Learn Go in ~5mins

This is inspired by A half-hour to learn Rust and Zig in 30 minutes.

Basics

Your first Go program as a classical "Hello World" is pretty simple:

First we create a workspace for our project:

$ mkdir hello
$ cd hello

Next we create and initialize a Go module:

$ go mod init hello

Then we write some code using our favorite editor in a file called main.go in the directory hello you created above:

package main

import "fmt"

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

And finally we build and produce a binary:

$ go build

You should now have a hello binary in your workspace, if you run it you should also get the output:

$ ./hello
Hello World!

Variables

You can create variables in Go in one of two ways:

var x int

Other types include int, int32, int64, float32, float64, bool and string (and a few others...), there are also unsigned variants of the integer types prefixed with u, e.g: uint8 which is the same as a byte.

Or implicitly with inferred types by creating and assigning a value with:

x := 42

Values are assigned by using the = operator:

x = 1

NOTE: Most of the time you don't need to use the var keyword unless you are creating a variable and assigning it a value later or want a zero or nil value for some reason (_like a nil map).

Functions

Functions are declared with the func keyword:

func hello(name string) string {
  return fmt.Sprintf("Hello %s", name)
}

Functions with a return type must explicitly return a value.

Functions can return more than one value (commonly used to return errors and values):

func isEven(n int) (bool, error) {
  if n <= 0 {
    return false, fmt.Errorf("error: n must be > 0")
  }
  return n % 2 == 0, nil
}

Go also supports functions as first-class citizens and as such supports many aspects of functional programming, including closures, returning functions and passing functions around as values. For example:

func AddN(n int) func(x int) int {
  return func(x int) int {
    return x + n
  }
}

Structs

As Go is a multi-paradigm language, it also support "object orientated" programming by way of "structs" (borrowed from C). Objects / Structs are defined with the struct keyword:

type Account struct {
  Id      int
  Balance float64
}

Fields are defined similar to variables and are accessed with the dot-operator .:

account := Account{}
fmt.Printf("Balance: $%0.2f", account.Balance)

Methods

Structs (objects) can also have methods. Unlike other languages however Go does not support multiple-inheritance nor does it have classes (you can however embed structs into other structs).

Methods are created like functions but take a "receiver" as the first argument:

type Account struct {
  id  int
  bal float64
}

func (a *Account) String() string {
  return fmt.Sprintf("Account[%d]: $0.2f", a.id, a.bal)
}

func (a *Account) Deposit(amt flaot64) float64 {
  a.bal += amt
  return a.bal
}

func (a *Account) Withdraw(amt float64) float64 {
  a.bal -= amt
  return a.bal
}

func (a *Account) Balance() float64 {
  return a.bal
}

These are called "pointer receiver" methods because the first argument is a pointer to a struct of type Account denoted by a *Account.

You can also define methods on a struct like this:

type Circle struct {
  Radius float64
}

func (c Circle) Area() float64 {
  return 3.14 * c.Radius * c.Radius
}

In this case methods cannot modify any part of the struct Circle, they can only read it's fields. They are effectively "immutable".

Arrays and Slices

Like other langauges Go has Arrays, but unlike other languages Go's arrays are more similar to C where they are of fixed size. You create a fixed sized array by specifying it's size and type like this:

xs := [4]int{1, 2, 3, 4}

Most of the time however you will deal with Slices, which behave more like lists in other languages like Python where they are resized automatically.

Sliaces are created by omitting the size:

xs := []int{1, 2, 3, 4}

Slices can also be created and appended to:

xs := []int{}
xs = append(xs, 1)
xs = append(xs, 2)
xs = append(xs, 3)

You can access an slice's elements by indexing:

xs[1]  // 2

You can also access a subset of an array or slice by "slicing" it:

ys := xs[1:] // [2, 3]

You can iterate over an array/slice by using the range keyword:

for i, x := range xs {
  fmt.Printf("xs[%d] = %d\n", i, x)
}

Maps

Go has a builtin data structure for storing key/value pairs called maps (called hash table, hash map, dictionary or associative array in other languages).

You create a map by using the keyword map and defining a type for keys and type for values map[Tk]Tv, for example a map with keys as strings and values as integers can be defined as:

var counts map[string]int

You can assign values to a map just like arrays by using curly braces {...} where keys and values are separated by a colon :, for example:

counts := map[string]int{
  "Apples": 4,
  "Oranges": 7,
}

Maps can be indexed by their keys just like arrays/slices:

counts["Apples"]  // 4

And iterated over similar to array/slices:

for key, value := range counts {
  fmt.Printf("%s: %d\n", key, value)
}

The only important thing to note about maps in Go is you must initialize a map before using it, a nil map will cause a program error and panic:

var counts map[string]int
counts["Apples"] = 7  // This will cause an error and panic!

