16/12/2020

Golang

Go or Golang is a programming language that designed at Google.

The language can be described by these following items:

it’s compiled language
similar to C language by syntax
the language has:
Official website

cli

**go run <module> - run the file/module as a program
go test - run tests
go mod init <module> - initialise a module

godoc

It provides an ability to open Go documentation locally and even offline

Install

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go install golang.org/x/tools/cmd/godoc@latest

Run

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godoc -http :8000

“Hello, World!” Example

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package main

import "fmt"

func main() {
	fmt.Println("Hello, World!")
}

Go has standard library that is built-in modules of functions
for example: fmt is a built-in package
Println(value) is a method that prints value in a console/terminal and also prints break line in the end
func main() is a function
- it is a main function of the module and it will be executed automatically when you run command go run hello.go

Variables

Declaring

Example 1:

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func main() {
	var greetingText string
	greetingText = "Hello, World!"

	fmt.Println(greetingText)
}

Example 2:

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func main() {
	var greetingText string = "Hello, World!"

	fmt.Println(greetingText)
}

Example 3 (with type inference):

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func main() {
	greetingText := "Hello, World!"

	fmt.Println(greetingText)
}

type inference is when you declare variable without specifying a type
the variables type is inferred by the value

Types: string

Declaring a string variable

Example 1:

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func main() {
	var greetingText string
	greetingText = "Hello, World!"

	fmt.Println(greetingText)
}

Example 2:

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func main() {
	var greetingText string = "Hello, World!"

	fmt.Println(greetingText)
}

Example 3 (with type inference):

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func main() {
	greetingText := "Hello, World!"

	fmt.Println(greetingText)
}

concatenating

An example:

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func main() {
	firstName := "Max"
	lastName := "K"

	fullName := firstName + " " + lastName

	fmt.Println(fullName) // Max K
}

add a number to a string

JavaScript devs be like:

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func main() {
	numb := "1"

	fmt.Println(numb + 7) // invalid operation: numb + 7 (mismatched types string and untyped int)
}

Types: int

Declaring an int (number) variable

Example 1:

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func main() {
	var luckyNumber int
	luckyNumber = 17

	fmt.Println(luckyNumber)
}

Example 2:

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func main() {
	var luckyNumber int = 17

	fmt.Println(luckyNumber)
}

Example 3 (with type inference)

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func main() {
	luckyNumber := 17

	fmt.Println(luckyNumber)
}

Interesting stuff

if I declare variable like this:

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func main() {
	var luckyNumber int

	fmt.Println(luckyNumber)
}

it will print the result: 0

Operations: add (+)

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func main() {
	luckyNumber := 17
	anotherOneLuckyNumber := luckyNumber + 8

	fmt.Println(luckyNumber) // 17
	fmt.Println(anotherOneLuckyNumber) // 25
}

Reassigning a variable

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func main() {
	luckyNumber := 17
	anotherOneLuckyNumber := luckyNumber + 8

	fmt.Println(luckyNumber)           // 17
	fmt.Println(anotherOneLuckyNumber) // 25

	// reassigning a variable
	anotherOneLuckyNumber = 33

	fmt.Println(anotherOneLuckyNumber) // 33
}

Operations: multiply (*)

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func main() {
	luckyNumber := 17
	anotherOneLuckyNumber := luckyNumber * 8

	fmt.Println(luckyNumber)           // 17
	fmt.Println(anotherOneLuckyNumber) // 136
}

Types: float64

An example:

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func main() {
	luckyNumber := 17
	anotherOneLuckyNumber := luckyNumber / 8

	fmt.Println(luckyNumber)           // 17
	fmt.Println(anotherOneLuckyNumber) // 3
}

anotherOneLuckyNumber will still be integer (int), no numbers after decimal point
we should use float64 type to have full number with part after point

Using float64

We need to declare that variable anotherOneLuckyNumber should be with type float not int:

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func main() {
	luckyNumber := 17

	var anotherOneLuckyNumber float64 = luckyNumber / 8 // cannot use luckyNumber / 8 (value of type int) as float64 value in variable

	fmt.Println(luckyNumber)
	fmt.Println(anotherOneLuckyNumber)
}

there will be an error
we need to convert luckyNumber into float64 but only in operation of dividing with 8 - for this we can use a command (function) float64

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func main() {
	luckyNumber := 17

	var anotherOneLuckyNumber float64 = float64(luckyNumber) / 8

	fmt.Println(luckyNumber)           // 17
	fmt.Println(anotherOneLuckyNumber) // 2.125
}

Types: float32

The difference between types float32 and float64 is how many numbers will you have after the point sign.

For example, the result of operation will be:

17 / 7 =
- float32
  - 2.4285715
- float64
  - 2.4285714285714284

An example with float32:

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func main() {
	luckyNumber := 17

	var anotherOneLuckyNumber float32 = float32(luckyNumber) / 7

	fmt.Println(anotherOneLuckyNumber) // 2.4285715
}

An example with float64:

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func main() {
	luckyNumber := 17

	var anotherOneLuckyNumber float64 = float64(luckyNumber) / 7

	fmt.Println(anotherOneLuckyNumber) // 2.4285714285714284
}

Types: rune

single unicode character

An example:

notice that there is a single quotes ('’) instead of double quotes ("") because double quotes are used for string type

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func main() {
	var someRune rune = '👍'

	fmt.Println(someRune) // 128077
}

128077 is a special identifier for the sign ‘👍’

to print the sign 👍 as it is instead of integer we need to convert it to a string by function string()

