json:"home_page,omitempty"` 
UpdateURL   string `json:"update-url,omitempty"`

Start by unmarshalling the global file in a struct value as below. I have left out error handling to reduce noise. We then unmarshal the custom product values.

var globalConfig Config
configFile, _ := ioutil.ReadFile("global.json") 
json.Unmarshal(configFile, &globalConfig)
var productConfig Config
productconfigFile,_ := ioutil.ReadFile("product.json") 
json.Unmarshal(productconfigFile, &productConfig)

Using reflect, I override the custom product values:

func mergeWithGlobal(destConfig, srcConfig *Config){
 st := reflect.TypeOf(*destConfig) 
 for i := 0; i < st.NumField(); i++ { 
  tag := strings.Split(field.Tag.Get("json"), ",") 
  v2 := reflect.ValueOf(destConfig).Elem().FieldByName(st.Field(i).Name) 
  if tag[0] != "" && v2.String() != "" { 
   v := reflect.ValueOf(srcConfig).Elem().FieldByName(st.Field(i).Name) 

Using the Config struct, I then populate the template files. To do this I read all files with extension .template in the source diectory, execute the template using the populated Config struct and save the result in the destination directory.

func populateTemplateFiles(source, destination string, globalConfig *Config) error { 
 templatefiles, _ := ioutil.ReadDir(source) 
 re := regexp.MustCompile(`(.*)\.template$`) 
 os.MkdirAll(destination, 0755) 
 for _, file := range templatefiles { 
  if re.MatchString(file.Name() ) == true { 
   buf, _ := ioutil.ReadFile(filepath.Join(source,file.Name())) 
   tmpl, _ := template.New("extensions").Parse(string(buf)) 
   targetfilename := strings.Split(file.Name(), ".template") 
   targetfile := filepath.Join(destination, targetfilename[0] ) 
   f, _ := os.OpenFile(targetfile, os.O_WRONLY|os.O_CREATE, 0755) 
   w := bufio.NewWriter(f) 
   tmpl.Execute(w, globalConfig) 
return nil

I also save customized images in the product directory. This way in the build script we can copy custom images in the destination directory. We can then upload a zipped version to the chrome webstore.



Launch a Golang web server using Docker

If you want to create a web server using Go the simplest way to deploy is using Docker.  Golang code is compiled to a binary and does not need a special environment to run.

Here is the simplest web server code in Go to get started. Save this as webserver.go

package main
import ( 
func main() { 
     http.HandleFunc("/", indexHandlerHelloWorld) 
     log.Fatal(http.ListenAndServe(":8080", nil))

func indexHandlerHelloWorld(w http.ResponseWriter, r *http.Request) { 
     fmt.Fprintf(w, "Hello world, I'm running search on %s with an %s CPU ", runtime.GOOS, runtime.GOARCH)

We can use the simplest docker image scratch and add a directive to copy the binary to the server. Save as Dockerfile.

FROM scratch
MAINTAINER Maria De Souza <maria.g.desouza@gmail.com>

ADD go-webserver go-webserver
ENTRYPOINT ["/go-webserver"]

We setup a start shell script to build and kick up the docker


GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -a -o go-webserver webserver.go || exit 1
if [ "$(docker ps -q -f name=go-web-server)" ]; then
 docker stop $(docker ps -a -q --filter name=go-web-server --format="{{.ID}}")
docker build -t go-web-server . || exit 1
docker run -p 8000:8080 go-web-server || exit 1

You can use the -h option to add the hostname to the webserver

Voila! Now your webserver is running. Navigate to http://localhost:8000/ to test.

Known Issue

If you make any SSL requests from your webserver, you will see the following error when running the webserver using Docker:

x509: failed to load system roots and no roots provided

This is because the /etc/ssl/certs/ca-certificates.crt, is missing from the scratch image that is read by the golang tls package.

To avoid this copy the cert to your docker image from your local system. I normally add this to my bash script that will copy it based on OS.

if [ ! -e ca-certificates.crt ]; then
 if [[ $(uname) = "Darwin" ]]; then
 cp /usr/local/etc/openssl/cert.pem ca-certificates.crt || exit 1
 cp /etc/ssl/certs/ca-certificates.crt ca-certificates.crt || exit 1

This is what your Dockerfile should then look like:

FROM scratch 
MAINTAINER Maria De Souza <maria.g.desouza@gmail.com> 

COPY ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
ADD go-webserver go-webserver 
ENTRYPOINT ["/go-webserver"] 





String manipulation in Go

Basic Operations

  • Get char array of a String

greeting := "Comment allez-vous"
greeingCharacterArr := []rune(greeting)
  • Get a char at the specific index

fmt.Printf("%c", greeting[5])

The result would be  “n“. However, without the character formatter, the result would be the Unicode decimal code 110.

  • Get string length

  • Substrings
func substring(s string, beginIndex int) string {
 return s[beginIndex:]

func substring2(s string, beginIndex int, endIndex int) string {
 return s[beginIndex:endIndex]
  • String to Integer
testN,_ := strconv.Atoi("1234") 
testN += 1 

strconv package

Above examples : https://play.golang.org/p/kds_Lu9HyTJ

Sorting and Searching Strings in Golang

The sort package in Go has some very useful methods for sorting and searching slices and strings.

