KucherenkoIhor / CompositionOfFunctionsInKotlin

This repository contains two implementation of function composition in Kotlin and benchmark tests.

Composition Of Functions In Kotlin

This repository contains two implementation of function composition in Kotlin and benchmark tests.

The implementation:

inline operator fun <P1, R1, R2> ((R1) -> R2).plus(crossinline f: (P1) -> R1): (P1) -> R2 {
    return { p1: P1 -> this(f(p1)) }
}

If you need combine several operations in one sequence you can implement it as follows:

@Benchmark
@CompilerControl(CompilerControl.Mode.DONT_INLINE)
fun firstCase(): Double {

    val prices = listOf(3.4, 5.6, 5.6, 3.4, 3.4, 3.4, 5.6, 5.6, 3.4, 3.4)

    return prices
            .map { it - it * 0.9 }
            .map { it + 0.3 }
            .map { it - 0.1 }
            .sum()
}

To increase readability of code you can extract code from maps to separate functions:

@Benchmark
@CompilerControl(CompilerControl.Mode.DONT_INLINE)
fun secondCase(): Double {

    val prices = listOf(3.4, 5.6, 5.6, 3.4, 3.4, 3.4, 5.6, 5.6, 3.4, 3.4)

    return prices
            .map(::discount)
            .map(::tax)
            .map(::aid)
            .sum()
}

fun discount(price: Double) = price - price * 0.9

fun tax(price: Double) = price + 0.3

fun aid(price: Double) = price - 0.1    

It still contains several invoking of map(). Now we can use the composition of functions approach:

@Benchmark
@CompilerControl(CompilerControl.Mode.DONT_INLINE)
fun thirdCase(): Double {

    val prices = listOf(3.4, 5.6, 5.6, 3.4, 3.4, 3.4, 5.6, 5.6, 3.4, 3.4)

    return prices
            .map(::aid + ::tax + ::discount)
            .sum()
}

It looks much better!

And what about performance!??

MyBenchmark.firstCase 415.542 ± 14.232 ns/op

MyBenchmark.secondCase 367.386 ± 8.428 ns/op

MyBenchmark.thirdCase 265.401 ± 5.976 ns/op

To run benchmarks:

1. Clone the repository
2. Build a .jar file using this command at the root of the repository: mvn clean install
3. To run the .jar with fast benchmarking: java -jar target/benchmarks.jar -wi 0 -i 1 -f 1 -tu ns -bm avgt To run the .jar with default benchmarking: java -jar target/benchmarks.jar

Changelog

Added inline and crossinline based on this [suggestion] (#1).

MyBenchmark.firstCase 399.855 ± 20.951 ns/op

MyBenchmark.secondCase 366.027 ± 12.553 ns/op

MyBenchmark.thirdCase 190.994 ± 8.581 ns/op

Added function to combine predicates:

@Benchmark
@CompilerControl(CompilerControl.Mode.DONT_INLINE)
fun filteringSecondCase(): List<Student> {
    return students
            .filter(::ageMoreThan20 and ::firstNameStartsWithE and ::theLengthOfSecondNameMoreThan5)
}

inline infix fun <P> ((P) -> Boolean).and(crossinline predicate: (P) -> Boolean): (P) -> Boolean {
    return { p: P -> this(p) && predicate(p) }
}

MyBenchmark.filteringFirstCase 172.909 ± 2.238 ns/op

MyBenchmark.filteringSecondCase 115.346 ± 1.488 ns/op