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Golang 2

Functions, Methods, and Interfaces in Go course notes by Oliver Frolovs, 2020.

Did this course in a day, speed-running videos at 1.25 to 1.5 speed. A lot of territory is familiar from general knowledge of programming. The only part which required thoughtful reading/listening was interfaces as they are quite peculiarly designed in Go. But I like them)

Minor complaints re course organisation:

Using function in Go

A function is a set of instructions with a name. The name is, actually, optional.

All programs in Go have to have the main() function. When the program is run, it is automatically invoked. All other functions have to be invoked manually.

func main() {
    // ...

Simplest function declaration

func HelloGruffalo() {

Why use functions?

Function parameters

Parameters provide a way to pass the data into the function to operate on.

In this example, x and y are parameters.

func Sum(x int, y int) {
    return x + y

And here, 42 and z are arguments.

Sum(42, z)

Function without any parameters still requires the parentheses ().

func HelloGruffalo() {
    fmt.Println("Hello, Gruffalo!")

Listing arguments of the same type.

func Sum(x, y int) {
    // ...

Function return value(s)

A function can return a value as a result.

The type of the return value is noted after the parameters in function declaration.

func Sum(x, y int) int {
    retunr x + y

Once the function is defined, it can be called.

Calling the Sum function:

z := Sum(1, 2) // z will be assigned the value of 3

The function can be executed, without the return value being assigned to anything. This is done for the side effects of the function, assuming the function has them defined.

Calling the function with no side effects and ignoring the return value:

Sum(1, 2) // the return value is quietly discarded

Declaring the function with side-effects – printing to the console:

func HelloGruffalo() {
    fmt.Println("Hello, Gruffalo!")

Calling the function for its side-effects outputs Hello, Gruffalo! to the console but does not return any value.


To conclude, nothing new, this is all familiar territory!

Multiple return values

Like Python, in Go functions can return multiple values.

// Define: return two next Fibonacci values for two given Fibonacci values
func fib2(a, b int) (int, int) {
    return a+b, b+(a+b)
// Call: a is assigned 2, and b is assigned 3
a, b := fib2(1, 2) 

FIXME this is an awful maths example, wtf Oliver

Call by value and call by reference

I am aware that there is some controversy about the meaning of “passing by pointer” vs “passing by reference” in the Go community. I don’t want to be part of the debate, so please don’t contact me about it.

Call by value and call by reference describe two different ways in which the arguments are bound to parameters during the function call.

Call by value

Means the argument values are copied into the parameters. Inside the functions, modifying the values of the parameters have no effect on the environment outside the function.

Call by reference

Is not built into the Go language. But it can be done manually by passing a pointer as an argument into the function. This signals that the programmer wants the original data to be altered by the function.

Then, called function has direct access to caller variable in memory.

package main

import "fmt"

// s is a pointer to an integer, where the sum is stored
func Sum(x, y int, s *int) {
    *s = x+y

func main() {
    // Define three integers. The third one is auto-initialised to zero.
    x := 1
    y := 2
    var z int

    // Call a function which adds the values of its first two arguments,
    // and stores the result in the third argument which is passed by reference.
    Sum(x, y, &z)

    // Inspect the result

The (+) and (-) of call by reference are the opposite of those by call by value, haha.

Passing arrays and slices

I am aware that there is some controversy about the meaning of “passing by pointer” vs “passing by reference” in the Go community. I don’t want to be part of the debate, so please don’t contact me about it.

Arguments are copied, even arrays.

