racket/set: add set-first' and set-rest'

collects/racket/set.rkt

@@ -3,12 +3,14 @@
          racket/serialize
          racket/pretty
          racket/contract/base
-         racket/contract/combinator)
+         racket/contract/combinator
+         (only-in "private/for.rkt" prop:stream))
 
 (provide set seteq seteqv
          set? set-eq? set-eqv? set-equal?
          set-empty? set-count
          set-member? set-add set-remove
+         set-first set-rest
          set-union set-intersect set-subtract set-symmetric-difference
          subset? proper-subset?
          set-map set-for-each 
@@ -84,7 +86,10 @@
                                 (=? (set-ht set1) (set-ht set2)))
                               (lambda (set hc) (add1 (hc (set-ht set))))
                               (lambda (set hc) (add1 (hc (set-ht set)))))
-  #:property prop:sequence (lambda (v) (*in-set v)))
+  #:property prop:sequence (lambda (v) (*in-set v))
+  #:property prop:stream (vector (lambda (s) (set-empty? s))
+                                 (lambda (s) (set-first s))
+                                 (lambda (s) (set-rest s))))
 
 ;; Not currently exporting this because I'm not sure whether this is the right semantics
 ;; for it yet, but it follows most closely the semantics of the old set/c implementation
@@ -265,6 +270,20 @@
 (define (proper-subset? one two)
   (subset* 'proper-subset? one two #t))
 
+(define (set-first set)
+  (unless (set? set) (raise-argument-error 'set-first "set?" set))
+  (define ht (set-ht set))
+  (if (zero? (hash-count ht))
+      (raise-arguments-error 'set-first "given set is empty")
+      (hash-iterate-key ht (hash-iterate-first ht))))
+
+(define (set-rest set)
+  (unless (set? set) (raise-argument-error 'set-rest "set?" set))
+  (define ht (set-ht set))
+  (if (zero? (hash-count ht))
+      (raise-arguments-error 'set-rest "given set is empty")
+      (make-set (hash-remove ht (hash-iterate-key ht (hash-iterate-first ht))))))
+
 (define (set-map set proc)
   (unless (set? set) (raise-argument-error 'set-map "set?" 0 set proc))
   (unless (and (procedure? proc)

collects/scribblings/reference/sequences.scrbl

@@ -635,7 +635,7 @@ in the sequence.
 @; ======================================================================
 @section[#:tag "streams"]{Streams}
 
-A @deftech{stream} is a kind of sequence that supports functional
+A @deftech{stream} is a kind of @tech{sequence} that supports functional
 iteration via @racket[stream-first] and @racket[stream-rest]. The
 @racket[stream-cons] form constructs a lazy stream, but plain lists
 can be used as stream, and functions such as @racket[in-range] and

collects/scribblings/reference/sets.scrbl

@@ -11,9 +11,10 @@ set, elements are equivalent via @racket[equal?], @racket[eqv?], or
 element-comparison procedure (@racket[equal?], @racket[eqv?], or
 @racket[eq?]) and have equivalent elements.
 
-A set can be used as a single-valued sequence (see
-@secref["sequences"]). The elements of the set serve as elements
-of the sequence. See also @racket[in-set].
+A set can be used as a @tech{stream} (see @secref["streams"]) and thus
+as a single-valued @tech{sequence} (see @secref["sequences"]). The
+elements of the set serve as elements of the stream or sequence. See
+also @racket[in-set].
 
 Operations on sets that contain elements that are mutated are
 unpredictable in much the same way that @tech{hash table} operations are
@@ -50,6 +51,17 @@ Returns @racket[#t] if @racket[v] is in @racket[st], @racket[#f]
 otherwise.}
 
 
+@defproc[(set-first [st (and/c set? (not/c set-empty?))]) any/c]{
+
+Produces an unspecified element of @racket[st]. Multiple uses of
+@racket[set-first] on @racket[st] produce the same result.}
+
+
+@defproc[(set-rest [st (and/c set? (not/c set-empty?))]) set?]{
+
+Removes @racket[(set-first st)] from @racket[st].}
+
+
 @defproc[(set-add [st set?] [v any/c]) set?]{
 
 @margin-note{Like operations on immutable hash tables, ``constant

collects/tests/racket/set.rktl

@@ -34,6 +34,10 @@
 (test #t set-member? (set 1 2 3) 3)
 (test #f set-member? (set 1 2 3) 4)
 
+(test #t stream? (set 1 2 3))
+(test (set-first (set 1 2 3)) set-first (set 1 2 3))
+(test (set-remove (set 1 2 3) (set-first (set 1 2 3))) set-rest (set 1 2 3))
+
 (let ([s (set 1 2 3)])
   (test #t equal? s (set-add (set-add (set-add (set) 1) 2) 3))
   (test #t equal? (seteq 1 2 3) (seteq 1 2 3))

doc/release-notes/racket/HISTORY.txt

@@ -1,6 +1,7 @@
 Version 5.3.0.24
 Added PLTCOMPILEDROOTS and --compiled/-R command-line flag
 Added `reroot-path'
+racket/set: added set-first and set-rest, sets are streams
 
 Version 5.3.0.23
 Changed make-log-receiver to accept a logger name as an