Asynchronous functions in functional programming

Now, using the chaining method, we are going to use the async and await keywords in functional programming. Suppose we have three tasks, as shown in the following code snippet, and we need to chain them together:

public async static Task<int> FunctionA( 
  int a) => await Task.FromResult(a * 1); 
public async static Task<int> FunctionB( 
  int b) => await Task.FromResult(b * 2); 
public async static Task<int> FunctionC( 
  int c) => await Task.FromResult(c * 3); 

For that purpose, we have to create a new extension method for Task<T> named MapAsync, with the following implementation:

public static class ExtensionMethod 
{ 
  public static async Task<TResult> MapAsync<TSource, TResult>( 
    this Task<TSource> @this, 
    Func<TSource, Task<TResult>> fn) => await fn(await @this); 
} 

The MapAsync() method allows us to define...