So, I really dislike truthiness and falsiness in other languages but somehow, in Lisp, it just seems to work. One thing that helps is that, in places where the

So, I really dislike truthiness and falsiness in other languages but somehow, in Lisp, it just seems to work. One thing that helps is that, in places where the distinction between null and false matters, other features of lisp help keep them distinct: so, for example, getting a value from a hash table returns nil if the value is not found and, consequently, you can't distinguish a missing key from a stored null. GETHASH solves this by returning multiple values: the first is the value stored (if there is one) while the second indicates whether or not the key was found in the hash table. Similarly, optional arguments [declared like (defun foo (&optional bar))] default to null but, if you want to no whether a value was actually passed, you can change the argument declaration to account for this: (defun foo (&optional (bar nil bar-p))) In this case, when bar isn't passed, bar-p will b nil but when bar is passed, it will be t.