Blockenspiel ↑

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.

This is an introduction to DSL blocks and the features of Blockenspiel.

What's a DSL block? ↑

A DSL block is an API pattern in which a method call takes a block that can provide further configuration for the call. A classic example is the Rails route definition:

ActionController::Routing::Routes.draw do |map|
  map.connect ':controller/:action/:id'
  map.connect ':controller/:action/:id.:format'
end

Some libraries go one step further and eliminate the need for a block parameter. RSpec is a well-known example:

describe Stack do
  before(:each) do
    @stack = Stack.new
  end
  describe "(empty)" do
    it { @stack.should be_empty }
    it "should complain when sent #peek" do
      lambda { @stack.peek }.should raise_error(StackUnderflowError)
    end
  end
end

In both cases, the caller provides descriptive information in the block, using a domain-specific language. The second form, which eliminates the block parameter, often appears cleaner; however it is also sometimes less clear what is actually going on.

How does one implement such a beast? ↑

Implementing the first form is fairly straightforward. You would create a class defining the methods (such as connect in our Rails routing example above) that should be available within the block. When, for example, the draw method is called with a block, you instantiate the class and yield it to the block.

The second form is perhaps more mystifying. Somehow you would need to make the DSL methods available on the “self” object inside the block. There are several plausible ways to do this, such as using instance_eval. However, there are many subtle pitfalls in such techniques, and quite a bit of discussion has taken place in the Ruby community regarding how–or whether–to safely implement such a syntax.

I have included a critical survey of the discussion in the document ImplementingDSLblocks.rdoc for the curious. Blockenspiel takes what I consider the best of the solutions and implements them in a comprehensive way, shielding you from the complexity of the Ruby metaprogramming while offering a simple way to implement both forms of DSL blocks.

So what is Blockenspiel? ↑

Blockenspiel operates on the following observations:

With that in mind, Blockenspiel provides a set of tools that allow you to take an implementation of the first form of a DSL block, one that takes a parameter, and turn it into an implementation of the second form, one that doesn't take a parameter.

Suppose you wanted to write a simple DSL block that takes a parameter:

configure_me do |config|
  config.add_foo(1)
  config.add_bar(2)
end

You could write this as follows:

class ConfigMethods
  def add_foo(value)
    # do something
  end
  def add_bar(value)
    # do something
  end
end

def configure_me
  yield ConfigMethods.new
end

That was easy. However, now suppose you wanted to support usage without the “config” parameter. e.g.

configure_me do
  add_foo(1)
  add_bar(2)
end

With Blockenspiel, you can do this in two quick steps. First, tell Blockenspiel that your ConfigMethods class is a DSL.

class ConfigMethods
  include Blockenspiel::DSL   # <--- Add this line
  def add_foo(value)
    # do something
  end
  def add_bar(value)
    # do something
  end
end

Next, write your configure_me method using Blockenspiel:

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

Now, your configure_me method supports both DSL block forms. A caller can opt to use the first form, with a parameter, simply by providing a block that takes a parameter. Or, if the caller provides a block that doesn't take a parameter, the second form without a parameter is used.

How does that help me? (Or, why not just use instance_eval?) ↑

As noted earlier, some libraries that provide parameter-less DSL blocks use a simple instance_eval, and they could even support both the parameter and parameter-less mechanisms by checking the block arity:

def configure_me(&block)
  if block.arity == 1
    yield ConfigMethods.new
  else
    ConfigMethods.new.instance_eval(&block)
  end
end

That seems like a simple and effective technique that doesn't require a separate library, so why use Blockenspiel? Because instance_eval introduces a number of surprising problems. I discuss these issues in detail in ImplementingDSLblocks.rdoc, but just to get your feet wet, suppose the caller wanted to call its own methods inside the block:

def callers_helper_method
  # ...
end

configure_me do
  add_foo(1)
  callers_helper_method  # Error! self is now an instance of ConfigMethods
                         # so this will fail with a NameError
  add_bar(2)
end

Blockenspiel employs a number of techniques to mitigate the ill effects of instance_eval. It delegates methods that are not part of the DSL, back to the enclosing context object, so that the caller retains access to helper methods. It also includes an optional experimental technique (not available on all ruby platforms) that temporarily mixes the DSL methods directly into the caller's self object, so that instance variable access is retained.

