Scope functions in Kotlin

The Kotlin standard library contains several functions whose sole purpose is to execute a block of code within the context of an object. When you call such a function on an object with a lambda expression provided, it forms a temporary scope. In this scope, you can access the object without its name. Such functions are called scope functions. There are five of them: let, run, with, apply, and also. [source]

When and how to use scope functions `run, let, apply, also, with` in Kotlin in order to improve

our productivity writing a clean, readable and easy to maintain code.

Objectives.

. Work with the same scope.

. Write clean, readable and easy to maintain code

– run:

Receives ‘this’ as an instance of the current object and returns a final value.

It is recommended for execute actions over the same scope.

```
val dog = Dog()
val result = dog.run { //this as instance of the object dog
	jump() // same as dog.jump() also you can use this.jump()
	sit()
        eat()
	sleep() // returns
}
```

Without “run” the code looks like:

```
dog.jump()
dog.sit()
dog.eat()
val result = dog.sleep()
```

– let:

Receives ‘it’ as an instance of the current object and returns a final value,

It is recommended for use the object scope several times with others functions.

```
val owner = dog?.let { 
  renameDog(it)
  paintDog(it, red)

  if(it.race == null) {
    it.race = generateRace()
  }

  assignOwner(it) // returns 
} ?: Dog().let{
  assignOwner(it) // returns 
}
```

– apply:

Receives ‘this’ as an instance of the current object and returns the object,

It is recommended for change the scope function data:

```
dog.apply {
  name = "Canelo"
  color = "Red"
  size = "Big"	
}
```

– also:

Receives ‘it’ as an instance of the object, returns the same scope(object) as result.

It is recommended for execute a code after a creation of the object or parameters assignations:

```
fun getInstance() : Dog {
  return instance ?: Dog().also {
    instance = it
    // returns 'Dog()' not 'instance'
  }
}
```

Also, you can chain with ‘apply’:

```
val selected = dog.apply {
  name = "Canelo"
  color = "Red"
  size = "Big"	
}.also {
  if(it.race == Race.PitBull) {
    it.size = big
  }
  // return dog
}
```

– with:

Similar to let, but using ‘this’ as scope.

It is recommended if you know the object is not null for example:

```
val dog = Dog()
with(dog) {
  println(name) //same as 'println(this.name)'
  eat() // this.eat()
}
```

If you need to check the object’s nullability you should use ‘let’ instead of ‘with’:

```
dog?.let {
  ...
}
```

Combining scope functions

Imagine you have two objects and you need to nest scope functions.

In this case you need to specify the name of the scope to avoid overlapping.

```
dog1.let { // it as instance of dog1
  dog2.let { d2 -> // you need use different names for the variable
    it.isBrotherOf(d2) //it stell an instance of dog1
    doSomething(arrayListOf(it,d2))
  }
  // d2 don't exist in this scope
  it.jump()
}
```