A list within a list is being passed as an empty list - Racket

Question

I'm stuck with a little problem here.

Say that I have a list of lists such as:

(define x '(("1" "one" "two" "2" "three") 
            ("2" "four" "five" "0" "six") 
            ("3" "seven" "eight" "4" "nine") 
            ("4" "ten" "eleven" "2" "twelve")))

So what I want to do is to get each of those lists and make the elements within each one become a single string, such as:

"1 one two 2 three"
"2 four five 0 six" ...

I have designed these functions:

(define (get-sublista-from-lista lista)
  (if (list? (car lista))
      (car lista)
      lista))  

and:

(define (string-from-lista lista)
  (define (list-to-string lista str)
    (if (null? (cdr lista))
        (string-append str (car lista))
        (list-to-string (cdr lista)
                        (string-append str (car lista) " "))))
  (list-to-string lista ""))  

which work properly by themselves. But whenever I try a combination of the two of them or of one of them with another, like in:

(define (string-from-lista1 lista)
  (define (list-to-string lista str)
    (if (null? (get-sublista-from-lista lista))
        (string-append str (car (get-sublista-from-lista lista)))
        (list-to-string (cdr (get-sublista-from-lista lista))
                        (string-append str (car (get-sublista-from-lista lista)) ""))))
  (list-to-string lista ""))

Then the individual lists within the master lists are passed as '(), resulting in a contract violation such as:

car: contract violation
  expected: pair?
  given: '()  

Can someone explain why am I getting such error?

Answer 1

A built-in function string-join works to join different strings in a list into one string separated by default character space. Use map to apply it all members of list x and you will have a short code to accomplish this task:

(map string-join x)

Output:

'("1 one two 2 three" "2 four five 0 six" "3 seven eight 4 nine" "4 ten eleven 2 twelve")