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


matz (Yukihiro Matsumoto) wrote:
> * Fundamentally accepted.

Hooray!

> * I am against #cause=

java.lang.Throwable does not have #setCause, so I agree. It does have #initCause, which can only be called once (if cause has not already been provided via constructor)...but I have never used it.

> * It's OK that #raise to have cause: keyword argument to specify cause

Agreed. Unfortunately this would still break backward-compatibility, since it will count as an extra argument on older Ruby versions. This might be the biggest justification for $! capturing.

> * I am not sure automagical capturing of $! would not cause wrong capturing or not

There are a few small risks:

* Many layers of exceptions being raised as an error propagates out would all be chained. This could cause more information/data/memory than necessary to be retained.
* Lower layers may not want higher layers to have access to the causing error for security or encapsulation reasons.
* Too much magic?

But I think the benefits may outweigh them:

* I can't think of any concrete examples of the above risks.
* Automatic adoption; all exceptions caused by other exceptions will immediately show their lineage.
* Reduced backward incompatibility (other than adding #cause) since users won't have to use the constructor to capture cause exception (of course, I still think we should have the cause constructor too).

I think the output of exception backtraces should also be enhanced to show cause exception's trace, as in the JVM. This could cause a problem for tools that parse the backtrace, though (IMO nobody should ever parse backtraces and expect that to be stable).
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Feature #8257: Exception#cause to carry originating exception along with new one
https://bugs.ruby-lang.org/issues/8257#change-42384

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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