Object Mentor Blog: Abstracting Away From Exceptions

Abstracting Away From Exceptions

Michael Feathers — Mon, 05 Jan 2009 23:20:00 -0600

I’ve been unsatisfied with exceptions for a long time. Sure, I use them and find them helpful, but I feel that they are a bit of a cop-out as a language construct. If an exception could talk, it would say “get me the hell out of here.” And, you have to admit, that’s a bit rude.

Exceptions seem to encourage “controlled abort” as an error handling policy. You throw an exception, it travels up the stack, and you catch it someplace else. All of your work is unwound (you hope) and then you are left with the task of logging the error or trying something else.

It is true – in many applications you do want to “fail fast” but not all applications are the same. Sometimes, you do want to go on, and it would be nice if exceptions had transactional semantics – it would be nice to have a guarantee that your program is in the same state it was before your try, in all of the ways that are important to you. Without that guarantee, programs devolve. “Log and fail” looks like the only option.

In this blog, I’m going to describe a first step toward rationalizing error handling in applications with pervasive exceptions. It’s a simple one, and now that I think about it, it probably doesn’t deserve all of the build up that I just gave it, but I suspect that I’ll be writing more about this topic. There’s definitely more to discuss.

A few days, I ago I tweeted that I was looking at condition systems for error handling. I was inspired by the fact that Ola Bini decided to add a condition system to his new language: Ioke. As far as I can tell, condition systems were first developed in Lisp. If I had to characterize the difference between an exception handling system and a condition system it would be that instead of saying “hey, get me the hell out of here”, a condition system says “I don’t know how to handle this, did you register something which could help?” All in all, it looks like a more reasonable approach.

After I tweeted this, Tracy Harms tweeted that the J Programming Language has a function called adverse which could be used as a the basis for a condition system. What does adverse do? Well as it turns out, it is very simple. Here’s a translation to C#:


public static class Adverse<T>
{
  public delegate T Function();
  public delegate T RecoveryFunction();

  public static T Call(Function attemptFunction,  
                       RecoveryFunction recoveryFunction)
  {
    try { return attemptFunction(); }
    catch (Exception e) { return recoveryFunction(); }            
  }
}

Adverse accepts two functions. It calls the first and returns its value. However, if there is any kind of an error, it calls the second function and returns its value instead.

This might not look like a big deal. Superficially, it looks like another way to swallow exceptions. But, you can (of course) write an adverse which passes the caught exception to the second function. You can also write an adverse which works for void functions:


public static class Adverse
{
  public delegate void Procedure();
  public delegate void RecoveryProcedure(Exception e);

  public static void Call(Procedure attempt,
                          RecoveryProcedure recovery)
  {
    try { attempt(); }
    catch (Exception e) { recovery(e); }
  }
}

How is adverse better than straight exception handling? Well, first of all, it forces us to separate responsibilities. And, in general, that’s good.

A long time ago, Bertrand Meyer described something called the Command/Query Separation Principle. The idea was that methods should either change something or return something back – they shouldn’t do both. The funny thing about exceptions is that they can do both and as a result we rarely think about structuring exceptions around the distinct actions of getting something or modifying something. And, that’s a shame. There are often cases where we are obliged to return something and it’s better that we return some sort of a Null Object rather than a null. In other cases, we need to focus our attention on what we need to do to patch up the state of our application. I suspect Adverse can help with that. There’s nothing that forces attention like the syntactic need to write some piece of code to use a construct.

I’ll end with some toy examples of Adverse in use (I haven’t put it through real paces yet):


   string line  = Adverse<string>.Call(GetLine, e => "");
   string line2 = Adverse<string>.Call(GetLine,
                    e => { LogError(new ReadError(e));
                           return "";});

   Adverse.Call(SomeWriteOperation, 
                    e => LogError(new WriteError(e));

I’ve used lambdas here to be brief, but imagine plugging in your own handler function. You could start to parameterize your code with error handling policies. It could be interesting.

"Abstracting Away From Exceptions" by outlet juicy couture

Sun, 08 Jan 2012 22:06:15 -0600

Shopping inside a incredibly outlet juicy couture sale in regards to the web can be incredibly a great offer handy especially once the regional shop outlet exactly where it is possible to buy the item that you desire is a good offer out of your area. Apart from this, going to malls demands some trip and of course, you would ought to devote resources to the fare of going to mall.

