how can i implement dart binary search algorithm?

how can I use this binary search algorithm in the dart main function?

int binarySearch<T>(List<T> sortedList, T value,{int Function(T, T)? compare}) {
 compare ??= defaultCompare(); // I can't understand this line code
 var min = 0;
 var max = sortedList.length;
 while (min < max) {
   var mid = min + ((max - min) >> 1);
   var element = sortedList[mid];
   var comp = compare(element, value); //I can't understand this line code 
   if (comp == 0) return mid;
   if (comp < 0) {
     min = mid + 1;
   } else {
     max = mid;
   }
 }
 return -1;
}

why use this line of code

{int Function(T, T)? compare} 
compare ??= defaultCompare(); 
var comp = compare(element, value);

Read more here: https://stackoverflow.com/questions/65703145/how-can-i-implement-dart-binary-search-algorithm