Implementing next_permutation in sml?

Question

How would I implement this in SML? Is it possible to change the inner for-loop to a recursive inner function?

void RecursivePermute(char str[], int k) {

 int j;

 // Base-case: All fixed, so print str.
 if (k == strlen(str))
     printf("%s\n", str);

 else {

     // Try each letter in spot j.
     for (j=k; j<strlen(str); j++) {

         // Place next letter in spot k.
         ExchangeCharacters(str, k, j);

         // Print all with spot k fixed.
         RecursivePermute(str, k+1);

         // Put the old char back.
         ExchangeCharacters(str, j, k);
     }
 }

}

Answer 1

You could write it like

val rec RecursivePermute = fn (str, k) => ...

or

fun RecursivePermute (str, k) = ...

You can also check k and length of str like

if (String.size str = k)
then ...
else ...

As inner-loop function, it would work in poor way, so its better to do something else but this It concerns with representation of String. If you want to swap characters, it will work O(n) at worst where n - the length of the String (due to you need to remove characters between them and then put them back). So generally, you need to use O(n^2) time which is quite ineffective.