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Item 5: Pay Attention to Run-Time Warnings

Ruby programmers enjoy a shortened feedback loop while writing, executing, and testing code. Being interpreted, the compilation phase isn’t present in Ruby. Or is it? If you think about it, Ruby must do some of the same things a compiler does, such as parsing our source code. When you give your Ruby code to the interpreter, it has to perform some compiler-like tasks before it starts to execute the code. It’s useful to think about Ruby working with our code in two phases: compile time and run time.

Parsing and making sense of our code happens at compile time. Executing that code happens at run time. This distinction is especially important when you consider the various types of warnings that Ruby can produce. Warnings emitted during the compilation phase usually have something to do with syntax problems that Ruby was able to work around. Run-time warnings, on the other hand, can indicate sloppy programming that might be the source of potential bugs. Paying attention to these warnings can help you fix mistakes before they become real problems. Before we talk about how to enable the various warnings in Ruby, let’s explore a few of the common warning messages and what causes them.

Warnings emitted during the compilation phase are especially important to pay attention to. The majority of them are generated when Ruby encounters ambiguous syntax and proceeds by choosing one of many possible interpretations. You obviously don’t want Ruby guessing what you really meant. Imagine what would happen if a future version of Ruby changed its interpretation of ambiguous code and your program started behaving differently! By paying attention to these types of warnings you can make the necessary changes to your code and completely avoid the ambiguity in the first place. Here’s an example of where the code isn’t completely clear and Ruby produces a warning:

irb> "808".split /0/
warning: ambiguous first argument; put parentheses or even spaces

When Ruby’s parser reaches the first forward slash, it has to decide if it’s the beginning of a regular expression literal, or if it’s the division operator. In this case, it makes the reasonable assumption that the slash starts a regular expression and should be the first argument to the split method. But it’s not hard to see how it could also be interpreted as the division operator with the output of the split command being its left operand. The warning itself is generic, and only half of it is helpful. But the fix is simple enough—use parentheses:

irb> "808".split(/0/)
---> ["8", "8"]

If you send your code through Ruby with warnings enabled you’re likely to see other warnings related to operators and parentheses. The reason is nearly always the same. Ruby isn’t 100% sure what you mean and picks the most reasonable interpretation. But again, do you really want Ruby guessing what you mean or would you rather be completely clear from the start? Here are two more examples of ambiguous method calls that are fixed by adding parentheses around the arguments:

irb> dirs = ['usr', 'local', 'bin']

irb> File.join *dirs
warning: '*' interpreted as argument prefix

irb> File.join(*dirs)
---> "usr/local/bin"

irb> dirs.map &:length
warning: '&' interpreted as argument prefix

irb> dirs.map(&:length)
---> [3, 5, 3]

Other useful warnings during the compilation phase have to do with variables. For example, Ruby will warn you if you assign a value to a variable, but then never end up using it. This might mean you’re wasting a bit of memory but could also mean you’ve forgotten to include a value in your calculation. You’ll also receive a warning if you create two variables with the same name in the same scope, so-called variable shadowing. This can happen if you accidentally specify a block argument with the same name as a variable that’s already in scope. Both types of variable warnings can be seen in this example:

irb> def add (x, y)
       z = 1
       x + y
warning: assigned but unused variable - z

irb> def repeat (n, &block)
       n.times {|n| block.call(n)}

warning: shadowing outer local variable - n

As you can see, these compile-time warnings don’t necessarily mean that you’ve done anything wrong, but they certainly could mean that. So the best course of action is to review the warnings and make changes to your source code accordingly. The same can also be said of warnings generated while your code is executing, or what I call run-time warnings. These are warnings that can only be detected after your code has done something suspicious such as accessing an uninitialized instance variable or redefining an existing method. Both of which could have been done on purpose or by accident. Like the other warnings we’ve seen, these are easy to remedy.

I think you get the point, so I won’t enumerate a bunch of descriptive, easy-to-fix run-time warnings for you. Instead, I’d rather show you how to enable warnings in the first place. Here again, it becomes important to distinguish between compile time and run time. If you want Ruby to produce warnings about your code as it’s being parsed, you need to make sure the interpreter’s warning flag is enabled. That might be as easy as passing the “-w” command-line option to Ruby:

ruby -w script.rb

For some types of applications, it’s not that simple. Perhaps your Ruby program is being started automatically by a web server or a background job processing server. More commonly, you’re using something like Rake to run your tests and you want warnings enabled. When you can’t enable warnings by giving the interpreter the “-w” command-line option, you can do it indirectly by setting the RUBYOPT environment variable. How you set this variable will depend on the operating system and how your application is being started. What’s most important is that the RUBYOPT environment variable be set to “-w” within the environment where your application is going to run before Ruby starts.

(I should also mention that if you’re using Rake to run your tests you have another option available for enabling warnings. Item 36 includes an example Rakefile that does just that.)

Now, there’s one last way to enable warnings. It’s poorly documented and as a result often causes a lot of confusion. Within your program you can inspect and manipulate the $VERBOSE global variable (and its alias, $-w). If you want all possible warning messages you should set this variable to true. Setting it to false lowers the verbosity (producing fewer warnings) and setting it to nil disables warnings altogether. You might be thinking to yourself, “Hey, if I can set $VERBOSE to true, then I don’t need to mess around with this ‘-w’ business.” This is where the distinction between compile time and run time really helps.

If you don’t use the “-w” command-line option with the Ruby interpreter, but instead rely upon the $VERBOSE variable, you won’t be able to see compile-time warnings. That’s because setting the $VERBOSE global variable doesn’t happen until your program is running. By that time, the parsing phase is over and you’ve missed all the compile-time warnings. So, there are two guidelines to follow. First, enable compile-time warnings by using the “-w” command-line option to the Ruby interpreter or by setting the RUBYOPT environment variable to “-w”. Second, control run-time warnings using the $VERBOSE global variable.

My advice is to always enable compile-time and run-time warnings during application development and while tests are running. If you absolutely must disable run-time warnings, do so by temporarily setting the $VERBOSE global variable to nil.

Unfortunately, enabling warning messages comes with a warning of its own. I’m disappointed to report that it’s not common practice to enable warnings. So, if you’re using any RubyGems and enable warnings, you’re likely to get a lot of warnings originating from within them. This may strongly tempt you to subsequently disable warnings. Thankfully, when Ruby prints warnings to the terminal it includes the file name and line number corresponding to the warning. It shouldn’t be too hard for you to write a script to filter out unwanted warnings. Even better, become a good open-source citizen and contribute fixes for any gems that are being a little sloppy and producing warnings.

Things to Remember

  • Use the “-w” command-line option to the Ruby interpreter to enable compile-time and run-time warnings. You can also set the RUBYOPT environment variable to “-w”.
  • If you must disable run-time warnings, do so by temporarily setting the $VERBOSE global variable to nil.
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