Issue #21359 has been updated by ioquatix (Samuel Williams).
Serializing exceptions properly without Marshal is probably quite hard yes, not only about the cause but also the internal backtrace representation, the backtrace_locations objects, other internal state for core exceptions that can't always be set in constructor, etc. But is there a need for that?
Yes, serialisation with msgpack is painful and impossible to do correctly in some situations, if you have custom serialisation rules for certain object types (using a msgpack factory), you can't use `Marshal.dump` as part of msgpack's serialisation because it won't correctly serialise the internal objects of the exception. ---------------------------------------- Feature #21359: Introduce `Exception#cause=` for Post-Initialization Assignment https://bugs.ruby-lang.org/issues/21359#change-113389 * Author: ioquatix (Samuel Williams) * Status: Open * Assignee: ioquatix (Samuel Williams) ---------------------------------------- Ruby currently allows an exception’s `cause` to be explicitly set **only at the time of raising** using `raise ..., cause: ...`. However, there are valid use cases where it would be convenient to set the cause when creating an exception. ## The Problem Ruby supports exception chaining via the `cause:` keyword during a `raise`, like so: ```ruby raise StandardError.new("failure"), cause: original_exception ``` ## Proposed Solution Introduce `Exception#cause=` as a public setter method on `Exception`, allowing post-initialization assignment of the cause: ```ruby cause = RuntimeError.new("low-level error") error = StandardError.new("higher-level error") error.cause = cause ``` It would be semantically equivalent to the following implementation: ```ruby error = StandardError.new("error") cause = RuntimeError.new("cause") class Exception def cause= value backtrace = self.backtrace_locations raise self, cause: value rescue Exception self.set_backtrace(backtrace) end end p error.cause # nil error.cause = cause p error.cause # => #<RuntimeError: cause> ``` ### Benefits * Simplifies creation of rich exception objects in frameworks, tools, and tests. * Enables structured error chaining in asynchronous and deferred execution environments. * Avoids misuse of `raise`/`rescue` for control flow and metadata setting. -- https://bugs.ruby-lang.org/