Disclaimer: I’ve taken the Rails i18n implementation because I’ve worked with it lately and because I think it serves as a good case study here. Personally, I think that all the guys have done a great work in realizing this i18n interface and with this article I don’t want to attack them in any way. You may want to read Sven Fuchs’ writeup on the Rails i18n features first in order to better understand some parts of the article.

The Stumbling Block

A few days ago when I tried to implement the i18n functionality for Rails’ Date and Time classes I stumbled across an interesting part in the i18n source or, to be more specific, in the simple backend’s localize method:

def localize ( locale , object , format = :default ) raise ArgumentError , "Object must be a Date, DateTime or Time object. #{ object . inspect } given." unless object . respond_to? ( :strftime ) type = object . respond_to? ( :sec ) ? 'time' : 'date' formats = translate ( locale , :" #{ type } .formats" ) format = formats [ format . to_sym ] if formats && formats [ format . to_sym ] # TODO raise exception unless format found? format = format . to_s . dup format . gsub! ( /%a/ , translate ( locale , :"date.abbr_day_names" ) [ object . wday ] ) format . gsub! ( /%A/ , translate ( locale , :"date.day_names" ) [ object . wday ] ) format . gsub! ( /%b/ , translate ( locale , :"date.abbr_month_names" ) [ object . mon ] ) format . gsub! ( /%B/ , translate ( locale , :"date.month_names" ) [ object . mon ] ) format . gsub! ( /%p/ , translate ( locale , :"time. #{ object . hour < 12 ? :am : :pm } " )) if object . respond_to? :hour object . strftime ( format ) end

What this code does is basically take a format, look it up in the dictionaries and use it as a format string for strftime. If it can’t find the format it will just convert the format parameter to a string and then use it in strftime directly.

Notice the comment! It’s basically a note from one developer to the other developers where they ask whether or not they should raise an exception if the format can’t be translated. And well, they should!

Why and When Raise Exceptions?

Over the years I’ve come up with a little rule of thumb for exceptions:

Libraries Should Raise, Applications Should Not Raise.

Yes, it’s as simple as that. To understand the reasons for my rule of thumb, we have to switch to the world of compiled programming languages for a second.

Lots of libraries in C/C++/Java/etc. come to you as compiled source which basically means you can’t just look at the implementation and figure out what the code does and how it behaves in different situations. This, on the one hand, forces library developers to write good documentation for their code and, on the other hand, also means they must provide sensible error handling since the developer can’t just – like in Ruby – re-open a class and change a method according to their needs.

Ask yourself a little question: If you pour water in a glass that’s already full, would you expect the glass to react to that all by itself or would you rather take it into your own hands and stop pouring or maybe empty the glass (you don’t like what’s in it anyway)?

In other words: The tool should not decide about error handling but instead delegate it to its user. At the moment of its inception, a programmer basically just can’t know how it’s going to be used and thus can’t make decisions about how errors should be handled.

Plus, the programmer of the library should apply the principle of least surprise: If I, as a user of the library, throw in an invalid value (i.e. a value that can’t be found in the dictionary), I don’t automatically expect that my value is just pushed into strftime no matter what.

Dynamic Typing and Error Handling

How do you actually define what’s valid or invalid for a given parameter?

Well, as long as you work with only one type of values, it’s usually pretty straightforward. Imagine a car with a pre-defined maximum speed and a setter for its current speed. If the setter is invoked with a speed that exceeds the defined maximum speed it is invalid and it can trigger error handling like setting the current speed to the maximum speed instead of raising an exception.

However, if you allow a parameter to have different types, things get a little more complicated in a dynamically typed language like Ruby. Where you would have interfaces and polymorphic methods in Java to ensure that all types are handled correctly, you need a full blown control structure in Ruby:

def method ( param ) case param when Symbol # do some symbol specific things when String # do some string specific things # etc. end end

With lots of different options, this quickly gets a bit bloated but it’s a thing we accept because we get added flexibility that comes with dynamic typing.

The real challenge in this case is to conceive error handling that works. Where in Java each polymorphic method usually has its own validation rules, we’d have to throw them inside the code blocks in Ruby.

def method ( param ) case param when Symbol raise ArgumentError , ":default is not a valid option" if param == :default # do some symbol specific things when String raise ArgumentError , "param is too long - use max. 5 characters" if param . strip . length > 5 # do some string specific things # etc. end end

Sidenote: Premshree Pillai has written a very good Introduction to Static and Dynamic Typing over at SitePoint, so check it out if you want.

Apply it to Some Real Code

To cut a long story a little shorter, let’s take a look at why I don’t like the current implementation of Simple#localize. Here’s the code that I’d need to write in order to support both, Rails’ existing behavior and i18n capabilities:

def to_formatted_s ( format = :default ) return DATE_FORMATS [ format ]. respond_to? ( :call ) ? DATE_FORMATS [ format ]. call ( self ) . to_s : strftime ( DATE_FORMATS [ format ] ) if DATE_FORMATS . keys . include? ( format ) type = self . respond_to? ( :sec ) ? 'time' : 'date' # could be TimeWithZone so I need to even check that in date # neet to check first if the format is defined - if not, fall back to default to_s I18n . translate ( :" #{ type } .formats" ) ? I18n . localize ( self , format ) : to_default_s end

To shortly explain the flow: Rails looks up standard formats in its DATE_FORMATS constant first. If it can find it it either calls it if it’s a proc or uses the format as a format string for strftime. If it can’t find the format in the constant it does localization. The I18n.translate is necessary because localize doesn’t raise an exception if it can’t find the supplied format but instead uses it as a string for strftime. We can’t have that since Rails falls back to to_default_s if it can’t find the format.

