Blockenspiel is a helper library designed to make it easy to implement DSL blocks. It is designed to be comprehensive and robust, supporting most common usage patterns, and working correctly in the presence of nested blocks and multithreading.

Blockenspiel is a helper library providing several different strategies for implementing DSL blocks. It supports both DSLs that take a block parameter and those that do not. For example:

# Call DSL block with parameter
configure_me do |config|
  config.add_foo(1)
  config.add_bar(2)
end

# Call DSL block without parameter
configure_me do
  add_foo(3)
  add_bar(4)
end

To support the above usage, you can do this:

# Implement DSL block methods
class ConfigMethods
  include Blockenspiel::DSL
  def add_foo(value)
    # do something
  end
  def add_bar(value)
    # do something
  end
end

# Implement configure_me method
def configure_me(&block)
  Blockenspiel.invoke(block, ConfigMethods.new)
end

By default, Blockenspiel uses a “delegation” technique (to my knowledge first proposed by Dan Manges) to support parameterless blocks while mitigating some of the issues with instance_eval. It supports nested blocks and multithreaded access, and provides a variety of tools for handling the typical issues you may encounter when writing DSLs. On some ruby platforms, Blockenspiel also supports a mixin technique (proposed by Why The Lucky Stiff).

For more detailed usage and examples, see Blockenspiel.rdoc.

For an extended analysis of different ways to implement DSL blocks, see ImplementingDSLblocks.rdoc.

• Ruby 1.9.3 or later, JRuby 1.5 or later, or Rubinius 1.0 or later.

gem install blockenspiel

• Implementing wildcard DSL methods using method_missing doesn’t work. I haven’t yet decided on the right semantics for this case, or whether it is even a reasonable feature at all.

• Including Blockenspiel::DSL in a module (rather than a class) is not supported, but this could appear in a future release.

• Find a way to implement mixin behavior reliably on MRI.

Documentation is available at dazuma.github.com/blockenspiel/rdoc

Source code is hosted on Github at github.com/dazuma/blockenspiel

Contributions are welcome. Fork the project on Github.

Build status: <img src=“https://secure.travis-ci.org/dazuma/blockenspiel.png” />

Report bugs on Github issues at github.org/dazuma/blockenspiel/issues

Contact the author at dazuma at gmail dot com.

Blockenspiel is written by Daniel Azuma (www.daniel-azuma.com/).

The mixin implementation is based on a concept by the late Why The Lucky Stiff, documented in his 6 October 2008 blog posting entitled “Mixing Our Way Out Of Instance Eval?”. The original link has disappeared along with its author, but you may find copies or mirrors out there.

The unmixer code is based on Mixology, version by Patrick Farley, anonymous z, Dan Manges, and Clint Bishop. The JRuby code is adapted from Mixology 0.1, and has been stripped down and modified to support JRuby >= 1.2. The Rubinius code was adapted from unreleased code in the Mixology source tree and modified to support Rubinius 1.0. I know Mixology 0.2 is now available, but its Rubinius support is not active, and I’d rather keep the unmixer bundled with Blockenspiel for now to reduce dependencies. Earlier versions of Blockenspiel also included a C extension, adapted from Mixology, to support mixins for MRI, but this code has been disabled due to issues with newer versions of Ruby.

The dsl_attr_writer and dsl_attr_accessor feature came from a suggestion by Luis Lavena.

Copyright 2008 Daniel Azuma.

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