Interesting kotlin 0x0a:fun with composition

Recently http://portal.kotlin-academy.com/#/ I see a lot about Kotlin An interesting topic . I think it's very suitable for Kotlin lovers , If you are interested, you can check it by yourself .
【 amusing Kotlin 】 Record your understanding of each question in the series .

0x0A：Fun with composition

val increment = {
     i: Int -> i + 1 }
val bicrement = {
     i: Int -> i + 2 }
val double = {
     i: Int -> i * 2 }
val one = {
     1 }

infix fun <T, R> (() -> T).then(another: (T) -> R) = {
     another(this()) }
operator fun <T, R1, R2> ((T) -> R1).plus(another: (T) -> R2) = {
     x: T -> this(x) to another(x) }

fun main(args: Array<String>) {
    
    val equilibrum = one then double then (increment + bicrement)
    print(equilibrum())
}

Above code , What is the result of the operation ？ optional ：

1. Nothing, it doesn’t compile
2. 5
3. (1,2)
4. (3,4)

Think about it , Record the answers in your mind .

analysis

Let's look at the variables first

Definition at the beginning of the topic 4 A variable , All are Function type and Lambda expression .

Function purpose ：

• increment： Add 1
• bicrement： Add 2
• double： double
• one： Return value after calling 1

Look at the function

4 After variables , Following the 2 A function , One is infix function , One is operator overloading function .

• then： Concatenate two functions . Execute first receiver, The result is passed in as a parameter another .
• plus： Combine the results of the two functions . perform receiver , perform another, And combine the two results into Pair .

Infix function then

then A function is an infix function , It is also a generic function , hold T, R Two generic types

• receiver：() -> T.

• Parameters ：(T) -> R.

• Return value ：() -> R.

Use infix The modified function is Infix function , Three conditions must be met ：

• Must be a member function or an extension function .
• Must have unique function arguments .
• The parameter cannot be a variable number of parameters (vararg ), And there can be no default value .

infix fun Int.shl(x: Int): Int {
     ... }

// calling the function using the infix notation
1 shl 2

// is the same as
1.shl(2)

Operator overload function plus

plus Is an operator overloaded function , It is also a generic function , hold T, R1,R2 Three generic types .

• receiver：(T) -> R1.

• Parameters ：(T) -> R2.

• Return value ：(T) -> Pair<R1, R2>.

Finally, look at the main logic

fun main(args: Array<String>) {
    
    val equilibrum = one then double then (increment + bicrement)
    println(equilibrum())
}

Restore the code logic

therefore , Last println(equilibrum()) Execute function type variables equilibrum Call logic , And print the results .

Pair Type of toString() Method public override fun toString(): String = "($first, $second)".

therefore , The correct answer is ：

Options 4 ：(3,4)

summary

The charm of grammar sugar is to express rich logic with concise code . But think back , It may also obscure logic .

therefore , Daily development , Choosing the right variable name and function is particularly important . The variable name and function name are used properly ,1 Double first , Add... Separately 1, Add 2, Last use + Represents a combination of .

 val equilibrum = one then double then (increment + bicrement)