Cannot have first parameter as optional

Question

Can you have a function with and optional parameter as the first and only parameter?

thus

(defn foo [& bar] (if (= bar) 1 2))

Answer 1

What & bar means at function definition is that all the rest of the arguments will be put into a list. It does not give any guarantees as to the size of the list, so it could be empty, with one or more items.

A better approach to having one and only optional argument, is to have it accept zero or one arguments:

(defn foo
   ([]    (foo 12))
   ([bar] (if (= bar 12) 1 2)))

In this example, if you call the first function definition, with zero arity, it will simply call the second 1-arity function definition with a default value of 12.