"Abstracting Away From Exceptions" by Natural Beauty

Wed, 14 Dec 2011 19:05:01 -0600

I was extremely pleased to find this website. I wanted to thank you for this well-written post. I definitely enjoyed every single small bit of it and we’ve bookmarked you to check out new stuff from your post in the future!

"Abstracting Away From Exceptions" by FM TRANSMITTER

Mon, 28 Nov 2011 20:19:48 -0600

Regulations to legalise the use of certain types of FM TRANSMITTER came into force on 8 December 2006. From the end of 2006 the iTrip and other FM TRANSMITTER, such as the popular Belkin Tunecast can be used without licence in the United Kingdom. To be legal, it must carry a CE mark which indicates their approval for sale in the European Union.

"Abstracting Away From Exceptions" by beats by dr dre

Tue, 15 Nov 2011 02:18:51 -0600

A university studentbeats by dr dre caught by the enemy, the enemy tied him at the poles,just beats solo headphones purple and then asked him: say, where are you? You do not say it electrocuted! Scheap dr.dre beats studio headphones balck/yellowtudents back to the enemy a word, the result was electrocuted, he said: I am TVU.Hot sale beats by dr dre pro headphones

"Abstracting Away From Exceptions" by alwadifa

Sat, 05 Nov 2011 18:01:51 -0500

I liked you blog so im going bookmark it with my prefered websites,

"Abstracting Away From Exceptions" by alwadifa

Sat, 05 Nov 2011 18:01:30 -0500

I liked you blog so im going bookmark it with my prefered websites, you have posted an amazing posts

"Abstracting Away From Exceptions" by canada goose coat

Thu, 20 Oct 2011 20:15:57 -0500

When it comes to feather dress, what appears in your mind? Which kind brand of down jacket do you like prefer? Though there are many down jackets for you to choose from, on the word, which one you really enjoy? I want to say that canada goose coats is really your best choice. I believe you can’t agree with me any more. When you take the quality into consideration, you will find that it is superior to any other kind of coat. Besides, discount canada goose jackets is a world well-known brand, which has gained high reputation in the world, which has accepted by our customers and some organization. Because of its high quality, some of our loyal customers have promoted it to the people around them. In their opinion, it is good to informing others to know it. Recently, Canada Goose Trillium Parka is on hot sale. What I have to inform you is that all the products there are made by hand, so they are elaborative and elegant enough. It is really beautiful once you dress in. So, if you are a lovely girl or woman, go to the store to buy one for you. You will appreciate it that you have such a coat.In addition, they also have any other products like canada goose Gloves and canada goose jacket supplier.Hope your will like its!

"Abstracting Away From Exceptions" by Tips For Bowling

Thu, 20 Oct 2011 13:22:22 -0500

Physiology seeks to derive the processes in our own nervous system from general physical forces, without considering whether these processes are or are not accompanied by processes of consciousness. Wilhelm Wundt

"Abstracting Away From Exceptions" by MTS Conversor

Tue, 11 Oct 2011 00:57:17 -0500

Some Twitter users thought the “Last Chance” message was a terrorist threat. Others assumed the Wall St. protesters were behind it. MTS Conversor Artist Kim Beck, a professor at Carnegie Mellon University in Pittsburgh who lives part time in New York, chose messages from advertising billboards as cryptic comments on the faltering economy. MTS Conversor Mac

"Abstracting Away From Exceptions" by robert.robot369@gmail.com

Sat, 01 Oct 2011 02:20:28 -0500

I am really very happy for the nice services in this blog and the amazing technology is visible in this blog. This is very much enjoyed for the nice services in this really very happy for the amazing services in this blog Program Coordinator Job Description|Hospital Administrator Job Description|Cook Job Description|Registered Nurse Job Description

"Abstracting Away From Exceptions" by canada goose coat

Wed, 28 Sep 2011 20:22:38 -0500

Canada Goose Outlet is Marmot 8000M Parka. The Marmot 8000M Parka is really a waterproof, breathable jacket with 800 fill canada goose jacket feathers. It truly is design and light colored shell is produced for trendy, but uncomplicated, protection from cold temperatures. Reinforced shoulders, elbows and adjustable waist and hem make the Marmot a perfect alternate for skiing and other outdoor sports that want fairly a bit of arm motion. The 8000M Parka weighs three lbs., comes in bonfire and black colours and might be stuffed and stored like a sleeping bag to your convenience.This is one of well-know and prime down jacket brands.Hope our friends like its!Like canada goose womens and Canada Goose Expedition Parka.There are wholesale canada goose.

