Issue #8257 has been updated by headius (Charles Nutter).


Unfortunately it doesn't look like anything other than $! logic for this made it into 2.1. I was hoping we'd get either the constructor or a one-time-only #cause= but there was still some debate about which way to go.

Since the base #cause and $! logic made it in, perhaps we should call this bug closed as of 2.1 and add a new bug for additional ways to initialize the exception cause.
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Feature #8257: Exception#cause to carry originating exception along with new one
https://bugs.ruby-lang.org/issues/8257#change-43938

Author: headius (Charles Nutter)
Status: Open
Priority: Normal
Assignee: matz (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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