# Help me design my exception classes

**URL:** <https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453>\
**Category:** Ruby\
**Created:** [November 28, 2006, 2:07am UTC](https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453 "2006-11-28T02:07:22Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![JSBSRainsberger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/3da27b/32.png) [@JSBSRainsberger](https://www.ruby-forum.com/u/JSBSRainsberger)\
**Post date:** [November 28, 2006, 2:07am UTC](https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453/1 "2006-11-28T02:07:22Z")

</div>

Everyone:

Being a Java programmer learning Ruby, I don’t want my Java-based  
prejudices about design pollute my thinking as I make the transition to  
being a fluent Ruby speaker. Accordingly, I’d like you to suggest to me  
how I should design my exception classes.

I Googled around for about 10 minutes, but couldn’t find anything on  
point, and I found it difficult to rummage through everything. If  
someone has written extensively about this already, please just point me  
there.

My concrete example is this: I’m “spooling” documents to be printed by  
sending them via FTP. There might be other ways to spool documents, such  
as by sending them via e-mail, to a printer, to the screen. I would like  
to represent an exception that occurs in the underlying transport  
mechanism (FTP, SMTP, printing subsystem…), but of course, I don’t  
want my clients to know about those things directly.

In Java, I would do something like

//…  
catch (FTPException wrapped) {  
throw new RuntimeException(“Unable to spool document”, wrapped);  
}

where the FTP library would throw me an FTPException to indicate that  
something went wrong. I only throw a RuntimeException because I don’t  
necessarily want to create a custom exception class when I only report  
generic, unrecoverable “I couldn’t spool” exceptions. At that point,  
there’s nothing (yet) to gain from distinguishing exceptions by their  
type.

What’s the corresponding exception class in Ruby to Java’s generic  
RuntimeException. Is it RuntimeError? StandardError? Exception? Does  
anyone want to tell me I’m nuts and show me a more “standard” way of  
designing exceptions in Ruby?

Thanks.

---

<div class="post-metadata">

**Author:** ![JSBSRainsberger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/3da27b/32.png) [@JSBSRainsberger](https://www.ruby-forum.com/u/JSBSRainsberger)\
**Post date:** [November 28, 2006, 3:09am UTC](https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453/2 "2006-11-28T03:09:32Z")

</div>

On Tue, 2006-11-28 at 10:07 +0900, J. B. Rainsberger wrote:

> Being a Java programmer learning Ruby,

Welcome! Your JUnit book rocks.

> What’s the corresponding exception class in Ruby to Java’s generic  
> RuntimeException. Is it RuntimeError? StandardError? Exception? Does  
> anyone want to tell me I’m nuts and show me a more “standard” way of  
> designing exceptions in Ruby?

I usually rescue a generic Exception:

# =========== $ irb irb(main):001:0\> begin irb(main):002:1\* raise “hey” irb(main):003:1\> rescue Exception =\> e irb(main):004:1\> puts e.class irb(main):005:1\> end RuntimeError =\> nil

You can create your own exceptions, of course:

# =========== irb(main):006:0\> class MyException \< Exception irb(main):007:1\> def initialize ; puts “hello” ; super ; end irb(main):008:1\> end =\> nil irb(main):009:0\> MyException.new hello =\> #\<MyException: MyException\>

Yours,

Tom

---

<div class="post-metadata">

**Author:** ![JSBSRainsberger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/3da27b/32.png) [@JSBSRainsberger](https://www.ruby-forum.com/u/JSBSRainsberger)\
**Post date:** [November 28, 2006, 8:18am UTC](https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453/3 "2006-11-28T08:18:06Z")

</div>

On Tue, 2006-11-28 at 15:41 +0900, Joel VanderWerf wrote:

> rescue StandardError =\> e
> 
> This suggests that StandardError was intended to be thought of as “generic”.

Hm, good point, StandardError probably is a better way to go. I’ve  
always used Exception, but I think I’ll go back and revisit some of  
those usages now 🙂

Yours,

Tom

---

<div class="post-metadata">

**Author:** ![JSBSRainsberger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/3da27b/32.png) [@JSBSRainsberger](https://www.ruby-forum.com/u/JSBSRainsberger)\
**Post date:** [November 28, 2006, 7:42am UTC](https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453/4 "2006-11-28T07:42:05Z")

</div>

Tom C. wrote:

> # I usually rescue a generic Exception:

Careful… that’s a wide net to cast. It will catch Interrupt and  
SystemExit, for example. StandardError is often (but not always) the  
right generic class to catch.

Note that the syntax

rescue =\> e

(with exception class omitted) is the same as

rescue StandardError =\> e

This suggests that StandardError was intended to be thought of as  
“generic”.

---

<div class="post-metadata">

**Author:** ![JSBSRainsberger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/3da27b/32.png) [@JSBSRainsberger](https://www.ruby-forum.com/u/JSBSRainsberger)\
**Post date:** [November 28, 2006, 9:32am UTC](https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453/5 "2006-11-28T09:32:14Z")

</div>

DRb does this:

```
class DRbError < RuntimeError; end

```

and uses DRbError as a superclass of all DRb error classes. Is this a  
standard convention for Ruby libraries?

Jeremy H.

---

<div class="post-metadata">

**Author:** ![JSBSRainsberger](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/3da27b/32.png) [@JSBSRainsberger](https://www.ruby-forum.com/u/JSBSRainsberger)\
**Post date:** [November 28, 2006, 4:03pm UTC](https://www.ruby-forum.com/t/help-me-design-my-exception-classes/77453/6 "2006-11-28T16:03:15Z")

</div>

On 11/27/06, J. B. Rainsberger [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> catch (FTPException wrapped) {  
> RuntimeException. Is it RuntimeError? StandardError? Exception?  
> You’re on the right track. I’d use the same model you’ve got going in  
> your Java design. For a usage like this, RuntimeError is the one  
> you’re looking for. Fortunately, Ruby uses RuntimeError as the default  
> when you just raise a string instead of a specific Exception object.

Also, in Ruby to raise specific exceptions you don’t need to call new  
on the specific exception class, just provide more arguments to raise.  
And unless you do your own custom wrapping in a subclass, you can’t  
wrap a previous exception – but you can subsume the backtrace as an  
additional parameter to raise. Unfortunately, you need to specify the  
exception class as the first parameter when you do so.

So your syntax could look like this:

begin  
# …  
# code that might raise FTPException  
# …  
rescue FTPException =\> wrapped  
raise RuntimeError, “Unable to spool document”, wrapped.backtrace  
end

Alternately, if you decide you do want to create your own class of  
exception for later discernment, you will inherit from RuntimeError:

class SpoolError \< RuntimeError; end

begin  
# …  
rescue FTPException =\> wrapped  
raise SpoolError, “Unable to spool document”, wrapped.backtrace  
end

Jacob F.