[ruby-core:64211] Re: Asking for clarification for exception handling usage

To answer the first half of your question, the "cause" is an underlying
exception (inherited from $!), so you can use it to chain errors. For
example:

  begin
    begin
      raise 'foo'
    rescue => foo
      raise 'bar'
    end
  rescue => bar
  end
  bar #=> #<RuntimeError: bar>
  bar.cause == foo #=> true

As to the second half, the third parameter to #raise is an
optionally-overridden backtrace:

  begin
    raise RuntimeError, 'baz', %w[a b c]
  rescue => ex
  end
  ex.backtrace #=> ["a", "b", "c"]

Passing an exception instance to #raise does the following:

  begin
    begin
      raise 'foo'
    rescue => foo
      raise foo # re-raise
    end
  rescue => bar
  end
  foo.__id__ # => 70351312183760
  bar.__id__ # => 70351312183760

  begin
    begin
      raise 'foo'
    rescue => foo
      raise foo, 'bar' # copy and raise
    end
  rescue => bar
  end
  foo.__id__ # => 70071384070800
  bar.__id__ # => 70071384070620
  bar.cause == foo # => true


In the first case the backtrace for both foo and bar lists line 3; in the
second they have line 3 and 5 respectively. I've never heard of an
exception's #cause returning anything but an Exception instance or nil.
​

On 6 August 2014 06:01, Rodrigo Rosenfeld Rosas <[e-mail]> wrote:

> I've created a ticket for that but didn't get any feedback so I decided to
> try asking in another way:
>
> https://bugs.ruby-lang.org/issues/10110
>
> I'd like to understand what is Exception#cause and what exactly does the
> third parameter of Kernel#raise.
>
> I've seen a code calling raise exception_instance, message rather than
> passing the exception class and the cause for the handled exception was a
> string rather than an Exception. I don't even know if this is a bug and if
> raise should fail if you pass an exception instance with a message as I
> don't really understand how exception handling works in Ruby and what to
> expect for Exception#cause and how to properly wrap an exception for
> logging purposes...
>
> If you could at least explain what Exception#cause is supposed to carry on
> and what the third argument of Kernel#raise does exactly by the specs I'd
> be a lot grateful.
>
> Thanks in advance,
> Rodrigo.
>
>


-- 
  Matthew Kerwin
  http://matthew.kerwin.net.au/