Feature #13224
closed
Add FrozenError as a subclass of RuntimeError
Description
Currently, attempting to modify a frozen object leads to a RuntimeError exception. Unfortunately, this means it is not easy to differentiate exceptions raised from attempting to modify frozen objects from generic exceptions such as calling Kernel#raise with no exception class.
The attached patch adds FrozenError as a subclass of RuntimeError, and uses FrozenError instead of RuntimeError for exceptions raised when there is an attempt to modify a frozen object. It should be backwards compatible in the sense that:
begin
"a".freeze << b
rescue RuntimeError
end
Will still function as before.
More people in the ruby community are starting to use frozen objects for the benefits of immutability (thread-safety, cache-ability, referential transparency), and having an exception class dedicated to misuse of frozen objects will make it easier to handle those exceptions.
The attached patch is a little long, mostly because ruby's test suite is currently brittle in regards to handling exception classes. For example, assert_raise(RuntimeError) needs to be changed to assert_raise(FrozenError), because assert_raise uses instance_of? instead of kind_of? if given an exception class and not a module. Both RSpec and recent versions of Minitest should automatically handle this type of change without code modifications.
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