Lisp was invented in 1958, which makes it the second-oldest computer programming language. It has spawned several modern derivatives, including Common Lisp, Emacs Lisp (Elisp), Clojure, Racket, Scheme, Fennel, and GNU Guile.
People who love thinking about the design of programming languages often love Lisp because of how its syntax and data share the same structure: Lisp code is essentially a list of lists, and its name is an acronym for LISt Processing. People who love thinking about the aesthetics of programming languages often hate Lisp because of its frequent use of parentheses for scoping; in fact, it's a common joke that Lisp stands for Lots of Irritating Superfluous Parentheses.
Whether you love or hate its design philosophies, Lisp is an interesting glimpse at the past and, thanks to Clojure and Guile, into the future. You might be surprised how much Lisp code there is lurking within big codebases in any given industry, so it's a good idea to have at least a passing familiarity with the language.
There are many implementations of Lisp. Popular open source versions include SBCL, GNU Lisp, and GNU Common Lisp (GCL). You can install any of these with your distribution's package manager, but for this article I use
On Fedora Linux:
$ sudo dnf install clisp
$ sudo apt install clisp
$ sudo port install clisp
Even though I'm using the
clisp command, most of the principles in this article apply to any Lisp. Should you choose to use a different Lisp implementation, the command to run Lisp code is different from what I use in this article (
sbcl instead of
clisp, for example) but everything else is the same.
The basic unit of Lisp source code is an expression, which is written as a list. For instance, this is a list of an operator (
+) and two integers (
(+ 1 2)
It's also a Lisp expression, using a symbol (
+) that evaluates to a function (addition) and two arguments (
2). You can run this expression and others in an interactive Common Lisp environment called REPL (read-eval-print loop). If you're familiar with Python's IDLE, Lisp's REPL should feel somewhat familiar to you.
To launch a REPL, launch Common Lisp:
At the REPL prompt, type a few expressions:
> (+ 1 2)
> (- 1 2)
> (- 2 1)
> (+ 2 3 4)
Now that you know the basic structure of a Lisp expression, you can utilize Lisp functions in useful ways. The
pprint function "pretty" prints it. There are other variations on the print function, but
pprint is nice in REPL:
> (pprint "hello world")
You can create your own functions with
defun function requires a name for your function and any parameters you want your function to accept:
> (defun myprinter (s) (pprint s))
> (myprinter "hello world")
You can create variables in Lisp with
> (setf foo "hello world")
> (pprint foo)
You can nest expressions within expressions in a kind of pipeline. For instance, you can pretty print the contents of your variable after invoking the
string-upcase function to convert its characters to uppercase:
> (pprint (string-upcase foo))
Lisp is dynamically typed in the sense that you don't have to declare variable types when setting them. Lisp treats integers as integers by default:
> (setf foo 2)
> (setf bar 3)
> (+ foo bar)
If you intend for an integer to be interpreted as a string, you can quote it:
> (setf foo "2")
> (setf bar "3")
> (+ foo bar)
*** - +: "2" is not a number
The following restarts are available:
USE-VALUE :R1 Input a value to be used instead.
ABORT :R2 Abort main loop
Break 1 >
In this sample REPL session, both
bar are set to quoted numbers, so Lisp interprets them as strings. Math operators can't be used on strings, so REPL drops into a debugger mode. To get out of the debugger, press Ctrl+D on your keyboard.
You can do some introspection on objects using the
typep function, which tests for a specific data type. The tokens
NIL represent True and False, respectively.
> (typep foo 'string)
> (typep foo 'integer)
The single quote (
integer prevents Lisp from (incorrectly) evaluating those keywords as variables:
> (typep foo string)
*** - SYSTEM::READ-EVAL-PRINT: variable STRING has no value
It's a shorthand way to protect the terms, normally done with the
> (typep foo (quote string))
> (typep foo (quote integer))
Unsurprisingly, you can also create lists in Lisp:
> (setf foo (list "hello" "world"))
Lists can be indexed with the
> (nth 0 foo)
> (pprint (string-capitalize (nth 1 foo)))
To end a REPL session, press Ctrl+D on your keyboard, or use the
quit keyword in Lisp:
Lisp can be compiled or used as an interpreted scripting language. The latter is probably the easiest option when you're starting, especially if you're already familiar with Python or shell scripting.
Here's a simple dice roller script written in GNU Common Lisp:
(defun roller (num)
(pprint (random (parse-integer (nth 0 num))))
(setf userput *args*)
(setf *random-state* (make-random-state t))
The first line tells your POSIX terminal what executable to use to run the script.
roller function, created with
defun, uses the
random function to print a pseudo-random number up to, and not including, the zeroth item of the
num list. The
num list hasn't been created yet in the script, but the function doesn't get executed until it's called.
The next line assigns any argument provided to the script at launch time to a variable called
userput variable is a list, and it's what becomes
num once it's passed to the
The penultimate line of the script starts a random seed. This provides Lisp with enough entropy to generate a mostly random number.
The final line invokes the custom
roller function, providing the
userput list as its sole argument.
Save the file as
dice.lisp and mark it executable:
$ chmod +x dice.lisp
Finally, try running it, providing it with a maximum number from which to choose its random number:
$ ./dice.lisp 21
$ ./dice.lisp 21
$ ./dice.lisp 21
You may notice that your emulated die has a pontential value of 0, and never reaches the maximum number you provide to it as an argument. In other words, this script never rolls 20 on a 20-sided die (unless you count 0 as 20). There's a simple fix to this, and you only need the knowledge gained from this article to do it. Can you fix this bug?
Whether you can imagine using Lisp as a utilitarian language for personal scripts, to advance your career, or just as a fun experiment, you can see some particularly inventive uses at the annual Lisp Game Jam (most submissions are open source, so you can view the code to learn from what you play).
Lisp is a fun and unique language with an ever-growing developer base and enough historic and emerging dialects to keep programmers from all disciplines happy.