[Michal Rostecki] Introduction to Golang

Presentation from GDG DevFest Ukraine 2016.
Learn more at: https://devfest.gdg.org.ua

Transcript

1. Copyright © 2015 Mirantis, Inc. All rights reserved www.mirantis.com Introduction to Golang Michał Rostecki | Software Engineer mrostecki@mirantis.com
2. Copyright © 2015 Mirantis, Inc. All rights reserved What is Go? Go is a programming language, created by Rob Pike at Google, which is: • Statically typed • Compiled • Multithreaded • Garbage collected • Created mostly for networked systems, parallel computing and web applications
3. Copyright © 2015 Mirantis, Inc. All rights reserved What’s written in Go?
4. Copyright © 2015 Mirantis, Inc. All rights reserved Hello world package main
5. Copyright © 2015 Mirantis, Inc. All rights reserved Hello world package main import "fmt"
6. Copyright © 2015 Mirantis, Inc. All rights reserved Hello world package main import "fmt" func main() { fmt.Println("Hello world") }
7. Copyright © 2015 Mirantis, Inc. All rights reserved Variables foo := 3 bar := “something”
8. Copyright © 2015 Mirantis, Inc. All rights reserved Variables foo := 3 bar := “something” var foo int foo = 3 var bar string bar = “something”
9. Copyright © 2015 Mirantis, Inc. All rights reserved Arrays var a [5]int fmt.Println(a) // [0 0 0 0 0]
10. Copyright © 2015 Mirantis, Inc. All rights reserved Arrays var a [5]int fmt.Println(a) // [0 0 0 0 0] a[2] = 100 fmt.Println(a) // [0 0 100 0 0]
11. Copyright © 2015 Mirantis, Inc. All rights reserved Arrays You can define the size of array once.
12. Copyright © 2015 Mirantis, Inc. All rights reserved Slices s := make([]int, 1) // {0}
13. Copyright © 2015 Mirantis, Inc. All rights reserved Slices s := make([]int, 1) // {0} s = append(s, 1) // {0 1}
14. Copyright © 2015 Mirantis, Inc. All rights reserved Slices s := make([]int, 1) // {0} s = append(s, 1) // {0 1} s = append(s, 2) // {0 1 2}
15. Copyright © 2015 Mirantis, Inc. All rights reserved Slices s := make([]int, 1) // {0} s = append(s, 1) // {0 1} s = append(s, 2) // {0 1 2} s = append(s, 3) // {0 1 2 3}
16. Copyright © 2015 Mirantis, Inc. All rights reserved Slices s := make([]int, 1) // {0} s = append(s, 1) // {0 1} s = append(s, 2) // {0 1 2} s = append(s, 3) // {0 1 2 3} fmt.Println(s[1:]) // {1 2 3}
17. Copyright © 2015 Mirantis, Inc. All rights reserved Slices s := make([]int, 1) // {0} s = append(s, 1) // {0 1} s = append(s, 2) // {0 1 2} s = append(s, 3) // {0 1 2 3} fmt.Println(s[1:]) // {1 2 3} fmt.Println(s[:2]) // {0 1 2}
18. Copyright © 2015 Mirantis, Inc. All rights reserved Slices s := make([]int, 1) // {0} s = append(s, 1) // {0 1} s = append(s, 2) // {0 1 2} s = append(s, 3) // {0 1 2 3} fmt.Println(s[1:]) // {1 2 3} fmt.Println(s[:2]) // {0 1 2} fmt.Println(s[1:2] // {1 2}
19. Copyright © 2015 Mirantis, Inc. All rights reserved Slices x := make([]int, 1) // {0} y := make([]int, 0, 5) // {} with capacity 5
20. Copyright © 2015 Mirantis, Inc. All rights reserved Slices Slices are resizable, despite having the length and capacity at the beginning. They support “slicing” operations (well known from Python), which get some defined part of slice.
