Kotlin的reduce()函数具有不同的类型 [英] Kotlin's reduce() function with different types

问题描述

我正在浏览数组扩展功能，发现reduce()一个

I was looking through array extension functions and found reduce() one

inline fun <S, T: S> Array<out T>.reduce(operation: (acc: S, T) -> S): S {
    if (isEmpty())
        throw UnsupportedOperationException("Empty array can't be reduced.")
    var accumulator: S = this[0]
    for (index in 1..lastIndex) {
        accumulator = operation(accumulator, this[index])
    }
    return accumulator
}

此处是类型S的accumulator变量，该变量分配了类型为T的数组中的第一个元素.

here the accumulator variable of type S assigned with first element from the array with type T.

不能用两种数据类型来解决reduce()函数的实际用例.这里的综合示例实际上没有任何意义.

Can't wrap my head around the real use case of reduce() function with two data types. Here synthetic example which actually doesn't make any sense.

open class A(var width: Int = 0)
class B(width: Int) : A(width)

val array = arrayOf(A(7), A(4), A(1), A(4), A(3))
val res = array.reduce { acc, s -> B(acc.width + s.width) }

似乎大多数具有此功能的现实生活用例都使用以下签名:

Seems most real life use cases with this function use this signature:

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

您能否提供一些示例，其中reduce()函数在不同类型上可能有用.

Can you help with providing some examples, where reduce() function can be useful with different types.

推荐答案

以下是示例:

interface Expr {
    val value: Int
}

class Single(override val value: Int): Expr

class Sum(val a: Expr, val b: Expr): Expr {
    override val value: Int
        get() = a.value + b.value
}

fun main(args: Array<String>) {
    val arr = arrayOf(Single(1), Single(2), Single(3));
    val result = arr.reduce<Expr, Single> { a, b -> Sum(a, b) }
    println(result.value)
}

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