Add in-slice sequences.

Patch from David Vanderson.

collects/tests/unstable/sequence.rkt

@@ -0,0 +1,15 @@
+#lang racket/base
+
+(require rackunit rackunit/text-ui unstable/sequence)
+
+(run-tests
+ (test-suite "sequence.rkt"
+   (check-true (sequence? (in-slice 1 '())))
+   (check-equal? '() (for/list ([v (in-slice 1 '())]) v))
+   (check-equal? '((0 1)) (for/list ([v (in-slice 3 (in-range 2))]) v))
+   (check-equal? '((0 1 2) (3 4 5))
+                 (for/list ([v (in-slice 3 (in-range 6))]) v))
+   (check-equal? '((0 1 2) (3 4 5) (6 7))
+                 (for/list ([v (in-slice 3 (in-range 8))]) v))
+   (check-exn exn:fail:contract?
+              (lambda () (for/list ([x (in-slice 0 (in-range 8))]) x)))))

collects/unstable/scribblings/sequence.scrbl

@@ -43,4 +43,18 @@ Produces the sequence of @racket[f] applied to each element of @racket[seq].
   x)]
 }
 
+
+@addition{David Vanderson}
+
+@defproc[(in-slice [length exact-positive-integer?] [seq sequence?])
+         sequence?]{
+  Returns a sequence where each element is a list with @racket[length]
+  elements from the given sequence.
+
+  @examples[#:eval the-eval
+  (for/list ([e (in-slice 3 (in-range 8))]) e)
+  ]
+}
+
+
 @close-eval[the-eval]

collects/unstable/sequence.rkt

@@ -1,10 +1,12 @@
 #lang racket/base
 
-(require (for-syntax racket/base))
+(require (for-syntax racket/base) racket/contract/base)
+
+;; Added by samth:
 
 (provide in-syntax in-pairs in-sequence-forever sequence-lift)
 
-(define-sequence-syntax in-syntax 
+(define-sequence-syntax in-syntax
   (lambda () #'(lambda (e) (in-list (syntax->list e))))
   (lambda (stx)
     (syntax-case stx ()
@@ -43,3 +45,25 @@
                (lambda (_) (more?))
                (lambda _ #t)
                (lambda _ #t))))))
+
+
+;; Added by stamourv (from David Vanderson (david.vanderson at gmail.com)):
+
+(provide/contract
+ [in-slice (exact-positive-integer? any/c . -> . any)])
+
+(define (in-slice k seq)
+  (unless (sequence? seq) (raise-type-error 'in-slice "sequence" seq))
+  (make-do-sequence
+   (lambda ()
+     (define-values (more? get) (sequence-generate seq))
+     (values
+      (lambda (_)
+        (for/list ((i k)
+                   #:when (more?))
+          (get)))
+      values
+      #f
+      #f
+      (lambda (val) (0 . < . (length val)))
+      #f))))