What (exactly) does the type keyword do in go?

Question

I've been reading A Tour of Go to learn Go-Lang and so far it's going good.

I'm currently on the Struct Fields Lesson and here is the sample code from the right hand side:

package main

import "fmt"

type Vertex struct {
  X int
  Y int
}

func main() {
  v := Vertex{1, 2}
  v.X = 4
  fmt.Println(v.X)
}

Take a look at line 3:

type Vertex struct {

What I don't understand is, what does the type keyword do and why is it there?

Answer 1

The type keyword is there to create a new type. This is called type definition. The new type (in your case, Vertex) will have the same structure as the underlying type (the struct with X and Y). That line is basically saying "create a type called Vertex based on a struct of X int and Y int".

Don't confuse type definition with type aliasing. When you declare a new type, you are not just giving it a new name - it will be considered a distinct type. Take a look at type identity for more information on the subject.

Answer 2

It's used to define a new type.

General format:
type <new_type> <existing_type or type_definition>

Common use cases:

• Create a new type for an existing type.
  Format:
  type <new_type> <existing_type>
  e.g
  type Seq []int
• Create a type while defining struct.
  Format:
  type <new_type> struct { /*...*/}
  e.g
  https://gobyexample.com/structs
• Define function type, (aka. by assigning name to a function signature).
  Format:
  type <FuncName> func(<param_type_list>) <return_type>
  e.g
  type AdderFunc func(int, int) int

In your case:

It define a type named Vertex for a new struct, so that later you can refer to the struct via Vertex.

Answer 3

Actually type keyword is same with class topology in PHP.

With type keyword as though you create class in GO

Example type in struct

type Animal struct {
  name string //this is like property
}

func (An Animal) PrintAnimal() {
  fmt.Println(An.name) //print properties
}

func main() {
  animal_cow := Animal{ name: "Cow"} // like initiate object

  animal_cow.PrintAnimal() //access method
}

OK let's move with type string (is same for int or float)

   type Animal string
        
   // create method for class (type) animal
   func (An Animal) PrintAnimal() {
      fmt.Println(An) //print properties
   }
    
   func main(){
      animal_cow := Animal("Cow") // like initiate object
    
      animal_cow.PrintAnimal() //access method
      //Cow
   }

Difference between struct and string, int, float just in struct you can add more properties with any different data type

Opposite in string, int, float you can only have 1 properties, which created when you initiate your type (ex: animal_cow := Animal("Cow")

But, all type which build using type keyword can definitely have more than 1 method

Correct me if I am wrong