In the following example, the whole array is copied, even if only one element is used inside the function, and the array is not even altered.

package main
import "fmt"

func ShowFirst(x [3]int) {

func main() {
    x := [...]int {1,2,3}

Altering the array does not change anything outside the function.

package main
import "fmt"

func ShowFirst(x [3]int) {
    x[0] = 121           // altering the argument has no effect on original array

func main() {
    x := [...]int {1,2,3}


Making x “global” (FIXME is it in file scope?) doesn’t change the situation.

package main
import "fmt"

var x[3]int = [...]int {1,2,3}

func ShowFirst(x [3]int) {
    x[0] = 121

func main() {

Passing the array by reference changes that.

package main
import "fmt"

// The function parameter now a pointer to a three element integer array.
func ShowFirst(x *[3]int) { 
    fmt.Println( (*x)[0] )         // the parens () are necessary
    (*x)[0] = 121                  // the parens () are necessary

func main() {
    x := [...]int {1,2,3}


This way of passing arrays by reference is doable, but messy and isn’t ideomatic in Go. Instead, slices are used. In general, slices are used in place of arrays in Go.

Pass slices instead!

A slice is like a window on array. Slices contain a pointer to the array.

Passing a slice copies the pointer. A slice is like a structure with three elements – the pointer, the length, and the capacity. By passing the slice, it is passed by value, but its value include the pointer, so the underlying array can be accessed!

Note, that when you decleare a slice you don’t have to specify the size. In fact, you can’t specify the size, you just give the square brackets.

package main
import "fmt"

// No array size in [] means it's a slice being passed as an argument, not array.
func IncFirst(sli []int) {
    sli[0] = sli[0] + 1

// The array size in [] means it's an array being passed by value
func ShowFirst(x [3]int) {

func main() {
    x := [3]int {1,5,6}



To reiterate, the only difference between passing in the array vs passing in the slice, is that when the slice is passed, no size has to be specified in the square brackets: xs []float or sli []int.

Well-written functions

Not specific to Go. The code is functions and data. Focus on understandability: when you are asked to find a feature, can you find it quickly? It’s a matter of organisation. Another aspect of understandability, is finding where the data is defined and used. Being able to trace through data is important.

A function crashes. Locally,

If the function is understandable and the data is traceable, it’s easier to identify and fix mistakes.

Global variables spoil this approach, so use them sparingly, if at all.

Guidelines for functions

First-class values

A concept from functional programming – functions can be treated like any other types:

In many ways, Go allows to treat the functions as any other data type.

Variables as functions

Declare a variable as a function.

package main
import "fmt"

// This function is defined statically. Its type is (int, int) -> int
func Sum(x, y int) int {
    return x + y

// Declare a function variable of the same type as the function above
var funcVar func(int, int) int

// Main is declared statically here, no surprises
func main() {
    // Assign a value to the function variable
    funcVar = Sum

    // Test the function by its new "name"

    // Can leave the type to be figured out by the compiler
    funcVarAsWell := Sum

    // Test the function by its new "name"

Functions as arguments

Functions can be passed to another function as an argument.

// f is a function of type (int) -> int
// v is an integer value
func apply(f func (int) int , v int) int {
    return f(v)

Anonymous functions

Don’t need a name to a function! The concept comes from “lambda calculus”. But I don’t care about it. It’s useful though. Using an example from the previous section, apply:

func main() {
    v := apply(func (x int) int { return x+1 }, 42)

The anonymous function definition also begins with func.

All in all, the syntax is very intuitive.

Functions as return values

Useful to make parameterisable functions. The “constructor” function get the values of parameters and returns another function set up using those parameters.

This works in the same way I’ve parametrised Python functions in my numerical simulations. No surprises.

TODO maybe set up an example from schientific Python sources that I have?

Every function has an environment, which includes all the names defined locally in the function. But also, this includes lexical scoping, that is all the names available in the block in which the function was defined.

Did someone just say closure? ))

var x int

// The function is defined in the scope which include the variable x (above)
func Boop(y int) {
    z := 1
    // The variable x is available inside this scope as well

FIXME he mentioned technicality about what is the scope and what is the environment, and said people often confuse the two; check back later…

The environment goes along with the function. This is important when functions get passed around as variables.

Now we geto to… closures!


A closure is a function plus its environment together. In Go, its actually implemented as a struct! When functions are passed/returned, their environment comes with them.

What implications does this have?

When a function is passed as an argument, it’s passed as a closure, that is together with its environment.

