Higher-order functions

A higher-order function is a function that takes at least one function as an argument, or returns a function as its result. It is fully supported in Kotlin, as functions are first-class citizens. Let's look at it in an example. Let's suppose that we need two functions: a function that will add all BigDecimal numbers from a list, and a function that will get the product (the result of multiplication between all the elements in this list) of all these numbers:

    fun sum(numbers: List<BigDecimal>): BigDecimal { 
        var sum = BigDecimal.ZERO 
        for (num in numbers) { 
            sum += num 
        } 
        return sum 
    } 
 
    fun prod(numbers: List<BigDecimal>): BigDecimal { 
        var prod = BigDecimal.ONE 
        for (num in numbers) { 
            prod *= num 
        } 
        return prod 
    } 
 
    // Usage 
    val numbers = listOf( 
        BigDecimal.TEN,  
        BigDecimal.ONE,  
        BigDecimal.valueOf(2) 
    ) 
...