add 'false?'

svn: r9849

collects/lang/private/beginner-funs.ss

@@ -186,6 +186,9 @@
       (boolean=? (boolean boolean -> boolean)
 	"to determine whether two booleans are equal")
 
+      (false? (any -> boolean)
+	"to determine whether a value is false")
+    
       ((beginner-not not) (boolean -> boolean)
 	"to compute the negation of a boolean value"))
    

collects/lang/private/teachprims.ss

@@ -116,6 +116,8 @@ namespace.
   (define (positive-real? v)
     (and (real? v) (>= v 0)))
 
+  (define (false? v) (eq? v #f))
+
   (define-teach beginner not
     (lambda (a)
       (unless (boolean? a)
@@ -336,7 +338,9 @@ namespace.
       (check-last/cycle 'append x)
       (apply append x)))
   
-  (provide beginner-not
+  (provide  
+    false?
+    beginner-not
     beginner-+
     beginner-/
     beginner-*

collects/scheme/bool.ss

@@ -1,6 +1,6 @@
 #lang scheme/base
 
-(provide true false
+(provide true false false?
          boolean=?
          symbol=?)
 
@@ -15,6 +15,8 @@
 (define-constant true #t)
 (define-constant false #f)
 
+(define (false? v) (eq? v #f))
+
 (define (boolean=? x y)
   (unless (and (boolean? x) (boolean? y))
     (raise-type-error 'boolean=? "boolean" (if (boolean? x) 1 0) x y))

collects/scribblings/reference/booleans.scrbl

@@ -135,3 +135,6 @@ Returns @scheme[(equal? a b)].}
 
 Returns @scheme[(equal? a b)].}
 
+@defproc[(false? [v any/c]) boolean?]{
+
+Returns @scheme[(not v)].}