Types?

Go is statically typed language means every variable's type is fixed at compile time, which helps catch errors early and makes code reliable.

Types of types

Type Description
1. Basic Type int, float64, bool, string

var a int = 42
str := "Go is awesome" // Type inference
b := true
var f float64 = float64(a)      //Type Conversion
2. Aggregate Types 1. Arrays:An array is a fixed-size sequence of elements of the same type.

colors := [3]string{"red", "green", "blue"}

2. struct

type Person struct {
    Name string
    Age  int
}
func (p Person) fun() string {  //Define function for struct
    return "Hello, my name is " + p.Name
}
Reference Types Pointers: variable that stores the memory address of another variable (&)
slices:
maps
functions
channels

4. Interface

Interface only declares the functions those should be implemented by concrete types. Similar to Abstract Class C++98, Concepts in C++20.
C++: A class must explicitly inherit from the abstract class (class Dog : public Animal)
Go follows duck typing(If it walks like a duck and quacks like a duck, it's a duck), What it means is if type in Go implements all methods of interface, it automatically implements the interface(no need to provide implements or extends keyword) 


package main
import "fmt"

// 1. Define the interface (The Contract)
type Speaker interface {      <<<<<< See Keyword interface
	Speak() string // Pure behavior, no implementation, no variables
}

// 2. Define a concrete type
type Dog struct {
	Name string // Structs hold the data
}

// 3. Implement the method (Implicitly!)
// There is no mention of "Speaker" here. Go just figures it out.
func (d Dog) Speak() string {
	return "Woof!"
}

type Robot struct{}

func main() {
	// A variable of interface type
	var s Speaker

	s = Dog{Name: "Buddy"} // Valid! Dog implements Speak()
	fmt.Println(s.Speak())

	// s = Robot{} 
	// ^ UNCOMMENTING THIS WILL CAUSE A COMPILER ERROR:
	// "cannot use Robot{} as Speaker value: Robot does not implement Speaker (missing method Speak)"
}

Restrictions on Go Interfaces

1. Interface can have Methods only, variables, constants, or data fields should not be present
2. Cannot write the body ({ ... }) of a function inside an interface.
3. Implementing type must match the method name, parameters, and return types exactly

Use case of interface

Use Case Explanation
Multiple Structs, One Interface ie (Polymorphism) Allowing different structs to be treated as the same type because they all implement the same functions.
Eg: A database interface. You might have a MySQL struct and a MongoDB struct. Both implement a Save() function.

type Saver interface {
    Save(data string) error
}

func DataProcessor(s Saver) { // Any database can be passed into this function
    s.Save("important info")
}
Decoupling and Testing (Mocking)

Interface Pollution Problem

Creating interfaces where they aren't actually needed. ie creating interface for every class
Symptoms of Interface Pollution
1. One-to-One Mapping: Every single struct has a corresponding interface
2.Harder Navigation: In an IDE, clicking "Go to Definition" on a method takes you to the interface definition rather than the actual code doing the work.
Polluted Interface Good Interface 

// Defined in the same package as the logic, 
// just for the sake of it.
type Worker interface {
    DoWork()
}

type MyWorker struct{}
func (m MyWorker) DoWork() { ... }
          

// Defined by the CONSUMER who needs the work done.
type Performer interface {
    DoWork()
}

func Execute(p Performer) {
    p.DoWork()
}