Move list-update and list-set from unstable/list to racket/list.

pkgs/racket-doc/scribblings/reference/pairs.scrbl

@@ -866,6 +866,27 @@ Returns a newly constructed list of length @racket[k], holding
 @mz-examples[#:eval list-eval
   (make-list 7 'foo)]}
 
+@defproc[(list-update [lst list?]
+                      [pos (and/c (>=/c 0) (</c (length lst)))]
+                      [updater (-> any/c any/c)])
+         list?]{
+
+Returns a list that is the same as @racket[lst] except at the specified index.
+The element at the specified index is @racket[(updater (list-ref lst pos))].
+
+@examples[#:eval list-eval
+(list-update '(zero one two) 1 symbol->string)]}
+
+@defproc[(list-set [lst list?]
+                   [pos (and/c (>=/c 0) (</c (length lst)))]
+                   [value any/c])
+         list?]{
+
+Returns a list that is the same as @racket[lst] except at the specified index.
+The element at the specified index is @racket[value].
+
+@examples[#:eval list-eval
+(list-set '(zero one two) 2 "two")]}
 
 @defproc[(take [lst any/c] [pos exact-nonnegative-integer?])
          list?]{

pkgs/racket-test-core/tests/racket/list.rktl

@@ -567,4 +567,21 @@
       cartesian-product '(4 5 6) '(d e f) '(#t #f))
 (err/rt-test (cartesian-product 3))
 
+;; ---------- list-update ----------
+
+(test '("zero" one two) list-update '(zero one two) 0 symbol->string)
+(test '(zero "one" two) list-update '(zero one two) 1 symbol->string)
+(err/rt-test (list-update '(zero one two) 3 symbol->string))
+(err/rt-test (list-update '(zero one two) -1 symbol->string))
+(err/rt-test (list-update '(zero one two) #f symbol->string))
+(err/rt-test (list-update #f 0 symbol->string))
+(err/rt-test (list-update '(zero one two) 0 #f))
+
+;; ---------- list-set ----------
+
+(test '(zero one "two") list-set '(zero one two) 2 "two")
+(err/rt-test (list-set '(zero one two) 3 "two"))
+(err/rt-test (list-set '(zero one two) -1 "two"))
+(err/rt-test (list-set '(zero one two) #f "two"))
+
 (report-errs)

racket/collects/racket/list.rkt

@@ -10,6 +10,9 @@
 
          make-list
 
+         list-update
+         list-set
+
          drop
          take
          split-at
@@ -102,6 +105,25 @@
   (let loop ([n n] [r '()])
     (if (zero? n) r (loop (sub1 n) (cons x r)))))
 
+(define (list-update l i f)
+  (unless (list? l)
+    (raise-argument-error 'list-update "list?" l))
+  (unless (exact-nonnegative-integer? i)
+    (raise-argument-error 'list-update "exact-nonnegative-integer?" i))
+  (unless (and (procedure? f)
+               (procedure-arity-includes? f 1))
+    (raise-argument-error 'list-update "(-> any/c any/c)" f))
+  (cond
+   [(zero? i) (cons (f (car l)) (cdr l))]
+   [else (cons (car l) (list-update (cdr l) (sub1 i) f))]))
+
+(define (list-set l k v)
+  (unless (list? l)
+    (raise-argument-error 'list-update "list?" l))
+  (unless (exact-nonnegative-integer? k)
+    (raise-argument-error 'list-update "exact-nonnegative-integer?" k))
+  (list-update l k (lambda (_) v)))
+
 ;; internal use below
 (define (drop* list n) ; no error checking, returns #f if index is too large
   (if (zero? n) list (and (pair? list) (drop* (cdr list) (sub1 n)))))