add set-get-one' and set-get-one/rest'

collects/racket/set.rkt

@@ -9,6 +9,7 @@
          set? set-eq? set-eqv? set-equal?
          set-empty? set-count
          set-member? set-add set-remove
+         set-get-one set-get-one/rest
          set-union set-intersect set-subtract set-symmetric-difference
          subset? proper-subset?
          set-map set-for-each 
@@ -147,6 +148,15 @@
   (unless (set? set) (raise-type-error 'set-remove "set" 0 set v))
   (make-set (hash-remove (set-ht set) v)))
 
+(define (set-get-one set)
+  (unless (set? set) (raise-type-error 'set-some-element "set" 0 set))
+  (for/first ([e (set-ht set)]) e))
+
+(define (set-get-one/rest set)
+  (unless (set? set) (raise-type-error 'set-some-element "set" 0 set))
+  (let ((element (for/first ([e (set-ht set)]) e)))
+    (values element (set-remove set element))))
+
 (define set-union
   (case-lambda
    ;; No 0 argument set exists because its not clear what type of set

collects/scribblings/reference/sets.scrbl

@@ -65,6 +65,17 @@ Produces a set that includes @racket[v] plus all elements of
 Produces a set that includes all elements of @racket[st] except
 @racket[v]. This operation runs in constant time.}
 
+@defproc[(set-get-one [st set?]) any/c]{
+Produces a random element from @racket[st]. This procedure is not guaranteed to
+produce the same element if it is called again with the same set. This operation
+runs in constant time.}
+
+@defproc[(set-get-one/rest [st set?]) (values any/c set?)]{
+
+Produces a random element from @racket[st] and a new set that does not contain
+that element. This procedure is not guaranteed to produce the same element if it
+is called again with the original set. This operation runs in constant time.}
+
 
 @defproc[(set-union [st set?] ...+) set?]{
 

collects/tests/racket/set.rktl

@@ -50,6 +50,12 @@
   (test #t set-member? (set-remove s 5) 3)
   (test #f set-member? (set-remove s 3) 3)
 
+  (test #t 'set-get-one (set-member? s (set-get-one s)))
+
+  (test #t 'set-get-one/rest (set-member? s (let-values (((e _) (set-get-one/rest s))) e)))
+  (test #f 'set-get-one/rest (let-values (((e s-no-e) (set-get-one/rest s)))
+                               (set-member? s-no-e e)))
+
   (test #t subset? (set 1 3) s)
   (test #t subset? (set 1 2 3) s)
   (test #f subset? (set 1 4) s)