lets-go-workshop

Let's Go workshop

Introduction

This workshop aims to give an introduction of the Go language by writing code and practising the language. Step by step more code will be introduced and the final product will be a tool that creates GIFs from an input text.

Tools needed for the workshop

• Go
  • Download at go.dev
  • Verify that it's working by running go version in the command line
• The IDE of your choice
  • Visual Studio Code with the Go extension
  • Goland (from JetBrains)
  • Vim plugin https://github.com/fatih/vim-go
• Git
  • To commit the different milestones
• pkg.go.dev
  • This is the link to the documentation of the various Go packages available
    • Inside or outside the standard library
  • Useful to learn about each symbol (types, functions, methods, and so on) and what they do for each package. It usually contains examples of usages of these symbols as well

Milestones

Here is the list of different milestones we are going to acheive during the workshop.

Hello, World! (milestone 0)

For the first step you are going to write the hello world, and this is also going to be the occasion to setup your Go project.

Steps

1. Create a folder, e.g. lets-go-workshop; This will contain the code to write for the workshop
2. Go inside the folder and run go mod init from the terminal
3. (optional) run git init if you want to store the code for the different milestones in a different commit
4. Open your IDE inside the workshop folder
5. Create a main.go file
6. Write the following code:

package main

import "fmt"

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

1. Run go run main.go

Explanations

The command go mod init initializes a project making it a Go module. A Go module represents a set of Go files that are supposed to be shipped altogether: e.g. an application or a library.

Every Go application starts at a main() function located in a main.go file.

Every Go file resides in a package, this information is declared as the first instruction of the file. The main function should reside in package main.

Packages can contain functions, constants, custom types and so on inside a specific namespace; to use a specific package it needs to be imported with the import keyword. An unused import results in a compilation error.

Any exported/public symbol inside a package can be invoked in the form of package name.symbol name. For example, in the call fmt.Println, fmt is the package name and Println is the symbol name.

In Go ; is not mandatory at the end of each statement.

Milestone 1

For this step you are going to improve on the hello world with some code that creates a file and writes the hello world on it. The goal is to see variable declaration and initialization, conditionals with the if statement, to start interacting with some packages the standard library and to introduce few Go specific features: the blank identifier, the multi-value returns, the error type and the defer keyword.

Steps for milestone 1

1. Create a file (use the os package), write the "hello, world!" string to the file (use the fmt package), close the file
2. if there are any errors during the file creation, log them and exit (use the log package)
3. Use the defer keyword to defer the execution of the Close() call
4. Use the command line arguments to take the text to print (use the flag package)
5. Run go run main.go "Hello, World!"

Notes for milestone 1

Use pkg.go.dev to search the content of the packages os, fmt, log and flag.

To declare and assign variables you can use two tools:

• The := operator (declare and assign)
  • a := 1
• The var keyword (declare), followed potentially by the = operator (assign)
  • var a int = 1

When the compiler can infer the type of a variable it can be omitted:

• var a int = 1 // => var a = 1
• a := 1
  • in this statement the type is already omitted

When the type name is present in a statement, it always goes after the name:

• var a int // 👍
• var int a // ❌ (compilation error)

You can declare and/or assign multiple variables, of the same or different type, at the same time:

• a, b, c := "hello", 1, "world"
• f, err := os.Create("myFile") // os.Create is a function that returns 2 values

If you don't need a specific value during an assignment you can ignore it using the blank identifier _. Any unused variables are considered complier errors.

• f, _ := os.Create("myFile") // following the previous example, I just need the value of f from the function

The if statement is similar to most languages, here are some differences:

• No () are required around the condition
• {} are always required around the code to execute if the condition is true, even if it fits in one line

The defer keyword instructs the function that follows to be called at the end of the function where it is deferred.

Generally, in a multi-value return the second argument or last argument is of type error which is an interface that declares one method: func Error() string

A value of a specific interface type can be compared only with another value of the same type or with nil which means: no value for that interface.

Milestone 2

In this milestone you'll create your first image (a PNG); in doing so you'll learn about creating your own package and function, invoke them and learn about for loops, types, zero values and the meaning of exported and unexported.

Steps for milestone 2

1. Create a new folder named myimage; in Go, this is a new package
2. Inside the myimage folder create a new file called myimage.go
3. Write a function func New(l, h int, c color.RGBA) *image.Paletted inside myimage.go that draws a rectangle of the specified size and color (use the image package, see functions Rect, NewPaletted and the Set method of the Paletted type)
4. Change the main.go to first call the function that draws the rectangle then use png.Encode to save the rectangle image in a PNG file
5. Run go run main.go with the appropriate parameter if you read them from the console

As a bonus you can add more input flags for the properties of the rectangle (length, height and color) and the output file name.

Notes for milestone 2

A package name and its containing folder do not have to have matching names, but it's good practice to do so.

