strengthen typing on `shift`

typed/sugar/list.rkt

@@ -166,25 +166,25 @@
 
 
 (define/typed+provide shift
-  (case-> ((Listof Any) (U Integer (Listof Integer)) -> (Listof Any))
-          ((Listof Any) (U Integer (Listof Integer)) Any -> (Listof Any))
-          ((Listof Any) (U Integer (Listof Integer)) Any Boolean -> (Listof Any)))  
+  (All (A) (case-> ((Listof (Option A)) (U Integer (Listof Integer)) -> (U (Listof (Option A)) (Listof (Listof (Option A)))))
+                   ((Listof (Option A)) (U Integer (Listof Integer)) (Option A) -> (U (Listof (Option A)) (Listof (Listof (Option A)))))
+                   ((Listof (Option A)) (U Integer (Listof Integer)) (Option A) Boolean -> (U (Listof (Option A)) (Listof (Listof (Option A)))))))  
   (case-lambda
     [(xs shift-amount-or-amounts)
      (shift xs shift-amount-or-amounts #f #f)]
     [(xs shift-amount-or-amounts fill-item)
      (shift xs shift-amount-or-amounts fill-item #f)]
     [(xs shift-amount-or-amounts fill-item cycle)
-     (define/typed (do-shift xs how-far)
-       ((Listof Any) Integer -> (Listof Any))
+     (define/typed (do-shift xs fill-item how-far)
+       (All (A2) ((Listof (Option A2)) (Option A2) Integer -> (Listof (Option A2))))
        (define abs-how-far (abs how-far))
        (cond 
          [(> abs-how-far (length xs)) (error 'shift "index is too large for list\nindex: ~a\nlist: ~v" how-far xs)]
          [(= how-far 0) xs]
          [(positive? how-far)
-          (append  (make-list abs-how-far fill-item) (drop-right xs abs-how-far))]
+          ((inst append (Option A2)) ((inst make-list (Option A2)) abs-how-far fill-item) (drop-right xs abs-how-far))]
          ;; otherwise how-far is negative
          [else  (append (drop xs abs-how-far) (make-list abs-how-far fill-item))]))
      (if (list? shift-amount-or-amounts)
-         (map (λ:([amount : Integer]) (do-shift xs amount)) shift-amount-or-amounts)
-         (do-shift xs shift-amount-or-amounts))]))
+         ((inst map (Listof (Option A)) Integer) (λ:([amount : Integer]) (do-shift xs fill-item amount)) shift-amount-or-amounts)
+         (do-shift xs fill-item shift-amount-or-amounts))]))