Is that it? ↑

Although the basic usage is very simple, Blockenspiel is designed to be comprehensive. It supports all the use cases that I've run into during my own implementation of DSL blocks. Notably:

By default, Blockenspiel lets the caller choose to use a parametered block or a parameterless block, based on whether or not the block actually takes a parameter. You can also disable one or the other, to force the use of either a parametered or parameterless block.

You can also let the caller use your DSL by passing you a string or a file rather than a block. That is, you can create file-based DSLs such as the Rails routes file.

You can control wich methods of the class are available from parameterless blocks, and/or make some methods available under different names. Here are a few examples:

class ConfigMethods
  include Blockenspiel::DSL

  def add_foo         # automatically added to the dsl
    # do stuff...
  end

  def my_private_method
    # do stuff...
  end
  dsl_method :my_private_method, false   # remove from the dsl

  dsl_methods false   # stop automatically adding methods to the dsl

  def another_private_method  # not added
    # do stuff...
  end

  dsl_methods true    # resume automatically adding methods to the dsl

  def add_bar         # this method is automatically added
    # do stuff...
  end

  def add_baz
    # do stuff
  end
  dsl_method :add_baz_in_dsl, :add_baz  # Method named differently
                                        # in a parameterless block
end

This is also useful, for example, when you use attr_writer. Parameterless blocks do not support attr_writer (or, by corollary, attr_accessor) well because methods with names of the form “attribute=” are syntactically indistinguishable from variable assignments:

configure_me do |config|
  config.foo = 1    # works fine when the block has a parameter
end

configure_me do
  # foo = 1     # <--- Doesn't work: looks like a variable assignment
  set_foo(1)    # <--- Fix it by renaming to this instead
end

# This is implemented like this::
class ConfigMethods
  include Blockenspiel::DSL
  attr_writer :foo
  dsl_method :set_foo, :foo=    # Make "foo=" available as "set_foo"
end

This is in fact a common enough case that Blockenspiel includes conveninence tools for a DSL-friendly attr_writer and attr_accessor, providing an alternate syntax for setting attributes within a parameterless block:

configure_me do
  # foo = 1             # This syntax wouldn't work, but
  foo 1                 # this syntax is now supported.
  puts "foo is #{foo}"  # The getter still works.
end

# This is implemented like this::
class ConfigMethods
  include Blockenspiel::DSL
  dsl_attr_accessor :foo     # DSL-friendly attr_accessor
end

In some cases, you might want to dynamically generate a DSL object rather than defining a static class. Blockenspiel provides a tool to do just that. Here's an example:

Blockenspiel.invoke(block) do
  add_method(:set_foo) do |value|
    my_foo = value
  end
  add_method(:set_things_using_block) do |value, &blk|
    my_foo = value
    my_bar = blk.call
  end
end

That API is itself a DSL block, and yes, Blockenspiel uses itself to implement this feature.

By default Blockenspiel uses mixins, which usually exhibit fairly safe and non-surprising behavior. However, there are a few cases when you might want the instance_eval behavior anyway. RSpec is a good example of such a case, since the DSL is being used to construct objects, so it makes sense for instance variables inside the block to belong to the object being constructed. Blockenspiel gives you the option of choosing instance_eval in case you need it. Blockenspiel also provides a compromise behavior that uses a proxy to dispatch methods to the DSL object or the block's context.

Blockenspiel also correctly handles nested blocks. e.g.

configure_me do
  set_foo(1)
  configure_another do     # A block within another block
    set_bar(2)
    configure_another do   # A block within itself
      set_bar(3)
    end
  end
end

Blockenspiel provides three strategies for doing parameterless DSL blocks.

The default strategy uses a proxy object that delegates unrecognized methods out to the calling context. It should work well for most cases.

Second, some applications might want to use the simple instance_eval behavior. RSpec is a good example of such a case, since the DSL is being used to construct objects, so it makes sense for instance variables inside the block to belong to the object being constructed.

Third, an experimental mixin strategy is provided, which adds the DSL methods directly to the context's self object, and removes them afterward. This is available on Rubinius and JRuby but not on MRI.

Finally, Blockenspiel is thread safe, correctly handling, for example, the case of multiple threads trying to mix methods into the same object concurrently.

Requirements ↑

Installation ↑

gem install blockenspiel

Known issues and to-do items ↑

Development and support ↑

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:

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

Contact the author at dazuma at gmail dot com.

Author / Credits ↑

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 is gone, 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.

License ↑

Copyright 2008 Daniel Azuma.

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.