This may be important for figuring out where the variable values are coming from. Remember, that Go is lexically scoped.

Variadic functions

One useful tool is to be able to pass a variable number of arguments into the function. Think fmt.Printf as an example.

Functions can take a variable number of arguments.

func Max(vs {
    maxV := -1 // FIXME valid integer, but cannot use nil here "untyped nil"
    for _, v := range vs {
        if v > maxV {
            maxV = v

    return maxV
vsli := []int {1,3,6,9}
Max(vsli...)              // Note the ellipsis in the suffix!

Thus, in Go one can pass a slice to a variadic function and it will be “unpacked” into variable arguments.

Deferred function calls

func main() {
    defer fmt.Println("Bye!")

    fmt.Println("Hello, Gruffalo!")

The defer-red call is not executed until main is complete.

Arguments of a deferred call are evaluated immediately!

Revision example

// The function fA is (statically) defined here. 
// It takes no arguments. 
// It returns a function.
// The returned function takes no arguments.
// The returned function returns an integer.
// Note, that fA forms a closure. 
// That is, the returned function keeps the environment in which it was defined.
// That environment includes the local (to fA) variable i.
// This variable is inaccessible directly from outside the returned function.
func fA() func() int {
    i := 0
    return func() int {
        return i

Onto object-orientation in Golang now.

OOP in Golang

Go doesn’t have classes exactly. But it has something similar.

Traditionally, classes are a collection of data fields and functions that share a well-defined responsibility. A class is a template, it contains the data field, but not data. An object is an instance of a class that contains actual data. Blah blah blah, let’s leave all that to the purists.

Encapsulation is hiding the data and implementation details behind the interface. It is associated with abstraction in general. It helps with keeping the data (internal state) consistent.

So, Go doesn’t have a “class” keyword and all that. But… there are different ways of associating data with methods in Go!

Associating methods with data in Go

Receiver type

It’s done using a receiver type. A method (function) has a receiver type that it is associated with. Use dot notation to call the method.

The type has to be defined in the same package as the method that it is associated with. So, can’t “monkey-patch” the built-in types.

type MyInt int

func (mi MyInt) Double() int {
    return int(mi*2)

func main() {
    v := MyInt(3)

Hmm… now that’s new. So, as defined above, MyInt is the receiver type for the function Double(). By listing a receiver type in the function definition, we let the compiler know that when a function Double is invoked using dot notation and the … entity of type MyType, this is the right function to invoke.

Implicit method arguments

It looks as if Double() has no arguments, but there is an implicit argument. So, this is similar to Python’s self, I suppose.

Every time there is a receiver type specified in function definition, it creates an implicit parameter of that type, which does not have to be listed in parentheses () with other parameters. It is implied.

Using dot notation, object v is an implicit argument to the method Double. Like other arguments in Go, it is passed by value. So, a copy gets passed. Interesting.

Weird, but let’s hold onto this notion. I hope the benefits of this approach will become obvious soon enough?

To build useful data types, it’s common to use struct-s as a receiver type.

type Point struct {
    x float64
    y float64

Structs with methods

Now, we can define a type using a struct, just like in the previous section. And we can use this type as a receiver type with methods to get something similar to classes in other languages.

Structs and methods together allow arbitrary data and functions to be composed.

// Using the Point struct defined in the previous section...

// Note that the receiver type is Point, 
// and the method gets an implicit parameter named p
func (p Point) DistToOrigin() {
    t := math.Pow(p.x, 2) + math.Pow(p.y, 2)
    return math.Sqrt(t)

// Example of using the method...
func main() {
    p1 := Point(3,4)

It looks a bit flimsy so far… but holding on to the idea for now…


Controlling access

Can define public functions to allow controlled access to hidden data.

package data
var x int = 100
func PrintX() {fmt.Println(x)}

To access it from main package:

package main
import "data"
func main() {

We can also define a function which would modify the private variable. We can do this with structures too!

