Thinking in Scala (6) ---- High-Order Function ---- returns a function

In thinking in Scala (5) ---- High-Order Function *, we demonstrate how to pass a function as a parameter to another function.

In this article, we will demonstrate another important feature of functional programming: return a function. First, let's look at the following code:

Code piece 1:

def sum(f:Int=>Int):(Int,Int)=>Int={       def sumF(a:Int,b:Int):Int=         if(a>b) 0         else f(a)+sumF(a+1,b)       sumF   }

In a bit, F: Int => int is a parameter received by the sum function, which is a function.

":" (INT, INT) => Int Is the return value of the sum function, because (INT, INT) => int represents an "anonymous function ",

That is, to call the sum function, the returned value is a function. This is applicable to programming languages such as C, C ++, and Java.

It is hard for programmers to understand.

Let's continue with the example. What will happen if we execute the following line of code?

sum(x=>x*x)(1,4)

30 is returned. Why is it 30? 30 = 1 ^ 2 + 2 ^ 2 + 3 ^ 2 + 4 ^ 2.

First, sum (x => X * X) is a function, and the sum (x => X * X) type is (INT, INT) => int, because sum (x => X * X) is a function,

So it can continue to receive parameters ).

Well, I may not have explained this clearly. It is too abstract.

Thinking in Scala (6) ---- High-Order Function ---- returns a function