not
Category: Boolean procedures
;;; (not val) -> boolean?
;;; val : any?
;;; If `val` is false (`#f`), return true (`#t`). If `val` is anything
;;; else, return false.
Unlike and and or, which are keywords, not is a procedure.
> (not #t)
#f
> (not #f)
#t
> (not 3)
#f
> (not "false")
#f
> (not)
. . not: arity mismatch;
the expected number of arguments does not match the given number
expected: 1
given: 0
> (not 1 2)
. . not: arity mismatch;
the expected number of arguments does not match the given number
expected: 1
given: 2
Why isn’t not a keyword?
Because it doesn’t have to be. That is, it has the normal evaluation strategy. We must evaluate the one parameter before we can decide what to return.