Controlling access to structs

package data

type Point struct {
    x float64
    y float64

func (p *Point) InitMe(xn, yn float64) {
    p.x = xn
    p.y = yn

Here, using the function InitMe, we can modify x and y values of the struct, without having direct access to them.

Another example of altering the hidden members of the structure:

func (p *Point) Scale(v float64) {
    p.x = p.x * v
    p.y = p.y * v

func (p *Point) PrintMe() {
    fmt.Println(p.x, p.y)

Revision: Point (pointer to Point type) is a *receiver type for these functions.

The methods have names starting with a capital letter so they are public.

FIXME is this a convention or is it actually enforced by the compiler?

Using the above examples from main:

package main
import "data"

// Access to hidden fields using only public methods.
func main() {
    var p data.Point
    p.InitMe(3, 4)

Hmm… how would you chain the method calls?!

FIXME yes, actually, how would you chain the method calls?

Limitations of methods in Go

In the following example, a copy of p1 is passed into OffsetX() method as an explicit parameter. Thus, the method cannot change the value of x inside p1!

func main() {
    p1 := Point(3,4)
    p1.OffsetX(5)     // cannot alter p1

We do what we did before. Need to call by reference. Instead of passing in the value of a type, pass a pointer.

Pointer receivers

When declaring a function, set the receiver type to a pointer. This achieves call by reference.

// Note the star which tells it's passed by reference
func (p *Point) OffsetX(v float 64) {
    p.x = p.x + v  // FIXME how come we don't need to dereference?

There is no need to dereference the pointer inside the method!?

This is because the technique is common enough for the compiler to do that for you. The compiler dereferences it automatically. Likewise, there is no need to reference manually either:

func main() {
    p := Point(3,4)
    p.OffsetX(5)      // but OffsetX expects pointer receiver, so (&p)

FIXME so is this basically syntactic sugar?

Using pointer receivers

When using pointer receivers, it’s a good programming practice to:

Revision point


Is an ability of an “object” to have many different forms depending on the context.


FIXME was Perl’s scalar and vector context for a variable… polymorphism?

Go does not have inheritance! Which I suppose is fine, because composition is so much more useful than inheritance.

Overriding is another concept traditionally associated with OOP.

In traditional OOP, inheritance and overriding (methods) is used to implement the polymorphism. Remember superclasses? Bleurgh!

The methods would have same signatures (in terms of superclasses) and this is how polymorphism would be implemented. Traditionally.

Enough, please. No more “traditional” OOP for me.

Interfaces in Go

Since Go does not have inheritance (thanks!), it uses interfaces to implement polymorphism. I am familiar with the concept from Objective-C and I like it. I agree with I an that this is a clean approach.

Example: Shape2D interface, providing Area() and Perimeter() methods.

How to define an interface in go

An interface is defined… using the interface keyword.

type Shape2D interface {
    Area() float64
    Perimeter() float64

// The Triangle data type SATISFIES the Shape2D interface.
type Triangle {...}
func (t Triangle) Area() float64 {...}
func (t Triangle) Perimeter() float64 {...}

NO NEED TO STATE IT XPLICITLY, unlike in other languages.

Interface types and concrete types

The interface types and concrete types are fundamentally different.

Concrete types

Interface types

When you create the interface, you declare an interface type. But then you can make values of that type! You can declare variables of that type. You can pass and return the values of that type. In the example given in previous sections, a variable of type Shape2D can be declared.

Now, the interface value, that is the value of the interface type has two components:

In the Shape2D example, the dynamic type could be a Triangle or a Rectangle. The dynamic value would be a value of type Triangle, instantiated in a program.

Subtle, but understandable so far…

So, and interface value is actually a pair, the dynamic type together with the dynamic value.