There are no specific keywords to define the scope of a Go symbol:

• Name is UPPERCASE
  • Symbol is exported, which means available to all packages (public-level scope)
• Name is lowercase
  • Symbol is unexported, which means available to the package where it is located (package-level scope)

There three kind of types in Go:

1. Primitives
2. Internal types
3. Custom types

Primitives in Go are bool (true or false), the literal type string and numeric types rune, byte (rune and byte can also represent literals), uint, uint8, uint16, uint32, uint64, int, int8, int16, int32, int64, float32, float64, complex64, complex128).

Internal types are array, function, slice, map, and channel.

Custom types are all Go types declared with the type and/or struct keywords.

For example:

type Age int

type Person struct {
  name string
  age  Age
}

Every variable is initialized in Go at the declaration time; if not explicitly initialized in the code, they will be initialized by Go with the zero value for the type of that variable.

Here is the zero value list:

var (
  b bool // false
  i int // 0 (same for all numeric types)
  s string // ""
  p Person // Person{}, all custom types are initialized with all fields set to their zero values
  v map[int]string // nil, also valid for all internal types
)

Looping in Go is similar to most languages with some differences:

• for is the only keyword used for loops
• No () are required around the loop instructions
• {} are always required around the code block to execute inside the loop, even if it fits in one line
• for condition {} replaces the while statement; there are no do/while statements in Go
• for true {} represents an infinite loop (to be interrupted with a break statement

Using flags in Go requires the usage of the flag package:

• first define all flags using the functions Int, String, Bool and so on, according to the type of the data you request in input
  • They all take flag name, default value and short description as inputs and return pointers of the same type as in the function name
  • They can be visualized when running go run main.go --help, without needing to define a flag for the help message
• then call the function flag.Parse() to parse all flags defined above

Pointers in Go are similar to C/C++. Use * declare or dereference them, use & to take the address of a value.

Milestone 3

In this milestone you'll write your first message on the previously created image; in doing so you'll learn interacting with external packages from the standard library and see how to import them in your project.

Steps for milestone 3

1. Write a function func Write(dst draw.Image, text string, c color.RGBA, fontPath string, fontSize float64) error inside myimage.go that draws a text on the rectangle created in milestone 2
2. Import the package github.com/golang/freetype inside myimage.go
3. In the terminal, at the root of the project run go mod tidy
4. Write the code to create and configure the freetype.Context object
5. Call the Write function inside the main function
6. Run go run main.go with the appropriate parameter if you read them from the console

As a bonus you can:

• keep adding input flags for the color of the string, the font file path and the font size.
• create a function that gives the appropriate size for the rectangle by taking into account the font size and the length of the text to draw

Notes for milestone 3

The difference between packages from the standard library and the external ones can be seen from its naming when getting or importing them. In fact, the main difference is that external packages include a complete path to external package containing:

• the registry: github, gitlab, bitbucket, and so on
• the package author: be it a username or a project group name
• the package name and path to subpackage if relevant
  • in local it will be the path to the package

the function Join of the strings package is a useful tool to join several strings, organized into a slice (array), in a single string.

Milestone 4

In this milestone you'll transform the previously created image into a blinking gif that alternates between two frames that switch their colors; by working on this milestone you'll learn to work with slices (mostly known as resizable arrays), the switch and for range statements, the make and append keywords and to interact with constants and interfaces.

Steps for milestone 4

1. Create a new folder in the project root named mygif
2. Inside the mygif folder create a new file called mygif.go
3. Write a function func New(text string, c1, c2 color.RGBA, fontPath string, fontSize float64) ([]*image.Paletted, error) inside mygif.go that creates a slice containing the frames to be stored in the gif and an error if something goes wrong (use the myimage package created in the previous milestones)
4. Write another function func Save(w io.Writer, frames []*image.Paletted, delay int) error inside mygif.go that stores the frames created in the New function into a gif file (use the gif package and don't forget to set the Delay field for the gif to store)
5. Call the New and Save functions inside the main function
6. Run go run main.go with the appropriate parameter if you read them from the console

As a bonus you can:

• derive the name of the output file from the input text
• create a custom type GIF inside mygif.go to hold the frames and the delay information and make the New function return GIF instead of []*image.Paletted, transform the Save function into a method of the GIF structure and adapt the calls in the main function to accomodate these changes

Notes for milestone 4

Constants in Go are declared with the const keyword followed by name = value. The value can only be hardcoded in constants or be taken from other constants.

Slices and arrays in Go can be associated with arrays from other programming languages. The main differences are:

• Arrays have fixed length
  • var array [2]int // array of int with length 2
• Slice have variable length
  • var elems []int // slice of int with length 0
• Can initialize a slice (not an array) with the make built-in function
  • elems = make([]int, 2) // {0, 0}
• Can add elements to the slice with append built-in function
  • elems = append(elems, 4) // {0, 0, 4}
• Arrays and slices can sliced to take a subset of their content

numbers := []int{0, 1, 2, 3}
numbers[1:3] // {1,2}
numbers[1:]  // {1,2,3}
numbers[:2]  // {0,1}

The switch statement is similar to most languages, here are some notable features:

