# Q: will this always work: rescuing with non-Exception classes

**URL:** <https://www.ruby-forum.com/t/q-will-this-always-work-rescuing-with-non-exception-classes/183434>\
**Category:** Ruby\
**Created:** [February 14, 2010, 12:45am UTC](https://www.ruby-forum.com/t/q-will-this-always-work-rescuing-with-non-exception-classes/183434 "2010-02-14T00:45:34Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![jeremyheiler](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/34f0e0/32.png) [@jeremyheiler](https://www.ruby-forum.com/u/jeremyheiler)\
**Post date:** [February 14, 2010, 12:45am UTC](https://www.ruby-forum.com/t/q-will-this-always-work-rescuing-with-non-exception-classes/183434/1 "2010-02-14T00:45:34Z")

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I wondered whether the clause rescue would accept any modules, not  
just exception classes. Well, at least in MRI 1.8.7, it does (example  
appended). But will this work in all Ruby implementations?

Regards,

Jeremy H.

Example (prints “Rescued!”)

```
module MyModule
end

class MyException < StandardError
  include MyModule
end

begin
  raise MyException
rescue MyModule
  puts 'Rescued!'
end
```

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**Author:** ![jeremyheiler](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/34f0e0/32.png) [@jeremyheiler](https://www.ruby-forum.com/u/jeremyheiler)\
**Post date:** [February 14, 2010, 1:23am UTC](https://www.ruby-forum.com/t/q-will-this-always-work-rescuing-with-non-exception-classes/183434/2 "2010-02-14T01:23:03Z")

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On Feb 13, 6:45 pm, Jeremy H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> ```
> module MyModule
> end
> 
> ```

That’s interesting. If it is using #=== to determine this then it  
should.

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<div class="post-metadata">

**Author:** ![jeremyheiler](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/34f0e0/32.png) [@jeremyheiler](https://www.ruby-forum.com/u/jeremyheiler)\
**Post date:** [February 14, 2010, 5:55am UTC](https://www.ruby-forum.com/t/q-will-this-always-work-rescuing-with-non-exception-classes/183434/3 "2010-02-14T05:55:44Z")

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Interesting idea. Seems to work pretty much everywhere.

~ ✖ ruby-1.9.1-p378 ▸ rvm ruby test.rb

macruby-nightly: MacRuby version 0.6 (ruby 1.9.0) [universal-  
darwin10.0, x86\_64]

Rescued!

mput-head: ruby 1.9.2dev (2010-02-14 trunk 26661) [x86\_64-  
darwin10.2.0]

Rescued!

rbx-1.0.0-rc2: rubinius 1.0.0-rc2 (1.8.7 release 2010-01-04 JI)  
[x86\_64-apple-darwin10.2.0]

Rescued!

ree-1.8.7-2009.10: ruby 1.8.7 (2009-06-12 patchlevel 174) [i686- darwin10.2.0], MBARI 0x6770, Ruby Enterprise Edition 2009.10

Rescued!

ruby-1.8.7-p248: ruby 1.8.7 (2009-12-24 patchlevel 248) [i686- darwin10.2.0]

Rescued!

ruby-1.9.1-p378: ruby 1.9.1p378 (2010-01-10 revision 26273) [i386- darwin10.2.0]

Rescued!  
~ ✖ ruby-1.9.1-p378 ▸

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<div class="post-metadata">

**Author:** ![jeremyheiler](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/34f0e0/32.png) [@jeremyheiler](https://www.ruby-forum.com/u/jeremyheiler)\
**Post date:** [February 14, 2010, 8:50am UTC](https://www.ruby-forum.com/t/q-will-this-always-work-rescuing-with-non-exception-classes/183434/4 "2010-02-14T08:50:43Z")

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(Following up to myself to summarise)

On 2010-02-13, Jeremy H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I wondered whether the clause rescue would accept any modules, not  
> just exception classes. Well, at least in MRI 1.8.7, it does  
> (example appended). But will this work in all Ruby implementations?

As reported elsewhere in the thread, this does seem to work  
everywhere.

MRI 1.8.7 raises “class or module required for rescue clause  
(TypeError)” if the handler is not a module. It tests whether the  
exception matches the handler with ::=== (I checked this by  
redefining self.#=== inside a module).

“Programming Ruby” (2nd ed.) explicitly says that the match test uses  
#=== . Its language implicitly presumes (at least as I read it) that  
the handlers should be Exception classes, not arbitrary modules.

The Draft Ruby Specification 20091201 disagrees with everyone! It  
says that a TypeError is raised if the handler is not an Exception  
class. Furthermore it says that the exception is handled if it is an  
instance of the handler, rather than if it satisfies ::=== .  
This rather goes against the specification’s stated intent that  
existing implementations should conform without modification.

Regards,

Jeremy H.