DSLs

• 7

  OCT
  2008

  DSL Block Styles

  There's an argument that rages in the Ruby camps: to instance_eval() or not to instance_eval(). Most often this argument is triggered by DSL discussions where we tend to want code like:

  configurable.config do
    width 100
    mode  :wrap
  end
  

  You can accomplish something like this by passing the block to instance_eval() and changing self to an object that defines the width() and mode() methods. Of course changing self is always dangerous. We may have already been inside an object and planning to use methods from that namespace:

  class MyObject
    include Configurable       # to get the config() method shown above
  
    def initialize
      config do
        width calculate_width  # a problem:  may not work with instance_eval()
      end
    end
  
    private
  
    def calculate_width        # the method we want to use
      # ...
    end
  end
  

  In this example, if width() comes from a different configuration object, we're in trouble. The instance_eval() will shift the focus away from our MyObject instance and we will get a NoMethodError when we try to call calculate_width(). This may prevent us from being able to use Configurable in our code.

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  In: Ruby Voodoo | Tags: DSLs & Style | 7 Comments