Here’s what it would look like if localize raised an exception if it can’t find a format:

def to_formatted_s ( format = :default ) # fall back to default to_s if localize raises an exception DATE_FORMATS . keys . include? ( format ) ? strftime ( DATE_FORMATS [ format ] ) : I18n . localize ( self , format ) rescue to_default_s end

Here’s the implementation for localize that I’ve proposed to the i18n team:

def localize ( locale , object , format = :default ) raise ArgumentError , "Object must be a Date, DateTime or Time object. #{ object . inspect } given." unless object . respond_to? ( :strftime ) format = case format when Symbol type = object . respond_to? ( :sec ) ? 'time' : 'date' translate ( locale , :" #{ type } .formats. #{ format } " ) # raises I18n::MissingTranslationData if format isn't found when Proc # allows for something like :long_ordinal => lambda { |date| "%B #{date.day.ordinalize}, %Y" }, # => "April 25th, 2007" # similar to current Rails functionality, see ActiveSupport::CoreExtensions::Date format . call ( object ) else format end . to_s format . gsub! ( /%a/ , translate ( locale , :"date.abbr_day_names" ) [ object . wday ] ) format . gsub! ( /%A/ , translate ( locale , :"date.day_names" ) [ object . wday ] ) format . gsub! ( /%b/ , translate ( locale , :"date.abbr_month_names" ) [ object . mon ] ) format . gsub! ( /%B/ , translate ( locale , :"date.month_names" ) [ object . mon ] ) format . gsub! ( /%p/ , translate ( locale , :"time. #{ object . hour < 12 ? :am : :pm } " )) if object . respond_to? :hour object . strftime ( format ) end

What happens here, exception-wise? First of all, I kept the ArgumentError that is raised if the object that is passed in doesn’t respond to strftime.

The code then makes an assumption about the value of format that is pretty common in Rails and also Ruby: If it’s a Symbol it’s most likely some kind of Hash key – therefore we try to look it up in our translations. If it can’t find the translation, the translate method correctly raises an exception which is passed back to the caller – in this case the to_formatted_s method in the Date/Time classes.

The second option is to use a Proc – also fairly common in both Rails and Ruby itself – to evaluate a closure.

Finally, the default behavior is just to return the format parameter converted to a String (notice the to_s at the end). Both options rely on Ruby’s internal error handling so if an error is raised it will – again – be passed back to the caller.

We’ve also opened the possibility to add other options later in the game so we could, say, treat an Array differently.

In short, we’ve basically set up a copule of rules for any programmer who wants to use the library:

You can pass in any parameter as an object as long as it has a strftime method.

The type of format you pass in affects how it’s going to be handled by the library.

We delegate all error handling to y so you, yourself, are responsible for handling all errors.

So now whoever uses the library can again decide whether they want to re-raise the error or provide a sensible default behavior – like Rails does by falling back to the original to_s method.

Why Applications Should Not Raise

Now that I’ve explained why I think that libraries should raise it’s time to explain why application code should not raise.

First, let me explain what I mean by application code. Roughly speaking, everything that you write yourself and (usually) exclusively for one application is what I consider application code. So your models, controllers and views are application code. Some stuff in the lib folder, although considered a library, might be application code, too. Even some plugins might be considered application code: Rick Olson’s popular restful_authentication plugin, for example, generates a user model and some other stuff for you that sits directly in your application and can – and usually even should – be changed.

I’ve seen some code lately that raises a comparatively high amount of application code exceptions. I think this is due to the rescue_from method that was added to Rails core in September 2007 because it makes it really easy to unify error handling.

Do I think that this is a valid approach for handling errors? Yes, absolutely. Do I think that it’s the best approach? Absolutely not.

While it may DRY up your code it usually also obfuscates it. If you look at code that raises an exception and the handling of the exception is handled somewhere else you’ll either have to think how you implemented the error handling or even look at the code again.

While this is perfectly acceptable and even natural for libraries because they don’t know anything about the code that’s going to use it, it feels awkward in application code. When you write application code that can potentially cause errors you already know how these errors are going to be handled which – by my definition – is a clear case against raising an exception.

Don’t get me wrong – I think DRY is a hell of a good rule. However, when it comes to handling application code errors DAMP code is just a lot more readable and meaningful.

What Do You Think?

I’ve told you about my opinions and my rule of thumb so now it’s time that you tell me about your experience with exceptions. Do you always/never raise exceptions? Is it merely a question of style or is it important to raise exceptions, especially in libraries that other people are going to use?

I look forward to hearing from you!