A practical example – a book publishing system

Let’s examine an example that demonstrates functional programming concepts using a book publishing system scenario:

public record Book(string Title, string Author, int Year, string Content);
// Pure function to validate a book
private static bool IsValid(Book book) =>
     !string.IsNullOrEmpty(book.Title) &&
     !string.IsNullOrEmpty(book.Author) &&
     book.Year > 1900 &&
     !string.IsNullOrEmpty(book.Content);
// Pure function to format a book
private static string FormatBook(Book book) =>
     $"{book.Title} by {book.Author} ({book.Year})";
// Higher-order function for processing books
private static IEnumerable<T> ProcessBooks<R>(
     IEnumerable<Book> books,
     Func<Book, bool> validator,
     Func<Book, T> formatter) =>
     books.Where(validator).Select(formatter);
public static void Main()
{
    var books = new List<Book>
         {
              new Book("The Great Gatsby", "F. Scott Fitzgerald", 1925, "In my younger and more vulnerable years..."),
              new Book("To Kill a Mockingbird", "Harper Lee", 1960, "When he was nearly thirteen, my brother Jem..."),
              new Book("Invalid Book", "", 1800, ""),
              new Book("1984", "George Orwell", 1949, "It was a bright cold day in April, and the clocks were striking thirteen.")
         };
    // Using our higher-order function to process books
    var formattedBooks = ProcessBooks(books, IsValid, FormatBook);
    Console.WriteLine("Processed books:");
    foreach (var book in formattedBooks)
    {
         Console.WriteLine(book);
    }
}

This example demonstrates several key functional programming concepts:

• Immutability: We use record for the Book type, which is immutable by default.
• Pure functions: IsValid and FormatBook are pure functions. They always return the same output for the same input and have no side effects.
• Higher-order function: ProcessBooks is a higher-order function that takes two functions as parameters (a validator and a formatter).
• Composability: We compose the validation and formatting operations in the ProcessBooks function.
• Declarative style: We describe what we want (valid, formatted books) rather than how to do it step by step.
• LINQ: We use LINQ methods (Where and Select) that align well with functional programming principles.

This example shows how functional programming can be applied to a real-world scenario such as a book publishing system.