how to use map append correctly with lists of list in Racket

Question

i have this code :

(: min&max-lists : (Listof (Listof Any)) -> (Listof Any))
    
(define (min&max-lists lst)
  (apply append 
         (map (lambda (slst)
                (sublist-numbers slst)              ;; **
                (cons (sublist-numbers slst) '())   ;; *
              )
              lst)))

if I run this code with the line with two ** everything works fine, but when I run the code above with the line with one * it shows me like that : Type Checker: type mismatch expected: (Listof Any) given: Any in: slst

how can I solve this? I see if i use the slst with another function it shows me that it is typed if Any and they need List of Any but if I use it like with two ** it works fine

Answer 1

The problem with (cons (sublist-numbers slst) '()):

(cons x '()) ;; => '(x)
;; which is nothing other than:
(list x)

If x is already a list - like in this case) you have instead of '(1 2 3) one list layer around it: '((1 2 3)). If you apply-append such results you get (Listof (Listof Any)) instead of just (Listof Any).

So, if you want to run using the line *, you have to change the typing to:

(: min&max-lists : (Listof (Listof Any)) -> (Listof (Listof Any)))