Functional Programming with C#

Chapter 5. Higher-Order Functions

Welcome back, my friends, to the show that never ends. In this chapter, we’ll look at uses for higher-order functions. I’m going to show you novel ways to use them in C# to save yourself effort and to make code that is less likely to fail.

But, what are higher-order functions? This slightly odd name represents something very simple. In fact, you’ve likely been using higher-order functions for some time if you’ve spent much time working with LINQ. They come in two flavors; here’s the first:

var liberatorCrew = new []
{
    "Roj Blake",
    "Kerr Avon",
    "Vila Restal",
    "Jenna Stannis",
    "Cally",
    "Olag Gan",
    "Zen"
};
var filteredList = liberatorCrew.Where(x => x.First() > 'M');

Passed into the Where() function is an arrow expression, which is just shorthand for writing out an unnamed function. The longhand version would look like this:

function bool IsGreaterThanM(char c)
{
    return c > 'm';
}

So here, the function has been passed around as the parameter to another function, to be executed elsewhere inside it.

This is another example of the use of higher-order functions:

public Func<int, int> MakeAddFunc(int x) => y => x + y;

Notice here that there are two arrows, not one. We’re taking an integer x and from that returning a new function. In that new function, references to x will be filled in with whatever was provided when MakeAddFunc() was called originally.

For example:

var addTenFunction = MakeAddFunc(10);
var answer = addTenFunction(5);
// answer is 15 ...