Dart generic function for Iterable<num>-CodePudding

I'm learning about generics in Dart and can't get the following example to work. The function takes an Iterable over any kind of num (int or double) and returns its sum. How can I fix this?

N sum<N extends num, T extends Iterable<N>>(T numbers) {
  return numbers.reduce((acc, n) => acc   n);
}

void main() {
  print(sum([1, 2, 3]));
}

CodePudding user response：

acc and n are both of type N, so acc n uses N.operator . Since N derives from num, N.operator must conform to num.operator , which is statically known to return a num. However, the compiler cannot deduce that the returned num must necessarily be an N since that would be a potentially unsafe downcast.

In practice, the only types allowed to derive from num are int and double, and each provides an override of operator that returns its own type. Therefore explicitly downcasting the result of N.operator should be safe, and you can fix your code by using:

  return numbers.reduce((acc, n) => (acc   n) as N);

CodePudding user response：

You have two options here:

//you have to specify types when calling this function(see example)
N sum<N extends num, T extends Iterable<N>>(T numbers) {
  return numbers.reduce((acc, n) => acc   n);
}

//you have to call cast() method
N sum2<N extends num, T extends Iterable<N>>(T numbers) {
  return numbers.cast<N>().reduce((acc, n) => acc   n);
}

void main() {
  print(sum<int, List<int>>([1, 2, 3]));//specify types
  print(sum2([1, 2, 3]));
}