Building a Simple Application in Lisp

June 28, 2024

Welcome to the final post of our Lisp course! In this post, we will bring together all the concepts we’ve covered so far and guide you through building a simple application using Lisp. It’s time to apply your knowledge and create a functional program from scratch.

Setting Up the Environment

Before we start coding our application, make sure you have a Lisp environment set up on your system. You can use an online REPL like repl.it or install a Lisp interpreter locally.

Defining the Problem

For our simple application, let’s create a program that calculates the factorial of a given number. We will define a function that takes an integer as input and returns its factorial.

Writing the Code

Let’s start by defining our factorial function:

(defun factorial (n)
  (if (= n 0)
      1
      (* n (factorial (- n 1))))
)

Next, we can prompt the user for input and call our factorial function:

(format t "Enter a number: ")
(finish-output)
(let ((num (parse-integer (read-line))))
  (format t "The factorial of ~A is ~A" num (factorial num))
)

Running the Application

Now that we have written our code, load it into your Lisp environment and run the program. You should see a prompt asking for a number, and the program will output the factorial of that number.

Conclusion

Congratulations on building your first Lisp application! This simple example demonstrates how you can leverage the concepts of Lisp to create functional programs. Feel free to explore more complex applications and continue learning about Lisp.