What is the difference between rethrows in Kotlin and regular throw, and what are its advantages?

In Kotlin, the keyword rethrows is used in the signature of higher-order functions that accept a lambda capable of throwing exceptions. It indicates that the function itself does not throw exceptions directly, but can only "rethrow" exceptions that occur inside the passed lambda.

Differences from the usual throw:

• throw is used for directly throwing an exception.
• rethrows is applied to functions that accept a block of code (lambda) and guarantees that the function does not generate new exceptions, but only propagates those that occurred inside the lambda.

Advantages of rethrows:

• Allows the compiler and calling code to know that exceptions can only originate from the passed lambda.
• Improves type safety and exception handling, as the function cannot throw exceptions itself.
• Simplifies error handling when working with higher-order functions.

Example:

fun <T> runOperation(operation: () throws Exception): T rethrows {
    return operation()
}

// Here, runOperation does not throw exceptions itself, but only rethrows exceptions from operation

Thus, rethrows helps explicitly indicate that the function does not generate exceptions, but only passes them further from the passed block.