TODO read this again later haha

Example of the shenanigans above

// An interface type is defined. It contains a single method (void) -> (void).
// It's a very simple interface, indeed.
type Speaker interface {

// A (concrete) type Dog, representing... a dog is defined.
// It contains only a single field of type string, which is the dog's name.
// It's a very simple data type, indeed.
type Dog struct {
    name string

// A method (!!!) with the "receiving type" Dog is defined.
// The method name and signature match those declared in the Speaker interface.
// Thus, this definition makes the Dog data type SATISFY the Speaker interface.
func (d Dog) Speak() {

func main() {

    // A variable of the interface type is declared. It has no value assgned yet.
    // But it is going to have an value of interface type Speaker assigned later.
    var s1 Speaker

    // A variable of (concrete) type and a value of that type is assigned to it.
    var d1 Dog{"Brian"} 

    // Since the (concrete) type Dog implements the interface Speaker,
    // this assignment is legal. Now, the dynamic type of s1 is Dog, 
    // and the dynamic value is that value of (concrete) type Dog which has
    // the name "Brian".
    s1 = d1

    // So, s1 is a PAIR: a "dynamic type" together with a "dynamic value".

Now, the interface can have a nil dynamic value, meaning no dynamic value. It can have a dynamic type but not a dynamic value:

func main() {
    var s1 Speaker
    var d1 *Dog       // FIXME why a pointer though?

    // d1 has no concrete VALUE yet
    s1 = d1
    // s1 now has a dynamic type, but no dynamic VALUE

How do you like that, Elon Musk?

nil dynamic value

FIXME but to what end?!

FIXME I’m still puzzled as to why that is beneficial?

Yes, you can (or rather could) make the call, but why would you want this to be legal?

func (d *Dog) Speak() {
    if d == nil {
    } else {
func main() {
    var s1 Speaker
    var d1 *Dog

    s1 = d1

FIXME is this some kind of “superclass” “overrideable” behaviour example?

For now, the point is that

It is legal to have an interface with a dynamic type but without a dynamic value.

There is also another concept, nil dynamic type, which describes an interface with a… nil dynamic type. And that’s a different situation – then you cannot call the methods on that interface.

Because without the dynamic type you don’t know which method you are referring to!

var s1 Speaker
// Cannot call a method s1.Speak(), 
// that would result in a runtime error

Hmm… ok, I understand how it works, but still don’t get why you’d want that “feature” to be legal?

TODO check back later in the course

Why interfaces?

This is a common way to use an interface. Hides the details between the types, emphasises important common features between the types.

Empty interface

An empty interface specifies no methods.

// Here, val can be ANY type.
func PrintMe(val interface{}) {

This is how you specify an empty interface: interface{}.

Type assertions

A lot of use for interfaces is to conceal the difference between the types. But sometimes one needs to differentiate based on a type. In some cases, concrete types matter. In those cases, you have to expose type differences.

How do you peel off the interface to look at the concrete types?

Example: graphics interface, for DrawShape(s Shape2D) any shape will do, but for implementation of the interface, I might want to know the concrete type of s, perhaps to map them to native graphics API.

In this case, type assertions can be used to determine and extract the underlying concrete type:

func DrawShape(s Shape2D) bool {

    rect, ok := s.(Rectangle)         // Concrete type in parentheses ()
    if ok {

    tri, ok := s.(Triangle)           // Concrete type in parentheses ()
    if ok {

Because there can be many different types to disambiguate, there is a switch construct which is meant precisely for that!

Type switch

Switch statement used with a type assertion.

func DrawShape(s Shape2D) bool {
    switch sh := s.(type) {
        case Rectangle:
        case Triangle:
            fmt.Fprintf(os.Stderr, "unknown type\n")

The type assertion part here is s.(type). The generic word type is used. Then, sh will be whatever the concrete type s represents.

Handling errors

The error interface.

Many Go programs (packages) return error interface objects to indicate errors.

type error interface {
    Error() string

Wow so sophistication much error handling wow haha

Lots of Go functions return error as a second return value.

f, err := os.Open("/etc/issue")
if err != nil {

The fmt package calls error() method to generate string to print. Handy!

The pattern is:

  1. check whether error is nil
  2. if it is not nil, handle it

Revision points