Common Lisp

Being that Common Lisp is just a set of standards defined for the language there are in fact many different implementations you can install. Due to the sheer number of implementations, it can be quite confusing where to start. Here's a small list of some of the them:

• SBCL
• CLISP
• Allegro CL
• LispWorks
• Clozure
• ECL
• CMUCL

Of these sbcl is probably the most popular and commonly used implementation and if you are just here to learn how to program with lisp may just be the quickest starting point for you.

We will start looking at a simple hello world program, not just because its a staple, but because it gives good insight into some of common lisp's syntax. So the most simple way to make a hello world "program" is to just write "Hello World" because again everything in lisp returns a value. For a more in-depth example:

(defun hello-world ()
  (format t "hello world"))

With that defined we can now run our hello world function with: (hello-world) which will print hello world. So as you may have guessed defun is how we define functions in lisp. defun its self is a function that accepts a function name as a its first argument and a list of a arguments for the function we are creating as its second argument. All arguments after that are considered the body of the function. We also used the format function for our hello world function. The format function is kind of a complicated function to describe, but our use case here isn't too hard to explain. the format function can have multiple arguments based to it but at a minimum it expects 2. The first argument is used to define where to print its output. In this case t is used which is shorthand for *standard-output*. The second argument format expects a string to format. You can have just direct text or you can supplement in variables with a syntax similar to printf in C. For more information on format please check the syntax specification docs: format.

One more basic thing to learn is that obviously programming in the repl isn't exactly ideal for full blown development. With that said although I'm not going to cover various development workflows as I use emacs with the sly package installed and everyones perfect development environment is different, but you can at least save your code in a file and run (load <filename>) to load your program into the repl. It is most common to end your lisp files in .lisp, but you can technically go with what ever you want; some people use .cl for common lisp.

In common lisp you use (defvar <variable_name> <initial_value>) to declare a variable and set and initial value for this variable. It is also important to note that to declare a global variable it is convention to use *'s around your variable name to signify it as a global variable: (defvar \*global\* 5).