The sort.StringSlice attaches the methods of Interface to []string, sorting in increasing order. https://golang.org/pkg/sort/#StringSlice


package main
import ( "fmt" "sort")
func main() { 
    countries := sort.StringSlice{"USA", "India", "South africa", "Libya"}
    n := countries.Search( "USA")
    fmt.Println(""Result: ", n, countries[n])

Result: 3 USA

Sorting Characters in a string

To sort characters in a string, we use the fact that strings in Go can be represented as a slice of runes. We could extend the generic sort function in Go to sort the slice of runes. To use the generic sort, we need to implement sort.Interface – Len, Swap, Less.

type sortRunes []rune

func (s sortRunes) Less(i, j int) bool {
    return s[i] < s[j]

func (s sortRunes) Len() int{
   return len(s)
func (s sortRunes) Swap(i, j int) {
   s[i], s[j] = s[j], s[i]

str1 := "sortthisstring"    
s := []rune(str1)

See the full working example here:


Note the sort package comments, Sort makes one call to data.Len to determine n, and O(n*log(n)) calls to   data.Less and data.Swap. 


Embedding in Go

Go supports defining is-a relationships using an embedded type.

Fields and methods in a struct have a has-a relationship

E.g.  Person has-a name and email.

type Person struct {
	Name   string
	Email  string

Embedding to define an is-a relationships  is declared with a type but no explicit field name. The unqualified type name acts as the field name.

type Employee struct { 
      EmployeeID string 

This way the Person struct can be accessed using the type name:

accountant := new(Employee)

We can also invoke any Person methods directly on the Employee object

accountant := new(Employee)

We cannot embed a slice or a map

type Employee struct { 
 EmployeeID string 

This will give  a syntax error: unexpected [, expecting field name or embedded type

A workaround will be defining a type and then embedding

type SpecialPeople []Person

type Employee struct { 
 EmployeeID string 

The spec in Go describes embedding as below:

EmbeddedField = [ "*" ] TypeName .

Embed by-pointer

The advantage of embedding by reference is that  you are embedding all the functionality of a type without needing to know when it is instantiated. The major application of this would be to have thousands of instances sharing a single underlying data structure. This can significantly reduce memory consumption.

type Image struct {
     data [5][5]

type Block struct {
   show bool

Because, the unqualified type name acts as the field name for an embedded struct, we can’t have an embedded struct and its pointer in the same struct

struct {
	T     // conflicts with embedded field *T and *P.T
	*T    // conflicts with embedded field T and *P.T
	*P.T  // conflicts with embedded field T and *T

Promotion of fields or methods

All field or method calls for embedded type objects are resolved at compile-time without the use of a virtual table. A field or method of an embedded field in a struct is called promoted.

type Person struct { 
      Name   string 
      Email  string 
func (p *Person) FreeGift() int64{ 
     return 100 

type Employee struct {       
     EmployeeID string  

In this example person.FreeGift()  and employee.FreeGift() is the same so the method is promoted. Promotion  occurs only at the first level.


However, if we have a method with the same name defined on the parent struct as below, then employee.FreeGift() will invoke the method on the Employee struct

func (e *Employee) FreeGift() int64{ 
      return 200

In the above example, calling person.FreeGift() and employee.Person.FreeGift() will invoke the method on the Person struct


One very important design aspect to remember is, if you plan to use an embedded struct and the embedded type has non-exported fields or methods,  those are completely inaccessible to you in a separate package.

Embedding Interfaces

Embedding an interface will add all (exported and non-exported) methods of the embedded interface to the enclosing interface.

type ImageWriter interface { 
      Read(b Buffer) bool 
      Write(b Buffer) bool 
type ImageFile interface { 
     ImageWriter  // same as adding the methods of ImageWriter 




Packages on the Go

Package management with Go is a very talked about issue. Unfortunately, go get does not support functionality to fetch specific tags or versions. It gets the package from the HEAD in git.

Recently we had a situation where a developer on our team used a package and then it got obsoleted. The package developer tagged the release before making breaking changes and the sources were still available but we had to do a git checkout and run it.

That is when I looked into package management. Go 1.5 introduced the “vendor” directory as an experiment and made it official  in Go 1.6

If you use third party packages in your product, copy it to the vendor directory and go searches for dependencies there.


package main 

import (



 |     |--mariadesouza
 |     |    |-- sftphelper
 |     |    |     |-- LICENSE
 |     |    |     |-- README.md 
 |     |    |     |--sftphelper.go

Pass by value or reference

The official Go site FAQ states,  “As in all languages in the C family, everything in Go is passed by value”. This is because the function gets a copy of everything that is passed in.

Is there such thing as pass by reference in Go?

There are different views  as to what is exactly pass by reference to Go.  Some strongly maintain there is no such thing as pass by reference. In C++ terms, the actual meaning of pass by reference is you pass a reference or a pointer to the actual data structure rather the data itself. The function then can modify the value of the argument using that reference.

In Go when I pass a pointer to a struct for example, whether its a copy to the pointer or not, I am not passing the struct itself but a pointer or a reference to it.  I can modify the actual struct using the pointer. In my view, that fits the definition of pass by reference.

When to pass a pointer?

We don’t need to pass pointers to map and slices  as they are already descriptors that contain pointers to the actual map or slice data.

Compelling arguments to use pointer receiver and pass by reference:

  • You want to modify the receiver. With value receivers you can’t modify the struct itself
  • Its is a big struct. It will cost to deep copy the struct.

When you pass a slice to a function, since it is a pointer to an array you just get a copy of the slice structure. It will still point to the same underlying array segment. So any modifications made to the slice within the function will be seen outside.


However, if you append an element, remember that a new slice is created and elements are copied over so you will lose the elements if this happens within a function so you must return a slice. E.g. append from the stdlib returns a new slice.

Whatever it is you choose to use, stay consistent. Coming from a C++ background, I always thought pass by reference is cheaper than passing by value. But apparently it is not always the case in Go.

Interesting Reads: