Discussion on: Is reduce() bad?

[CaelumF]

Cool article! I think Reduce is especially handy in constructing more human-friendly functions.

Kotlin:

fun main(args: Array<String>) {
    val arr1 = arrayOf(8,5, 12,90,65,1,0,768,8).asIterable()
    val arr2 = arrayOf(34,3,0,45,23,67,1,5, 15, 67,9).asIterable()
    val arr3 = arrayOf(344,23,5).asIterable()
    println(arr1.intersect(arr2).intersect(arr3))
}

[Jasterix]

My first time reading any Kotlin code, but it's pretty clean. How would this block of code look different with reduce? Is intersect a native method?

[CaelumF]

intersect is an extension function of Collections provided by Kotlin's stdlib, it would work on Lists, Sets, Maps (popularly implemented as ArrayList, HashSet and HashMap)

See: kotlinlang.org/api/latest/jvm/stdl...

My code could have been clearer by writing "arrayListOf" actually

Here is the code using reduce:

fun main(args: Array<String>) {
    val arr1 = arrayListOf(8,5,12,90,65,1,0,768,8)
    val arr2 = arrayListOf(34,3,0,45,23,67,1,5,15,67,9)
    val arr3 = arrayListOf(344,23,5)

    fun intersection(vararg lists: List<Int>) = lists.reduce {acc, arr -> acc.filter {arr.contains(it)} }

    println(intersection(arr1, arr2, arr3))
}

The types of acc and arr don't need to be declared, because lists is of the type List, and reduce uses type parameters in its declaration:

inline fun <S, T : S> Array<out T>.reduce(
    operation: (acc: S, T) -> S
): S

So the types passed to it must be S and T, and it must return of type S as well. Some people may choose to explicitly declare the type when type inference comes from a diffferent file.

Like you said, Kotlin is so clean! I love it. The perfect blend of concise and safe : )

If you're interested, a great way to learn is to automatically convert a Java file to Kotlin and refer to kotlinlang.org/docs/kotlin-docs.pdf whenever you're editing the file anyway, and have fun optimizing and continuing development without much downtime or needing to start again.