Small error and sl-store-head added.

private/store.rkt

@@ -11,6 +11,7 @@
          sl-store-grep-level
          sl-store-grep
          sl-store-tail
+         sl-store-head
          sl-store-enqueue!
          sl-store->list
          sl-store->display
@@ -105,7 +106,7 @@
        (if (> (queue-length queue) ml)
            (begin
              (dequeue! queue)
-             (set-sl-store-length! store (- sl-store-length store 1))
+             (set-sl-store-length! store (- (sl-store-length store) 1))
              (loop))
            #t))
      )
@@ -195,8 +196,12 @@
                       (queue-length nq)
                       nq))))
 
+
+(define (int>0? a)
+  (and (integer? a) (> a 0)))
+
 (define/contract (sl-store-tail store n)
-  (-> sl-store? number? sl-store?)
+  (-> sl-store? int>0? sl-store?)
   (let* ((q (sl-store-queue store))
          (from (if (>= n (queue-length q))
                    0
@@ -210,5 +215,18 @@
                                   (set! i (+ i 1))
                                   #f))))))
       (make-sl-store* (sl-store-max-length store) (queue-length nq) nq))))
+
+(define/contract (sl-store-head store n)
+  (-> sl-store? int>0? sl-store?)
+  (let* ((q (sl-store-queue store))
+         (until (if (>= n (queue-length q))
+                    (queue-length q)
+                    n))
+         (i 0))
+    (let ((nq (copy-queue q
+                          (λ (item)
+                            (set! i (+ i 1))
+                            (<= i n)))))
+      (make-sl-store* (sl-store-max-length store) (queue-length nq) nq))))