There are also unary operators, that serve as a prefix for a value and perform an operation on it. ! inverts a boolean value (so true becomes false and vice-versa), while - negates a number.

The operator does not affect the actual value the operator is applied to. In this case, we have a pair of val values, which are read-only ("immutable") and cannot be changed. The unary operators affect the use of the values where the operator appears.

You can learn more about this in:

• the documentation
• Elements of Kotlin