How to compute an int from a recursive method in Dart?

I want to compute the depth of a recursive object in Dart (kind of tree). Root node has a null as its parent.

class MyNode {
  MyNode? parent;
  int get depth => 1 + ((parent == null) ? -1 : parent.depth);
  }

This code should return 0 for the root node, 1 for roots children and so on. But it does not compile due to error

A value of type 'double' can't be returned from the function 'depth' because it has a return type of 'int'.

How to rewrite method depth to work as expected?

Dart version is 3.1.3

Solution

I am not sure I understand the error message you are getting since that does not seems right. But the error message you should get are the following on your parent.depth call:

class MyNode {
  MyNode? parent;
  int get depth => 1 + ((parent == null) ? -1 : parent.depth);
  // The property 'depth' can't be unconditionally accessed
  // because the receiver can be 'null'.
}

Reason being that even if you have checked if parent might be null, in theory you could have replaced this variable with a getter in a subclass which does not return the same value every time you request the value.

Instead, I suggest rewrite the code to the following:

class MyNode {
  MyNode? parent;
  int get depth => 1 + (parent?.depth ?? -1);
}

Here we make use of two null-aware operators, ?. and ??:

parent?.depth means that if parent is null, we will just return null. If parent is not null, we return the value of calling parent.depth.
?? means that if the left-side are null, we use the value to the right. Otherwise, we use the value to the left.

So parent?.depth ?? -1 ends up meaning that if parent are null, we use the -1 value. Otherwise, we use the value of parent.depth. And all of this can be done without ending up asking parent twice about it being null and it is therefore safe to do so.