You must initialize a map before use by using the make() function:

counts := make(map[string]int)
counts["Apples"] = 7

Flow control structures

Go only has one looping construct as seen in the previous sections:

sum := 0
for i := 0; i < 10; i++ {
  sum += i
}

The basic for loop has three components separated by semicolons:

the init statement: executed before the first iteration
the condition expression: evaluated before every iteration
the post statement: executed at the end of every iteration

If you omit the condition you effectively have an infinite loop:

for {
}
// This line is never reached!

Go has the usual if statement along with else if and else for branching:

N := 42
func Guess(n int) string {
  if n == 42 {
    return "You got it!"
  } else if n < N {
    return "Too low! Try again..."
  } else {
    return "Too high! Try again..."
  }
}

Note: The last else could have been omitted and been written as return "Too high~ Try again...", as it would have been functionally equivalent.

There is also a switch statement that can be used in place of multiple if and else if statements, for example:

func FizzBuzz(n int) string {
  switch n {
  case n % 15 == 0:
    return "FizzBuzz"
  case n % 3 == 0:
    return "Fizz"
  case n % 5 == 0:
    return "Buzz"
  default:
    return fmt.Sprintf("%d", n)
  }
}

Functions can be executed at the end of a function anywhere in your function by "deferring" their execution by using the defer keyword. This is commonly used to close resources automatically at the end of a function, for example:

package main

import (
  "os"
  "fmt"
)

func Goodbye(name string) {
  fmt.Printf("Goodbye %s", name)
}

func Hello(name string) {
  defer Goodbye(name)
  fmt.Printf("Hello %s", name)
}

func main() {
  user := os.Getenv("User")
  Hello(user)
}

This will output when run:

$ ./hello
Hello prologic
Goodbye prologic

Error handling

Errors are values in Go and you return them from functions. For example opening a file with os.Open returns a pointer to the open file and nil error on success, otherwise a nil pointer and the error that occurred:

f, err := os.Open("/path/to/file")

You check for errors like any other value:

f, err := os.Open("/path/to/file")
if err == nil {
  // do something with f
}

It is idiomatic Go to check for non-nil errors from functions and return early, for example:

func AppendFile(fn, text string) error {
  f, err := os.OpenFile(fn, os.O_CREATE|os.O_APPEND|os.WR_ONLY, 0644)
  if err != nil {
    return fmt.Errorf("error opening file for writing: %w", err)
  }
  defer f.Close()
  
  if _, err := f.Write([]byte(text)); err != nil {
    return fmt.Errorf("error writing text to fiel: %w", err)
  }
  
  return nil
}

Creating and import packages

Finally Go (like every other decent languages) has a module system where you can create packages and import them. We saw earlier In Basics how we create a module with go mod init when starting a new project.

Go packages are just a directory containing Go source code. The only difference is the top-line of each module (each *.go source file):

Create a Go package by first creating a directory for it:

$ mkdir shapes

And initializing it with go mod init:

$ cd shapes
$ go mod init github.com/prologic/shapes

Now let's create a source module called circle.go using our favorite editor:

package shapes

type Circle struct {
  Radius float64
}

func (c Circle) String() string {
  return fmt.Sprintf("Circle(%0.2f)", c.Radius)
}

func (c Circle) Area() float64 {
  return 3.14 * c.Radius * c.Radius
}

It is important to note that in order to "export" functions, structs or package scoped variables or constants, they must be capitalized or the Go compiler will not export those symbols and you will not be able access them from importing the package.

Now create a Git repository on Github called "shapes" and push your package to it:

$ git init
$ git commit -a -m "Initial Commit"
$ git remote add origin [email protected]:prologic/shapes.git
$ git push -u origin master

You can import the new package shapes by using it's fully qualified "importpath" as github.com/prologic/shapes. Go automatically knows hot to fetch and build the package given its import path.

Example:

Let's create a simple program using the package github.com/prologic/shapes:

$ mkdir hello
$ go mod init hello

And let's write the code for main.go using our favorite editor:

package main

import (
  "fmt"

  "github.com/prologic/shapes"
)

func main() {
  c := shapes.Circle{Radius: 5}
  fmt.Printf("Area of %s: %0.2f\n", c, c.Area())
}

Building it with go build:

$ go build

And finally let's test it out by running the resulting binary:

$ ./hello
Area of Circle(5.00): 78.50

Congratulations! 🎉

Now you're a Gopher!

That's it! Now you know a fairly decent chunk of Go. Some (pretty important) things I didn't cover include:

Writing unit tests, writing tests in Go is really easy! See testing
The standard library, Go has a huge amount of useful packages in the standard library. See Standard Library.
Goroutines and Channels, Go's builtin concurrency is really powerful and easy to use. See Concurrency.
Cross-Compilation, compiling your program for other architectures and operating systems is super easy. Just set the GOOS and GOARCH environment variables when building.

For more details, check the latest documentation, or for a less half-baked tutorial, please read the official Go Tutorial and A Tour of Go.

kmatt/LearnGoIn5mins.md