Use fold-left instead of map for an example of a nonuniform vararg function.

Fixes PR12014.

original commit: 4f761ddb2f44f4cd4961984c8c2cb65fa39f71ae

collects/typed-scheme/scribblings/guide/varargs.scrbl

@@ -1,6 +1,6 @@
 #lang scribble/manual
 
-@begin[(require "../utils.rkt" (for-label typed/racket/base))]
+@begin[(require "../utils.rkt" (for-label typed/racket/base) (for-label (only-in rnrs/lists-6 fold-left)))]
 
 @title[#:tag "varargs"]{Variable-Arity Functions: Programming with Rest Arguments}
 
@@ -43,63 +43,74 @@ of the rest parameter is used at the same type.
 @section{Non-Uniform Variable-Arity Functions}
 
 However, the rest argument may be used as a heterogeneous list.
-Take this (simplified) definition of the Racket function @racket[map]:
+Take this (simplified) definition of the R6RS function @racket[fold-left]:
 
 @racketmod[
 racket
-(define (map f as . bss)
+(define (fold-left f i as . bss)
   (if (or (null? as)
           (ormap null? bss))
-      null
-      (cons (apply f (car as) (map car bss))
-            (apply map f (cdr as) (map cdr bss)))))
+      i
+      (apply fold-left
+             f
+             (apply f i (car as) (map car bss))
+             (cdr as)
+             (map cdr bss))))
 
-(map add1 (list 1 2 3 4))
-(map cons (list 1 2 3) (list (list 4) (list 5) (list 6)))
-(map + (list 1 2 3) (list 2 3 4) (list 3 4 5) (list 4 5 6))]
+(fold-left + 0 (list 1 2 3 4) (list 5 6 7 8))
+(fold-left + 0 (list 1 2 3) (list 2 3 4) (list 3 4 5) (list 4 5 6))
+(fold-left (λ (i v n s) (string-append i (vector-ref v n) s))
+           ""
+           (list (vector "A cat" "A dog" "A mouse")
+                 (vector "tuna" "steak" "cheese"))
+           (list 0 2)
+           (list " does not eat " "."))]
 
 Here the different lists that make up the rest argument @racket[bss]
 can be of different types, but the type of each list in @racket[bss]
 corresponds to the type of the corresponding argument of @racket[f].
 We also know that, in order to avoid arity errors, the length of
-@racket[bss] must be one less than the arity of @racket[f] (as
-@racket[as] corresponds to the first argument of @racket[f]).
+@racket[bss] must be two less than the arity of @racket[f].
+The first argument to @racket[f] is the accumulator,
+and @racket[as] corresponds to the second argument of @racket[f].
 
-The example uses of @racket[map] evaluate to @racketresult[(list 2 3 4 5)],
-@racketresult[(list (list 1 4) (list 2 5) (list 3 6))], and
-@racketresult[(list 10 14 18)].
+The example uses of @racket[fold-left] evaluate to @racketresult[36],
+@racketresult[42], and @racketresult["A cat does not eat cheese."].
 
-In Typed Racket, we can define @racket[map] as follows:
+In Typed Racket, we can define @racket[fold-left] as follows:
 
 @racketmod[
 typed/racket
-(: map
+(: fold-left
    (All (C A B ...)
-        ((A B ... B -> C) (Listof A) (Listof B) ... B
+        ((C A B ... B -> C) C (Listof A) (Listof B) ... B
          ->
-         (Listof C))))
-(define (map f as . bss)
+         C)))
+(define (fold-left f i as . bss)
   (if (or (null? as)
           (ormap null? bss))
-      null
-      (cons (apply f (car as) (map car bss))
-            (apply map f (cdr as) (map cdr bss)))))]
+      i
+      (apply fold-left
+             f
+             (apply f i (car as) (map car bss))
+             (cdr as)
+             (map cdr bss))))]
 
 Note that the type variable @racket[B] is followed by an
 ellipsis.  This denotes that B is a dotted type variable
 which corresponds to a list of types, much as a rest
 argument corresponds to a list of values.  When the type
-of @racket[map] is instantiated at a list of types, then
+of @racket[fold-left] is instantiated at a list of types, then
 each type @racket[t] which is bound by @racket[B] (notated by
 the dotted pre-type @racket[t ... B]) is expanded to a number
 of copies of @racket[t] equal to the length of the sequence
 assigned to @racket[B].  Then @racket[B] in each copy is
 replaced with the corresponding type from the sequence.
 
-So the type of @racket[(inst map Integer Boolean String Number)]
+So the type of @racket[(inst fold-left Integer Boolean String Number)]
 is
 
-@racket[((Boolean String Number -> Integer)
-         (Listof Boolean) (Listof String) (Listof Number)
+@racket[((Integer Boolean String Number -> Integer)
+         Integer (Listof Boolean) (Listof String) (Listof Number)
          ->
-         (Listof Integer))].
+         Integer)].