• No () are required around the condition or value to switch on
• {} are always required around the code block to execute inside the switch
• No break statement is necessary after each case as the execution will exit the switch statement after the selected case or default statement is completed
• Fallthrough is the keyword to use before the case statement completes if you want to keep checking other cases
• A plain switch, e.g. switch {} is generally used instead of an if/else block

Continuing on the loops in Go, the for range statement is used on symbols that can be looped upon, the syntax of the for range is the following:

for i, e := range numbers {
  fmt.Println(i, e)
}

When looping on:

• strings, i is the index and e is the character at position i
• arrays and structs, i is the index and e is the value (e.g. the value of number[i])
• maps, i is the key of the map and e is the value corresponding to that key

Interfaces in Go are methods sets that are declared with the type and interface keywords.

type Stringer interface {
  String() string
}

To implement an interface a (custom) type has just to implement the methods that are listed in its method set. There are no keywords like implements or extends like in other languages.

type Person struct {
  name string
  age  int
}

func (p Person) String() string { // Person now implements the Stringer interface
  return fmt.Sprintf("%s,%d", p.name, p.age)
}

The (p Person) between the func keyword and the name of the function is called the receiver type and it means that function becomes a method attacched to the type Person.

Milestone 5

In this milestone you'll update the code to be able to accept a file with a list of sentences and create a gif with the same method for each sentence in the list. This milestone can be achieved with a sequential algorithm but your final objective is to do it using concurrency. In this way you'll get introduced to how Go manages concurrent code, learn about the go keyword to spawn goroutines, how to work with keyword chan to create channels and pass data between goroutines using the arrow operator <-, the close keyword to close a channel from the sender goroutine, the usage if sync.WaitGroup to create wait points where to wait a group of goroutines to end and the way to use the for range syntax to loop on channels.

Steps for milestone 5

1. Create a new file called input and write some sentences in several lines
2. Create a new function func readInput(r io.Reader) chan string inside main.go that reads from the reader and returns a channel of strings with the content of the reader. To read from the reader look at the bufio package for the Scanner type. To fill the channel create a goroutine with the go keyword and fill the channel to return with the <- operator. When the channel is filled with all the values use the close keyword on the channel to signal that no more values are expected to be sent
3. Inside the main function use os.Open to open the input file and pass it to the readInput function just created
4. Wrap the code previously written in a goroutine go func() {}() and then inside a for range loop for text := range readInput(r) {} that reads all values coming from the channel returned by readInput inside the text variable and uses it in the Goroutine
5. Use a sync.WaitGroup to make sure that the main goroutine waits that all other goroutines execute before closing the program
6. Run go run main.go with the appropriate parameter if you read them from the console

As a bonus you can:

• accept the name of the file to read as an input flag
• switch between the input file and the command line argument if no input file is provided (using a switch statement and leveraging the io.Reader interface to store the values coming from the file or from a strings.NewReader)

Notes for milestone 5

To start a function concurrently with another you can prefix it with the go keyword. This keyword instructs the creation of a goroutine that runs that function. The function itself can be named like in a go fmt.Println("hello concurrent world") or anonymous like in a go func() { fmt.Println("hello concurrent world") }(). Notice that the final () denotes the fact that the function is actually called and not just being defined.

In Go it is possible to pass data between goroutines using channels; they are a specific type defined with the chan keyword followed by the type that is passed in the channel. E.g. var a chan int is a channel that passes values of int type. To initialize a channel the make keyword should be used, make is a function that takes the channel as first input and the size of the channel as second one (if any).

A channel of size 0 is called unbuffered and it will always wait that the receiving end is ready before sending a value; a channel of size 1 or more is called buffered, with a buffer of the specified size, and it will be sending values until all the buffer is full and values are not read yet.

Here is a code example of interacting with a channel:

numbers := make(chan int)
go func() {
  numbers <- 1
}
fmt.Println(<-numbers) // will print 1 as soon as the goroutine puts 1 into the channel numbers

In this example you see the <- (arrow) operator being used twice:

• on the right of the variable (numbers <-)
  • in this direction the arrow operator is sending a value into the channel
• on the left of the variable (<-numbers)
  • in this direction the arrow operator is receiving a value from the channel

Channels can also be read inside a for range loop here is an example:

numbers := make(chan int)
go func() {
  for i := 0; i < 10; i++ {
    numbers <- i
  }
  close(numbers)
}
for n := range numbers {
  fmt.Println(n) // will print 0 to 10 as soon as the goroutine puts their values into the channel numbers
}

This is the only case where the for range loop assigns one value because for slices, maps and strings it assigns two values. This kind of for range loop doesn't stop unless the sender goroutine uses the close keyword to close the channel on the sender end. This function signals the receiving end of the channel (the for range loop) that it will not receive any more values and can exit. It will not exit without receiving the close call. It is not a call made to free a channel resource.

The sync.WaitGroup is a specific custom type defined in the standard library to create wait points where to wait a group of goroutines to end. It defines three useful methods:

• Add(int); to add a number of goroutines to wait in a counter
• Done(): to signal the WaitGroup that one goroutine has completed. The counter decrements of 1
• Wait(): which lets the main goroutine wait until the internal counter reaches 0