R – Does it make sense to create a Ruby gem that consists of only Rails template partials

Ruby has the concepts of blocks, which are essentially syntactic sugar around a section of code; they are a way to create closures and pass them to another method which may or may not use the block. A block can be invoked later on through a yield statement.

For example, a simple definition of an each method on Array might be something like:

class Array
  def each
    for i in self  
      yield(i)     # If a block has been passed, control will be passed here.
    end  
  end  
end  

Then you can invoke this like so:

# Add five to each element.
[1, 2, 3, 4].each{ |e| puts e + 5 }
> [6, 7, 8, 9]

Python has anonymous functions/closures/lambdas, but it doesn't quite have blocks since it's missing some of the useful syntactic sugar. However, there's at least one way to get it in an ad-hoc fashion. See, for example, here.

To avoid the risk of Over-engineering, I usually start with the feature I need directly into the application. Then, as soon as I need to use the same feature into another project, I check whether it is worth to extract it into a plugin or even a separate application providing an API.

Plugins and Gems are often interchangeable. Gems provides several significant advantages in terms of reusability and maintainability. On the other side, there are some specific known issue. For instance, a Rails app actually can't load rake tasks defined into a plugin packaged as a Gem.

Almost every Rails plugin can be packaged as a Gem. For instance, take my tabs_on_rails plugin.

You can install it as a Gem specifying the dependency on environment.rb. Or you can use script/plugin install command as you would expect.

If you want to achieve the same result, make sure to follow the standard Gem layout and provide the init.rb initialization script required by Rails. Also, you might want to create an install.rb and uninstall.rb file to include the post-install and post-uninstall hooks when the plugin is installed as a standard Rails plugin.

Last but not least, if you package a plugin as Gem you can reuse it in non-Rails projects and provide Rails-specific initializations using the init.rb file. Non-Rails applications will simply ignore it.