21. Copyright © 2015 Mirantis, Inc. All rights reserved What’s wrong with the other languages? • Inheritance, polymorphism, templating • Languages like C, C++ or Java are quite old • Dependencies in C and C++ scale very poorly • Python or Scala are easy and readable, but very slow • No system-based threading/concurrency in many standard libraries • Many modern languages don’t have pointers, only non-explicit references
22. Copyright © 2015 Mirantis, Inc. All rights reserved What Go got rid of? • Inheritance • Classes • Exceptions (?)
23. Copyright © 2015 Mirantis, Inc. All rights reserved Structs Defining structs is similar to C: type Foo struct { Name string Quantity int }
24. Copyright © 2015 Mirantis, Inc. All rights reserved Struct methods in C In C you define functions which get the instance of struct as an argument.
25. Copyright © 2015 Mirantis, Inc. All rights reserved Struct methods in C struct foo { char *name; int quantity; }
26. Copyright © 2015 Mirantis, Inc. All rights reserved Struct methods in C char *hello(struct foo) { char dest[256]; sprintf(dest, “Hello %s!”, foo); return dest; }
27. Copyright © 2015 Mirantis, Inc. All rights reserved Struct methods in Go In Go you don’t receive the struct as an argument. Writing methods for structs is very similar to writing methods for classes in object-oriented languages.
28. Copyright © 2015 Mirantis, Inc. All rights reserved Struct methods in Go type Foo struct { Name string Quantity int }
29. Copyright © 2015 Mirantis, Inc. All rights reserved Struct methods in Go func (f Foo) Hello() string { return fmt.Sprintf(“Hello %s!”, foo.Name) }
30. Copyright © 2015 Mirantis, Inc. All rights reserved Value vs pointer &myVariable *myVariable
31. Copyright © 2015 Mirantis, Inc. All rights reserved Value vs pointer myValue := SomeStruct{}
32. Copyright © 2015 Mirantis, Inc. All rights reserved Value vs pointer myPointer := &SomeStruct{} myValue := *myPointer
33. Copyright © 2015 Mirantis, Inc. All rights reserved Value receiver vs pointer receiver Methods can have two kind of receivers.
34. Copyright © 2015 Mirantis, Inc. All rights reserved Value receiver func (f Foo) SomeMethod() { f.Name = “new value” // This will not modify anything! }
35. Copyright © 2015 Mirantis, Inc. All rights reserved Pointer receiver func (f *Foo) SomeMethod() { f.Name = “new value” }
36. Copyright © 2015 Mirantis, Inc. All rights reserved Value receivers vs pointer receivers Value receiver will not modify the contents, because it receives the copy of the struct. Pointer receiver will modify the contents, because it has access to the original object in the memory.
37. Copyright © 2015 Mirantis, Inc. All rights reserved Value receiver vs pointer receiver You can use both kind of receivers regardless of what kind of variable you have. That means that both: f = Foo{} f.SomeMethod() and f = &Foo{} f.SomeMethod() will work. However, it’s not recommended to mix pointer and value receivers in one struct, one of the next slides will explain why.
38. Copyright © 2015 Mirantis, Inc. All rights reserved Composition vs inheritance Composition is “has-a” relationship. The object A can contain the object B. Type of A can contain a property of type of B. Inheritance is “is-a” relationship. The object A can have the same properties as the object B. Type of A is a subset of type of B.
39. Copyright © 2015 Mirantis, Inc. All rights reserved Composition type FooCommons struct { Name string Quantity int }
40. Copyright © 2015 Mirantis, Inc. All rights reserved Composition type Foo struct { Commons *FooCommons Annotations string[] }
41. Copyright © 2015 Mirantis, Inc. All rights reserved Composition type Bar struct { Commons *FooCommons Weight int }
42. Copyright © 2015 Mirantis, Inc. All rights reserved Inheritance class Base { protected: string name; int quantity; }
43. Copyright © 2015 Mirantis, Inc. All rights reserved Inheritance class Foo: public Base { protected: std::vector<string> annotations; }
44. Copyright © 2015 Mirantis, Inc. All rights reserved Inheritance class Bar: public Base { protected: int weight; }
45. Copyright © 2015 Mirantis, Inc. All rights reserved Interfaces Interface is a set of method signatures which may be implemented by some structure. Unlike in Java, you don’t declare the interface implementation explicitly.
