Issue #13224 has been reported by Jeremy Evans.

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Feature #13224: Add FrozenError as a subclass of RuntimeError
https://bugs.ruby-lang.org/issues/13224

* Author: Jeremy Evans
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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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.

---Files--------------------------------
0001-Add-FrozenError-as-a-subclass-of-RuntimeError.patch (27.2 KB)


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