Reverse and Add Palindrom - https://www.geeksforgeeks.org/reverse-and-add-function/

reverse_and_add.sml

(* Reverse the digits in a number *)
fun reverse_num(num : int) =
    Int.fromString( implode (rev (explode (Int.toString(num)))) )

(* Test if a string is a palindrome ie. 'abba' *)
fun palindrome (str : string) =
    let val str_list = explode str
    in
        str_list = rev str_list
    end

(* The reverse and add function starts with a number, reverses its digits, and
adds the reverse to the original. If the sum is not a palindrome (meaning it
does not give the same number read from left to right and right to left), we
repeat this procedure until it does. For example, if we start with 195 as the
initial number, we get 9,339 as the resulting palindrome after the fourth
addition *)
fun reverse_and_add (num : int) =
    let val reverse_num = reverse_num(num)
        val sum = valOf reverse_num + num
        val str_sum = Int.toString(sum)
    in
        if num > valOf Int.maxInt (* max int is 1073741823 *)
        then "No palindrome exists"
        else if palindrome(str_sum)
        then str_num
        else reverse_and_add(sum)
    end