46. Copyright © 2015 Mirantis, Inc. All rights reserved Interfaces type Helloer interface { Hello() string }
47. Copyright © 2015 Mirantis, Inc. All rights reserved Interfaces type Foo struct { Name string }
48. Copyright © 2015 Mirantis, Inc. All rights reserved Interfaces func (f Foo) Hello() string { return fmt.Sprintf(“Hello %s”, f.Name) }
49. Copyright © 2015 Mirantis, Inc. All rights reserved Interfaces func main() { var f Helloer f = Foo{Name: “foo”} fmt.Println(f.Hello()) }
50. Copyright © 2015 Mirantis, Inc. All rights reserved Pointers and values matter in interfaces // Pointer receiver func (f *Foo) Hello() string { return fmt.Sprintf(“Hello %s”, f.Name) }
51. Copyright © 2015 Mirantis, Inc. All rights reserved Pointers and values matter in interfaces func main() { /* that will fail, because pointer impelents interface */ var h Helloer = Foo{} }
52. Copyright © 2015 Mirantis, Inc. All rights reserved Pointers and values matter in interfaces func main() { var h Helloer = &Foo{} // that will work }
53. Copyright © 2015 Mirantis, Inc. All rights reserved The empty interface interface{} means the same as void* in C. This function can take every type as an argument: func Foo(bar interface{}) { }
54. Copyright © 2015 Mirantis, Inc. All rights reserved Type assertions var i interface{} = “something”
55. Copyright © 2015 Mirantis, Inc. All rights reserved Type assertions var i interface{} = “something” s := i.(string)
56. Copyright © 2015 Mirantis, Inc. All rights reserved Type assertions var i interface{} = “something” s := i.(string) s, ok := i.(string)
57. Copyright © 2015 Mirantis, Inc. All rights reserved Type assertions var i interface{} = “something” s := i.(string) s, ok := i.(string) s, ok := i.(float64) // ok will be false
58. Copyright © 2015 Mirantis, Inc. All rights reserved Type assertions var i interface{} = “something” s := i.(string) s, ok := i.(string) s, ok := i.(float64) // ok will be false if !ok { fmt.Printf(“it’s not float!”) }
59. Copyright © 2015 Mirantis, Inc. All rights reserved Errors error is a type you can return in your function, check for it when calling the other functions and handle it in appropriate place.
60. Copyright © 2015 Mirantis, Inc. All rights reserved Errors func DoSomethingWithNumber(n int) (*int, error) { If n == 10 { return nil, fmt.Errorf(“Cannot work with number 10”) } result := n + 2 return &result, nil }
61. Copyright © 2015 Mirantis, Inc. All rights reserved Errors func main() { for _, i := range []int{2, 10, 24} { n, err := DoSomethingWithNumber(i) if err != nil { fmt.Fprintln(os.Stderr, “Error occured %#v”, err) } } }
62. Copyright © 2015 Mirantis, Inc. All rights reserved Goroutines By writing go someFunction(arg1, arg2, …) You call this function as a goroutine, which is a thread with shared memory.
63. Copyright © 2015 Mirantis, Inc. All rights reserved Goroutines func say(msg string) { time.Sleep(100) fmt.Println(msg) }
64. Copyright © 2015 Mirantis, Inc. All rights reserved
65. Copyright © 2015 Mirantis, Inc. All rights reserved Goroutines func main() { go say(“world”) fmt.Println(“hello”) // “world” will be printed before “hello” }
66. Copyright © 2015 Mirantis, Inc. All rights reserved Channels Channel is a type through which you can send and receive values.
67. Copyright © 2015 Mirantis, Inc. All rights reserved Channels func foo(messages chan string) { messages <- “ping” }
68. Copyright © 2015 Mirantis, Inc. All rights reserved Channels func main() { messages := make(chan string) go foo(messages) msg := <- messages fmt.Println(msg) // will print “ping” }
69. Copyright © 2015 Mirantis, Inc. All rights reserved Interested in Golang? This presentation was just a brief outlook and introduction. If you want to play with Go, take the http://tour.golang.org!
70. Copyright © 2015 Mirantis, Inc. All rights reserved Thank you for your attention