Extend List with method exists(int index)

[chreck]

It is inconvenient to check if the list has an item on this index or not. The only way to solve this, is to add an extension on List implementation.

extension ListExtension on List {  
  bool exists(int index) {
    return index >= 0 && index < this.length;
  }
}

void main() {
  var list = <String>[];
  list.add("First");
  var index = 1;
  if(list.exists(index)) {
   list[index];
  }
  else
  {
    print('Index $index does not exist in list $list');
  }
} 

Otherwise an exception is immediately thrown.

Uncaught Error: RangeError (index): Index out of range: index should be less than 1: 1

[lrhn]

We can't add methods to the List interface, it would break too many third-party implementations of the interface.

Adding an extension method is possible, but I don't think this particular method is important enough that it needs to belong in dart:core or dart:collection. If anything, it would be package:collection.

It's not a method I think I'd personally ever use.
If I have an integer, and I don't know whether it's less than zero, I'm very, very unlikely to want to use it as a list index.
The only case where that's likely to happen is if someone else passes me an integer as argument, and in that case I'll just require the index argument to be valid for the list. Probably using:

RangeError.checkValidIndex(index, list, "index", list.length);

If the number is known to be non-negative, using this method is harder, and no more readable, than just doing index < list.length.

Others might have use-cases where it makes more sense, but I personally don't.