From: Paul Brannan <paul@...>
Date: 2001-12-07T00:30:56+09:00
Subject: [ruby-talk:27693] Re: Controlling the error messages of eval.

On Fri, Dec 07, 2001 at 12:13:06AM +0900, Phlip wrote:
> 'rescue' is really 'rescue Exception', and Exception is not the root 
> throwable. It derives from the root throwable class and then gets thrown 
> for runtime errors, including syntax errors but not including things like 
> signals.

'rescue' is actually 'rescue StandardError'; if you use 'rescue
Exception', then you will also catch ScriptError, Interrupt and some
others (which is generally not desirable).

The root throwable (from a Ruby script, since I think you can break the
rules from an extension) is Exception (eval.c:3467):

    if (argc > 0) {
        if (!rb_obj_is_kind_of(mesg, rb_eException))
            rb_raise(rb_eTypeError, "exception object expected");
        set_backtrace(mesg, (argc>2)?argv[2]:Qnil);
    }

> More important style wise, if I were your programmer-user, I want my system 
> to drop dead at the first error. If it "recovers" and continues I might not 
> notice the error message, because it might be high above a console full of 
> output.

This isn't always desirable.  As a user, I may want to be able to
manually clean things up when an exception occurs.  Or, it may be that I
forgot to catch an expected exception, and the exception is not a real
problem.  However, dropping dead could be a real problem; an IRC bot,
for example, is more useful disabled but connected than it is if it is
dead due to an exception.

Paul