ruby-core

Issue #8257 has been updated by Benoit Daloze.


`raise ErrorClass, msg, cause: cause` was implemented with the rest in r44473.

    e = (
    begin
      raise ArgumentError, "arg error"
    rescue
      nie = NotImplementedError.new("nie")
      raise StandardError, "stderr", cause: nie
    end) rescue $!
    e.cause # => #<NotImplementedError: nie>

But the cause is not shown in the error output, which I think is now the most important step forward.

----------------------------------------
Feature #8257: Exception#cause to carry originating exception along with new one
https://bugs.ruby-lang.org/issues/8257#change-44891

* Author: Charles Nutter
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* Category: 
* Target version: Ruby 2.1.0
----------------------------------------
Often when a lower-level API raises an exception, we would like to re-raise a different exception specific to our API or library. Currently in Ruby, only our new exception is ever seen by users; the original exception is lost forever, unless the user decides to dig around our library and log it. We need a way to have an exception carry a "cause" along with it.

Java has getCause/setCause and standard constructors that take a cause exception. Printing out an exception's backtrace then reports both that exception and any "cause" exception.

Rubinius has added a similar feature: https://gist.github.com/dbussink/b2e01e51d0c50b27004f

The changes required for this feature are pretty benign:

* Exception#cause and #cause= accessors.
* A new set of Kernel#raise overloads that accept (as a trailing argument, probably) the "cause" exception.
* Modifications to backtrace-printing logic to also display backtrace information from the "cause" exception (and in turn, from any nested "cause" exceptions).

There's some discussion here about alternatives to #cause, none of which are quite as elegant as having it built in: http://www.skorks.com/2013/04/ruby-why-u-no-have-nested-exceptions/



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