current-future now returns #f if not inside a future, instead of null. Added documentation for current-future.

collects/scribblings/reference/futures.scrbl

@@ -66,6 +66,13 @@ in parallel. See also @guidesecref["effective-futures"].
 (let ([f (future (lambda () (+ 1 2)))])
   (list (+ 3 4) (touch f)))
 ]}
+  
+  
+@defproc[(current-future) any]{
+  Returns the descriptor for the future that is evaluating the current thunk.
+  If not currently running inside a future thunk (or 
+  futures are disabled), returns @racket[#f].
+}
 
 
 @defproc[(future? [v any/c]) boolean?]{

collects/tests/future/future.rkt

@@ -149,8 +149,7 @@ We should also test deep continuations.
          (continuation-mark-set->list (current-continuation-marks) 'x))))
 
 ;Tests for current-future
-(check-equal? '() (current-future)) 
-(check-equal? #t (null? (current-future)))
+(check-equal? #f (current-future)) 
 (check-equal? #t (equal? (current-future) (current-future)))
 
 (let ([f (future (λ () (current-future)))])