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The - operator in Dart functions as both a binary arithmetic operator for subtraction and a prefix unary operator for numeric negation. Its behavior and precedence are determined by the arity of the expression in which it is invoked.

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Binary Subtraction

When used as a binary operator, - calculates the difference between a left operand and a right operand. It operates at the additive precedence level with left-to-right associativity. In Dart’s core numeric types (int and double), the return type depends on the operands: if both are integers, the result is an int; if either is a floating-point number, the result is a double. 
int difference = 10 - 3;       // Evaluates to 7
double floatDiff = 10.5 - 2;   // Evaluates to 8.5

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Unary Negation

When used as a prefix unary operator, - reverses the algebraic sign of a single numeric operand. It operates at the unary prefix precedence level, which is higher than multiplicative and additive operators. 
int value = 5;
int negated = -value;          // Evaluates to -5
int doubleNegated = -(-value); // Evaluates to 5

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Operator Overloading

Dart permits custom classes to override the - operator. Because the operator serves two distinct arities, Dart uses different syntax signatures to differentiate between binary subtraction and unary negation during operator overloading.
  • Binary Overload: Defined using operator - with a single parameter representing the right operand.
  • Unary Overload: Defined using the specific syntax operator unary- with empty parentheses to indicate the prefix negation.
class Vector {
  final int x;
  final int y;

  const Vector(this.x, this.y);

  // Overriding binary subtraction
  Vector operator -(Vector other) {
    return Vector(x - other.x, y - other.y);
  }

  // Overriding unary negation
  Vector operator unary-() {
    return Vector(-x, -y);
  }
}

void main() {
  Vector v1 = Vector(4, 5);
  Vector v2 = Vector(1, 2);

  Vector v3 = v1 - v2; // Invokes binary operator -(Vector other)
  Vector v4 = -v1;     // Invokes unary operator unary-()
}
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