How to optionally load gems

I ran into an interesting problem today at work. I wanted to optionally require a ruby gem. For example, I wanted to include the metric_fu rake tasks, but if a user doesn’t have metric_fu installed, it’s no big deal.

Here’s how I implemented it. I added a method to the Kernel space (where require lives) called desire. I desire this gem to be installed, but I should just see a warning if it’s not installed, instead of an error.

Here’s the desire method (I put this in RAILS_ROOT/Rakefile).

[ruby]
module Kernel
def desire(library)
require library
return true
rescue LoadError
STDERR.puts "warning: #{library} gem desired but not found."
return false
end
end
[/ruby]

And here’s how I optionally include the metric_fu gem.

[ruby]
desire ‘metric_fu’
[/ruby]

If a user has the gem installed, rake -T will show the tasks.

[text]
jasonn:~/sources/my_files [master] $ rake -T
(in ~/sources/my_files)
rake metrics:all
[…]
jasonn:~/sources/my_files [master] $
[/text]

If the user does not have the gem installed, rake -T will show a simple warning and continue.

[text]
jasonn:~/sources/my_files [master] $ rake -T
(in ~/sources/my_files)
warning: metric_fu gem desired but not found.
[…]
jasonn:~/sources/my_files [master] $
[/text]

I don’t remember where I first heard the desire idea, but it works pretty well.