Using metaprogramming to refactor many similar Rake tasks?

I inherited a project with a lot of badly-written Rake tasks that I
need to clean up a bit. Because the Rakefiles are enormous and often
prone to bizarre nonsensical dependencies, I’m simplifying and
isolating things a bit by refactoring everything to classes.

Specifically, that pattern is the following:

namespace :foobar do
  desc "Frozz the foobar."
  task :frozzify do
    unless Rake.application.lookup('_frozzify')
      require 'tasks/foobar'
      Foobar.new.frozzify
    end
    Rake.application['_frozzify'].invoke
  end

  # Above pattern repeats many times.
end

# Several namespaces, each with tasks that follow this pattern.

In tasks/foobar.rb, I have something that looks like this:

class Foobar
  def frozzify()
    # The real work happens here.
  end

  # ... Other tasks also in the :foobar namespace.
end

For me, this is great, because it allows me to separate the task
dependencies from each other and to move them to another location
entirely, and I’ve been able to drastically simplify things and
isolate the dependencies. The Rakefile doesn’t hit a require until
you actually try to run a task. Previously this was causing serious
issues because you couldn’t even list the tasks without it blowing up.

My problem is that I’m repeating this idiom very frequently. Notice
the following patterns:

  • For every namespace :xyz_abc, there is a corresponding class in
    tasks/... in the file tasks/[namespace].rb, with a class name that
    looks like XyzAbc.

  • For every task in a particular namespace, there is an identically
    named method in the associated namespace class. For example, if
    namespace :foo_bar has a task :apples, you would expect to see
    def apples() ... inside the FooBar class, which itself is in
    tasks/foo_bar.rb.

  • Every task :t defines a “meta-task” _t (that is, the task name
    prefixed with an underscore) which is used to do the actual work.

I still want to be able to specify a desc-description for the tasks
I define, and that will be different for each task. And, of course, I
have a small number of tasks that don’t follow the above pattern at
all, so I’ll be specifying those manually in my Rakefile.

I’m relatively sure that this can be refactored with metaprogramming
in some way so that I don’t have to keep repeating the same idiom over
and over, but I lack the experience to see how it could be done, or
whether I’m even on the right track at all. Any thoughts or advice to
point me in the right direction?

Thanks!

– James

On 4/22/10, James W. [email protected] wrote:

  end

  # Above pattern repeats many times.
end

# Several namespaces, each with tasks that follow this pattern.

There may be some feature of rake that helps dry up code like this…
I wouldn’t know. But if not, maybe something like this would help:
(untested, you need to customize it anyway)

def declare_tasks namespace, *tasks
eval %{
namespace :#{namespace.downcase} do
#{tasks.map{|task|
<<-END
desc("#{task.capitalize} the #{namespace}")
task :#{task}ify do
unless Rake.application.lookup(’#{task}ify’)
require ‘tasks/#{namespace.downcase}’
#{namespace}.new.#{task}ify
end
Rake.application[’
#{task}ify’].invoke
end
END
}.join
}
end
}
end

declare_task “Foobar”, “frozz”, “frotz”, “xyzzy”

Eval is the heavy guns of meta-programming. Instead of downcase, you
need to use the rails camel-case-to-snake-case conversion method. I
forget the name. Also, you could dry it up more by scanning the tasks/
directory for namespaces and tasks to define… but that might be
kind of hard.

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