dazuma / blockenspiel

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.

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 debate in the document ImplementingDSLblocks.txt 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:

• Implementing a DSL block that takes a parameter is straightforward.
• Safely implementing a DSL block that doesn’t take a parameter is tricky.

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 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.txt, 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 by default does not use the instance_eval technique. Instead, it implements a mechanism using mixin modules, a technique first proposed by Why. In this technique, the add_foo and add_bar methods are temporarily mixed into the caller’s self object. That is, self does not change, as it would if we used instance_eval, so helper methods like callers_helper_method still remain available as expected. But, the add_foo and add_bar methods are also made available temporarily for the duration of the block. When called, they are intercepted and redirected to your ConfigMethods instance just as if you had called them directly via a block parameter. Blockenspiel handles the object redirection behind the scenes so you do not have to think about it. With Blockenspiel, the caller retains access to its helper methods, and even its own instance variables, within the block, because self has not been modified.

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 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)    # <--- Renamed 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

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, :receive_block => true) do |value, blk|
     my_foo = value
     my_bar = blk.call
   end
 end

That API is in 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

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

Requirements

• Ruby 1.8.7 or later, or JRuby 1.2 or later. Ruby 1.9 compatible.
• Rubygems

Installation

 gem install blockenspiel

Known issues and limitations

• Implementing wildcard DSL methods using method_missing doesn’t work. I haven’t yet figured out the right semantics for this case.

Development and support

Documentation is available at virtuoso.rubyforge.org/blockenspiel

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

Report bugs on RubyForge at rubyforge.org/projects/virtuoso

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 Why The Lucky Stiff. See his 6 October 2008 blog posting, Mixing Our Way Out Of Instance Eval? for further discussion.

The unmixer code is based on Mixology, by Patrick Farley, anonymous z, Dan Manges, and Clint Bishop. The code has been stripped down and modified to support MRI 1.9 and JRuby 1.2.

License

Copyright 2008-2009 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:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
• Neither the name of the copyright holder, nor the names of any other contributors to this software, may be used to endorse or promote products derived from this software without specific prior written permission.

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.