Conditionals

• The basic macro to know: if

(if test
    (progn
    then
    continuing)
    else)

• If you don't need an else clause, then you should use when instead. It also has an implicit progn.

(when test
    then
    continuing)

• When you want something like when (not test), use unless. It is the contrary of when.

(unless test 
    then
    continuing)

• At some point, when you have some imbricated ifs, you can use cond.

(cond 
    (test
    logic
    logic2)
    (test 2
    logic)
    (t :default))

• There is also case.

(let ((a 1))
  (case a
    (1 :its1)
    (t :default)))

• There is another feature that helps you do conditionals, but a bit higher level, it's the *features* variables.

(push :udemy *features*)

(defun test ()
  #+unix 
  (print "we are on Linux"))

• If you use something that cannot be truthy, like NIL:

#+nil (assert (= 2 (+ 1 1)))

• and and or can be used as shortcuts for conditionals.

(if :truthy
    :truthy
    :falsy)

;; can be shortened to:

(or :truthy
    :falsy)