diff --git a/lens-data/lens/private/list/improper.rkt b/lens-data/lens/private/list/improper.rkt
new file mode 100644
index 0000000..9c119b9
--- /dev/null
+++ b/lens-data/lens/private/list/improper.rkt
@@ -0,0 +1,129 @@
+#lang racket/base
+
+(provide list-proper-lens
+         list-improper-lens)
+
+(require racket/contract
+         lens/private/base/contract
+         lens/common)
+
+(module+ test
+  (require rackunit lens/common lens/private/test-util/test-lens))
+
+;; (-> (Rec R (U (Pairof A R) A)) (Pairof A (Listof A))
+(define (make-proper l)
+  (if (pair? l)
+      (cons (car l)
+            (make-proper (cdr l)))
+      (list l)))
+
+;; (-> (Pairof A (Listof A)) (Rec R (U (Pairof A R) A)))
+(define (make-improper l)
+  (if (null? (cdr l))
+      (car l)
+      (cons (car l)
+            (make-improper (cdr l)))))
+
+(define improper-contract
+  (flat-rec-contract r
+    (cons/c any/c (or/c r (not/c pair?)))))
+
+(define proper-contract
+  (flat-rec-contract r
+    (cons/c any/c (listof any/c))))
+
+(define/contract list-proper-lens
+  (lens/c improper-contract
+          proper-contract)
+  (make-lens make-proper
+             (λ (target view)
+               (unless (= (length (make-proper target)) (length view))
+                 (raise-argument-error 'stx-flatten/depth-lens
+                                       (format "a list of length ~v"
+                                               (length (make-proper target)))
+                                       1
+                                       target
+                                       view))
+               (make-improper view))))
+
+(define/contract list-improper-lens
+  (lens/c proper-contract
+          improper-contract)
+  (make-lens make-improper
+             (λ (target view)
+               (unless (= (length (make-proper view)) (length target))
+                 (raise-argument-error 'list-improper-lens
+                                       (format "an improper list of length ~v (plus the last element)"
+                                               (length target))
+                                       1
+                                       target
+                                       view))
+               (make-proper view))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(module+ test
+  (test-case "list-proper-lens and list-improper-lens: lens laws"
+    (test-lens-laws list-proper-lens
+                    (list* 'x 'y 'z)
+                    (list 'a 'b 'c)
+                    (list "a" "b" "c"))
+    )
+  (test-case "list-proper-lens and list-improper-lens: lens-view"
+    (check-equal? (lens-view list-proper-lens (list* 'x 'y 'z))
+                  (list 'x 'y 'z))
+    (check-equal? (lens-view list-proper-lens (list 'x 'y 'z))
+                  (list 'x 'y 'z '()))
+    (check-equal? (lens-view list-improper-lens (list 'x 'y 'z))
+                  (list* 'x 'y 'z))
+    (check-equal? (lens-view list-improper-lens (list 'x 'y 'z '()))
+                  (list 'x 'y 'z))
+    )
+  (test-case "list-proper-lens and list-improper-lens: lens-set"
+    (check-equal? (lens-set list-proper-lens
+                            (list* 'x 'y 'z)
+                            (list 1 2 3))
+                  (list* 1 2 3))
+    (check-equal? (lens-set list-proper-lens
+                            (list 'x 'y)
+                            (list 1 2 3))
+                  (list* 1 2 3))
+    (check-equal? (lens-set list-improper-lens
+                            (list 'x 'y 'z)
+                            (list* 1 2 3))
+                  (list 1 2 3))
+    (check-equal? (lens-set list-improper-lens
+                            (list 'x 'y '())
+                            (list* 1 2 3))
+                  (list 1 2 3))
+    (check-equal? (lens-set list-improper-lens
+                            (list 'x 'y 'z)
+                            (list 1 2))
+                  (list 1 2 '()))
+    )
+  (test-case "list-proper-lens and list-improper-lens: lens-set length check"
+    (check-exn #px"expected: a list of length 3"
+               (λ () (lens-set list-proper-lens
+                               (list* 'x 'y 'z)
+                               (list 1 2))))
+    (check-exn #px"expected: a list of length 3"
+               (λ () (lens-set list-proper-lens
+                               (list* 'x 'y 'z)
+                               (list 1 2 3 4))))
+    (check-exn #px"expected: a list of length 3"
+               (λ () (lens-set list-proper-lens
+                               (list 'x 'y)
+                               (list 1 2 3 4))))
+    (check-exn #px"expected: an improper list of length 3 \\(plus the last element\\)"
+               (λ () (lens-set list-improper-lens
+                               (list 'x 'y 'z)
+                               (list* 1 2))))
+    (check-exn #px"expected: an improper list of length 3 \\(plus the last element\\)"
+               (λ () (lens-set list-improper-lens
+                               (list 'x 'y 'z)
+                               (list* 1 2 3 4))))
+    (check-exn #px"expected: an improper list of length 3 \\(plus the last element\\)"
+               (λ () (lens-set list-improper-lens
+                               (list 'x 'y '())
+                               (list* 1 2 3 4))))
+    ))
diff --git a/lens-unstable/unstable/lens/improper.rkt b/lens-unstable/unstable/lens/improper.rkt
new file mode 100644
index 0000000..3eb873e
--- /dev/null
+++ b/lens-unstable/unstable/lens/improper.rkt
@@ -0,0 +1,2 @@
+#lang reprovide
+lens/private/list/improper
diff --git a/lens-unstable/unstable/lens/main.rkt b/lens-unstable/unstable/lens/main.rkt
index 04ddfff..d31e4c0 100644
--- a/lens-unstable/unstable/lens/main.rkt
+++ b/lens-unstable/unstable/lens/main.rkt
@@ -5,6 +5,7 @@
 "flatten.rkt"
 "hash-filterer.rkt"
 "if.rkt"
+"improper.rkt"
 "isomorphism.rkt"
 "join-assoc.rkt"
 "lazy.rkt"