Question

Does Ruby’s “raise” modify exception?

Or, are following snippets:

some_method(MyException.new)

and

begin
  raise MyException.new
rescue MyException => e
  some_method(e)
end

equivalent? If not, what are the differences?

Answer 1

No, they are not equivalent.

begin
  e = Exception.new "AAA"
  puts e.backtrace.nil?
  raise e
rescue Exception => e
  puts e.backtrace.nil?
end

gives

true
false

Does anyone know other differences?

EDIT:

Kernel#raise WILL NOT modify exception unless raise is called with no params and $! is defined (that means: except re-raising exception in rescue clause with no params). In following example:

begin
  raise "exception" #1
rescue
  raise #2
end

exception will be modified (its context and backtrace altered) in #1 raise but untouched in #2 raise. It WILL be modified in any other case, including re-raising exception with some param like:

begin
  raise "exception" #1
rescue Exception => e
  raise e #2
end

Here exception will be altered in both #1 and #2.

See https://github.com/rubinius/rubinius/blob/master/kernel/delta/kernel.rb for details.

Above applies to Rubinius. It should be the same in MRI 1.9 and JRuby, but I haven’t checked it.