From: "ioquatix (Samuel Williams) via ruby-core" <ruby-core@...>
Date: 2025-05-22T23:44:04+00:00
Subject: [ruby-core:122243] [Ruby Feature#21359] Introduce `Exception#cause=` for Post-Initialization Assignment

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.

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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.




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