3.2. conditionals

if can be used to make choices at evaltime. It takes an evaltime constant boolean as its first argument and then either evaluates its second argument (the ‘then’ expression, if the condition was truthy) or its third argument (the ‘else’ expression, if it wasn’t). The ‘else’ expression can be omitted if it is not needed.

(def print-first true)
(print (if print-first
  "First!"
  "Second :("))

#(prints nothing)
(if false
  (print "Another message"))

(empty)

If multiple expressions (with side effects) need to be switched, when can be used instead: it evaluates all arguments as a block, only if the condition is truthy:

(def enable-things true)

(when enable-things
  (print "the things are enabled.")
  (print "they should happen soon.")
  (print "thank you for enabling them."))

the things are enabled.
they should happen soon.
thank you for enabling them.

This is equivalent to using a do block inside the if expression:

(def enable-things true)

(if enable-things
  (do
    (print "the things are enabled.")
    (print "they should happen soon.")
    (print "thank you for enabling them.")))