"Abstracting Away From Exceptions" by moncler coats men

Thu, 22 Sep 2011 03:09:23 -0500

Very good idea of article.I wondered idea of article is quite good and impresive to think about this article and future. Kind regards .

"Abstracting Away From Exceptions" by moncler down jacket

Fri, 26 Aug 2011 01:59:41 -0500

nike air max 2011 womennike air max 2011 mennike air max kidsnike air max 90 women

"Abstracting Away From Exceptions" by beats by dre store

Tue, 23 Aug 2011 03:16:11 -0500

some of the most respected names in a variety of the arts, The Atlantic has a got a series of first-person essays on the creative process of some of today’s most brilliant minds.high quality headphones new design headphones

"Abstracting Away From Exceptions" by Buy Womens True Religion Flare Jeans

Sat, 20 Aug 2011 04:24:49 -0500

womens true religion bootcut jeans Buy Womens True Religion Flare Jeans Well, if anyone can shed some light on how genius does work, it’s people like Paul Simon, Frank Gehry, Tim Burton and Sarah Ruhl. Gathering together some of the most respected names in a variety of the arts, The Atlantic has a got a series of first-person essays on the creative process of some of today’s most brilliant minds.

"Abstracting Away From Exceptions" by DVD Rip to iPad

Mon, 15 Aug 2011 04:26:20 -0500

I hope things work out. Your sons are adorable. I spy a tribble!

"Abstracting Away From Exceptions" by cartier bangle

Thu, 04 Aug 2011 22:23:23 -0500

http://www.alijewelry.com/burberry-earring-c-10.html"> Burberry Earring ,
http://www.alijewelry.com/burberry-bangle-c-11.html"> Burberry Bangle ,
http://www.alijewelry.com/bvlgari-earring-c-12.html"> Bvlgari Earring ,

"Abstracting Away From Exceptions" by mac cosmetics

Wed, 18 May 2011 23:22:57 -0500

I’m happy for you. I hope things work out. Your sons are adorable. I spy a tribble!

"Abstracting Away From Exceptions" by real estate advertising

Sat, 14 May 2011 13:01:12 -0500

I love your article its very informative Thanks :-)

"Abstracting Away From Exceptions" by okey oyunu oyna

Thu, 28 Apr 2011 06:17:26 -0500

Thanks a lot Michael.

internette görüntülü olarak okey oyunu oyna, gerçek kisilerle tanis, turnuva heyecanini yasa.

"Abstracting Away From Exceptions" by dswehfhh

Wed, 09 Mar 2011 14:03:23 -0600

We are the professional t-shirts manufacturer. t-shirts supplier. t-shirts factory, custom t-shirts.

"Abstracting Away From Exceptions" by replica tag heuer carrera

Fri, 14 Jan 2011 02:18:09 -0600

It is proven time and time again that information’s worth is not the main factor which impacts article promotion results.

"Abstracting Away From Exceptions" by Hot Water Recirculation System

Sun, 09 Jan 2011 19:42:27 -0600

It is proven time and time again that information’s worth is not the main factor which impacts article promotion results.

"Abstracting Away From Exceptions" by protein rich foods

Fri, 24 Dec 2010 00:30:47 -0600

If I had to characterize the difference between an exception handling system and a condition system it would be that instead of saying “hey, get me the hell out of here”, a condition system says

"Abstracting Away From Exceptions" by Pandora

Thu, 02 Dec 2010 01:41:09 -0600

it probably doesn’t deserve all of the build up that I just gave it, but I suspect that I’ll be writing more about this topic. There’s definitely more to discuss.