@@ -1,7 +1,8 @@

 (defpackage :data-lens.lenses

   (:shadow :set)

   (:use :cl)

-  (:export :over :set :view :make-alist-lens :make-plist-lens :make-hash-table-lens))

+  (:export :over :set :view :make-alist-lens :make-plist-lens :make-hash-table-lens

+           :make-list-lens))

 (in-package :data-lens.lenses)

 #+fw.dev

@@ -168,6 +169,16 @@ contain the new value at the location focused by the lens."

                                  :from-end t))

             (funcall cb (serapeum:assocdr key alist))))))

+(defun make-list-lens (index)

+  "A lens for updating a sequence"

+  (lambda (cb)

+    (lambda (seq)

+      (fmap (lambda (new)

+              (let ((result (copy-seq seq)))

+                (prog1 result

+                  (setf (elt result index) new))))

+            (funcall cb (elt seq index))))))

+

 (defun make-plist-lens (key)

   "A lens for updating a plist, preserving previous values"

   (lambda (cb)