Issue #8257 has been updated by Charles Nutter.


I agree with Benoit that the cause exception should be reflected in the error output, as on the JVM. What are the objections?

I do *not* believe this should show up in #backtrace since I have no idea how it would be formatted. This also lines up with the JVM (printStackTrace and friends print cause, but getStackTrace only returns trace for target exception).

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Feature #8257: Exception#cause to carry originating exception along with new one
https://bugs.ruby-lang.org/issues/8257#change-47939

* Author: Charles Nutter
* Status: Closed
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* Category: 
* Target version: Ruby 2.1.0
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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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