Kotlin: Scope Function Categorization

Table of Contents

I categorized scope functions, let, with, also, apply, run in Kotlin.

Classification

Scope functions can be classified with 3 aspects.

Identifier	Extension Function	The Object represented as	Return Value
also	Yes	it	The Object
let		it	Result
apply		this	The Object
run			Result
with	No		Result

Is Extension Function or not

This difference is affect to null handling. When it is an extension function, we can use .? before calling. with is not an extension function, it can’t be called as nullable?.with.

As you can see in above table, with is similar to run for non-null value. The following code explains.

lateinit var message: String
// change message
message.run {
toUpperCase()
toLowerCase()
}
with(message) {
toUpperCase()
toLowerCase()
}

lateinit var message: String

// change message

message.run {

toUpperCase()

toLowerCase()

}

with(message) {

toUpperCase()

toLowerCase()

}

But for nullable value.

lateinit var message: String?
// change message
message?.run {
toUpperCase()
toLowerCase()
}
with(message) {
this?.toUpperCase()
this?.toLowerCase()
}

lateinit var message: String?

// change message

message?.run {

toUpperCase()

toLowerCase()

}

with(message) {

this?.toUpperCase()

this?.toLowerCase()

}

Thus, extension function can easily handle nullable type, using .?.

The object is represented as

it or this represents the object in scope functions.

In also, let, it can be used, and we can call the class with this if it’s in a class, and call other functions in the class. We can rename the object from it to other. The disadvantage is that we have to write it every time we call the object’s method and property. From another aspect, it is easy to understand whose method is called because it appears every time.

In with, apply and run, the object is represented as this. So, the object’s method and property is called directly. If the scope function is used in a class, the class’s method and property also can be called directly. In other words, it is not so easy to distinguish the object’s method and property from class’s method and property at a glance.

Identifier	Advantage	Disadvantage
it	`it`以外のわかりやすい名前にすることができる。オブジェクト自身の関数・プロパティを呼び出す際に`it`を毎回記述するため、どのメソッドを呼び出しているかわかりやすい。	オブジェクト自身の関数・プロパティを呼び出す際には`it`を毎回記述する必要がある。
this	オブジェクト自身の関数・プロパティを直接呼び出すことができる。	オブジェクト自身の関数・プロパティとクラスの関数・プロパティとの区別が同じように呼び出されるようになるため、区別がつけづらい。

Return Value

スコープ関数は、自分自身の値が返り値になるのか、ブロックの実行結果の値が返り値になるのかでわかれます。

This effects when we build method chain.

1.
apply { plus(1) } .
apply { plus(1) } .
apply { plus(1) } .
apply { println(this) }
// => 1
1.
run { plus(1) } .
run { plus(1) } .
run { plus(1) } .
run { println(this) }
// => 4