Common Lisp Cheat Sheet

Essential Common Lisp syntax covering function and variable definitions, control flow, list operations, formatted output, and macros.

2 PagesIntermediateMar 25, 2026

Functions & Variables

Defining functions and global/local bindings.

lisp

(defun square (x)  (* x x))(defvar *count* 0)          ; global, special variable(defparameter *name* "Lisp") ; global, expects reassignment(let ((x 1) (y 2))          ; local bindings  (+ x y))(setf *count* (+ *count* 1))

Control Flow

Conditionals and iteration constructs.

lisp

(if (> x 0) "positive" "negative")(cond ((> x 0) "positive")      ((< x 0) "negative")      (t "zero"))(when (> x 0)  (print "positive"))(unless (> x 0)  (print "not positive"))(loop for i from 1 to 5      do (print i))

List Operations

Core functions for working with lists.

(car lst)- returns the first element of a list
(cdr lst)- returns the list minus its first element
(cons a b)- constructs a new cons cell
(list a b c)- creates a new list from arguments
(mapcar #'fn lst)- applies fn to each element, returns a new list
(append lst1 lst2)- concatenates lists
(reverse lst)- returns a reversed copy of the list
(nth n lst)- returns the nth element (zero-indexed)

Format & I/O

The format function's most common directives.

lisp

(format t "Hello, ~a!~%" "World")   ; ~a = aesthetic, ~% = newline(format t "~d + ~d = ~d~%" 1 2 3)   ; ~d = decimal integer(format t "~s~%" "quoted")          ; ~s = write with slashify(format t "~,2f~%" 3.14159)         ; ~,2f = float, 2 decimal places

Macros

Defining simple code-transforming macros.

lisp

(defmacro my-if (test then else)  `(cond (,test ,then)         (t ,else)))(my-if (> 5 3) (print "yes") (print "no"))(defmacro while (test &body body)  `(loop while ,test         do (progn ,@body)))

Pro Tip

Use `defparameter` (not `defvar`) when you want redefining the source file to actually update the variable's value — `defvar` only sets it if it is not already bound, which is convenient for state but confusing during iterative development.

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