Issue #21359 has been updated by matz (Yukihiro Matsumoto). I am basically against `cause=` (or `set_cause` that is). It makes exceptions more complex. Is it impossible to defer exception creation til it is absolutely needed? Matz. ---------------------------------------- Feature #21359: Introduce `Exception#cause=` for Post-Initialization Assignment https://bugs.ruby-lang.org/issues/21359#change-113621 * Author: ioquatix (Samuel Williams) * Status: Assigned * 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/