t3x.org / sketchy / library / booleanp.html
SketchyLISP
Reference
  Copyright (C) 2006
Nils M Holm

boolean?

Conformance: R5RS

Purpose: Check whether an expression x is a truth value. Return #t for x = #t and x = #f and otherwise #f.

Arguments:
X - expression

Implementation:

(define (boolean? x)
  (or (eq? x #t)
      (eq? x #f)))

Example:

(boolean? #f) 
=> #t

See also:
list?, null?.