This gem allows to build an instance_eval based DSL without losing access to the object calling the block. It features:

• Fallback to the object calling the DSL (which we will call the main provider) if a method was called that the DSL does not implement
• Fallback to an arbitrary amount of objects (which we call the additional providers) before falling back to the object calling the DSL
• Accessing instance variables within the object calling the DSL

It can also act as a context for renderers such as the HAML engine (see below).

Note that when accessing instance variables from within a DSL block, you are looking at a shallow copy of the object. Writing to primitive types will not affect the main provider, unless instance variable backfiring is enabled (see below). Altering complex types will however alter the object that the main providers is also referencing to, regardless of the backfire setting.

...because such abstract concepts are easier to show than to explain ;-)

class Testdsl < Dslblend::Base
  def foo
    puts 'foo was called'
  end
end

class Mainprovider
  def run
    @myvariable = 42

    Testdsl.new.evaluate do
      foo # normal DSL call
      bar # calling a method of Mainprovider, which would normally fail
      puts @myvariable # accessing an iunstance variable of the main provider
    end
  end

  def bar
    puts 'bar was called'
  end
end

class Testdsl < Dslblend::Base
  def foo
    puts 'foo was called'
  end
end

class Horse
  def gallop
    puts 'wheee!'
  end
end

class Dog
  def bark
    puts 'woof!'
  end
end

class Mainprovider
  def run
    @myvariable = 42
    horse = Horse.new
    dog = Dog.new

    Testdsl.new(horse, dog).evaluate do
      foo # normal DSL call
      bar # calling a method of Mainprovider, which would normally fail
      gallop # calling a method of Horse
      bark # calling a method of Dog
      puts @myvariable # accessing an iunstance variable of the main provider
    end
  end

  def bar
    puts 'bar was called'
  end
end

If a method is implemented by more than one providers, the precedence is as follows:

• DSL
• Additional providers in given order
• Main provider, i.e. the object calling the DSL

The main provider is normally automatically set to the context in which the evaluated block was defined. This can be undesirable, for instance in a nested DSL, when providing a block at an inner level that should run in the context of the class that calls the DSL in the first place.

To mitigate this, the main provider can be given as follows on instanciation:

TestDsl.new(horse, dog, main_provider: object_of_your_choice_e_g_self)

As stated above, adding or overwriting instance variables within the evaluation of the DSL will normally not alter the main provider. However, Dslblend can transfer the DSLs instance variables back to the main provider through a mechanism called backfire (the name sounds dangerous on purpose, use this feature with caution to avoid side effects).

To enable the feature, pass the argument backfire_vars: true to the evaluate call:

class Testlab
  def backfire_demo
    @foo = 42
    Testdsl.new.evaluate(backfire_vars: true) do
      @foo = 123
      @bar = 3.14
    end
    @foo == 123 # true
    @bar == 3.14 # true
  end
end

An interesting use/abuse (pick your opinion) of Dslblend is using it as a context for rendering engines such as HAML, similar to Rails' ActionView. Since HAML::Engine uses its own instance_eval, it does not call Dslblend's evaluate and thus, the Dslblend object is not initialized properly. To address it, you can call the initialization methods yourself. To avoid namespace collisions, they are prefixed with _dslblend_. Here is an example:

# Assuming you have the instance of the Rails controller and wish to use it to render some HAML in `@content`.
# The goal is to be able to call controller methods such as `link_to`, `form_for` etc. from inside the HAML.
def render_content_to_string(controller, locals)
  controller.helpers.extend Haml::Helpers
  controller.helpers.init_haml_helpers
  # Explicitely setting the main provider on instanciation allows us to skip `_dslblend_detect_main_provider`.
  context = Dslblend::Base.new(controller, main_provider: self)
  request_context._dslblend_transfer_inst_vars_from_main_provider # This is necessary because HAML does not call Dslblend's `evaluate`.
  return Haml::Engine.new(@content.strip_heredoc, format: :html5).render(context, { **locals })
end

This gem is written and maintained by Sandro Kalbermatter. It is based on the following two resources:

• https://www.dan-manges.com/blog/ruby-dsls-instance-eval-with-delegation
• http://djellemah.com/blog/2013/10/09/instance-eval-with-access-to-outside-scope