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func main() {
	var someRune rune = '👍'

	fmt.Println(string(someRune)) // 👍
}

Types: byte

single byte (ASCII character)

An example:

notice that there is a single quotes ('’) instead of double quotes ("") because double quotes are used for string type

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func main() {
	var someByte byte = 'c'

	fmt.Println(someByte) // 99
}

To print a ‘c’ value as it is we need to convert value to a string by using function string()

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func main() {
	var someByte byte = 'c'

	fmt.Println(string(someByte)) // c
}

Types: byte vs rune

A type byte is a smallest type in Go so you can initialise a variable only with one character (letter or number) but not with something like emoji or special character/symbol like ‘€’

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var someByte byte = '👍' // cannot use '👍' (untyped rune constant 128077) as byte value in variable declaration (overflows)

so you need to use type rune for such characters/symbols

Formatting strings (fmt)

(package) fmt
- print the argument str in command line (terminal)
  - .Print(str) - prints *str as it is
  - .Printf(formatStr, args…)
  - .Println(str) - as .Print(str) but adds “\n” in the end of string
- return a new formatted string
  - .Sprint(str) - returns a new formatted string, uses default formats
  - .Sprintf(formatStr, args…)
  - .Sprintln(str) - as .Sprint(str) but adds “\n” in the end of string
- write a new formatted string to specified writer (for example: a file or command-line/terminal)
  - .Fprint(writer, str) - writes a new formatted string to a writer (file, command-line, etc)
  - .Fprintf(writer, formatStr, args…)
  - .Fprintln(writer, str) - as .Fprint(str) but adds “\n” in the end of string

Examples

Printf(formatStr, args…)

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func main() {
	nickname := "byteski"
	level := 77

	fmt.Printf("Greetings, %v (%v lvl)!", nickname, level) // Greetings, byteski (77 lvl)!
}

Sprintf(formatStr, args…)

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func main() {
	nickname := "byteski"
	level := 77
	greeting := fmt.Sprintf("Greetings, %v (%v lvl)!", nickname, level)

	fmt.Printf(greeting) // Greetings, byteski (77 lvl)!
}

Fprintf(writer, formatStr, args…)

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import (
	"fmt"
	"os"
)

func main() {
	nickname := "byteski"
	level := 77

	fmt.Fprintf(os.Stdout, "Greetings, %v (%v lvl)!", nickname, level) // Greetings, byteski (77 lvl)!
}

Modules vs Packages

Go code is organised in modules and packages
- modules
  - are bigger than packages
  - has unique identifier (name)
  - can be distributed (as a library)
  - every Go project is a module
  - projects can use multiple modules
  - created and managed by commands go mod <smth> and file go.mod
- packages
  - modules contains packages (at least one package - for example main)
  - package can have multiple files
  - packages are stored in subfolders in a module
  - can be imported

Pascal Case vs Camel Case naming of variables

pascal case
- var Greeting string = "Hello there!"
  - in Go it means that variable will be used also outside of the package where was declared
camel case
- var greeting string = "Hello there!"
  - in Go it means that variable will be used only inside the same package as where it was declared

An example:

camel case

variables.go

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package greeting

var greetingText = "Hello there!"

hello.go

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package main

import (
	"fmt"
	"new-module/greeting"
)

func main() {
	// Error: greetingText not exported by package greeting
	fmt.Println(greeting.GreetingText)
}

pascal case

variables.go

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package greeting

var GreetingText = "Hello there!"

hello.go

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package main

import (
	"fmt"
	"new-module/greeting"
)

func main() {
	fmt.Println(greeting.GreetingText) // Hello there!
}

Constants

Constant is like a variable but it cannot be reassigned. Constant will have the same value.

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func main() {
	const COUNT = 10
	
	COUNT = 15 // Error: cannot assign to COUNT (untyped int constant 10)

	fmt.Println(COUNT)
}

Writing tests in Go

File should have name like “xxx_test.go”
The test function must start with word “Test”

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func TestHello(t *testing.T) {
}

The test function has one argument “t *testing.T”
Need to import “testing” to use argument “t *testing.T”

t.Errorf()

t.Errorf() - prints message fail the test

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func TestHello(t *testing.T) {
	result := Hello()
	expectedValue := "Hello, World!2"

	if result != expectedValue {
		t.Errorf("expected `%s` but got `%s`", expectedValue, result)
		/* will print:
		--- FAIL: TestHello (0.00s)
		hello_test.go:10: expected `Hello, World!2` but got `Hello, World!`
		*/
	}
}

t.Run()

it seems like an equivalent of “describe” or “it” in [[JestJS]]

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func TestHello(t *testing.T) {
	t.Run("should return `Hello, <name>!`", func(t *testing.T) {
		result := Hello("Max")
		expectedValue := "Hello, Max!"

		if result != expectedValue {
			t.Errorf("expected `%s` but got `%s`", expectedValue, result)
		}
	})
}

t.Helper()

it declares that the method where it was called is a helper function
because of this the line number of the assertion (where helper function is called) will be printed out

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assert := func(t testing.TB, expectedValue string, resultValue string) {
	t.Helper()

	if resultValue != expectedValue {
		t.Errorf("expected `%s` but got `%s`", expectedValue, resultValue)
	}
}

// ...

t.Run("should return `Hello, <name>!`", func(t *testing.T) {
	result := Hello("Max")
	expectedValue := "Hello, Max!"

	assert(t, expectedValue, result)
})