Docile

Ruby makes it possible to create very expressive Domain Specific Languages, or DSL's for short. However, it requires some deep knowledge and somewhat hairy meta-programming to get the interface just right.

"Docile" means Ready to accept control or instruction; submissive [1]

Instead of each Ruby project reinventing this wheel, let's make our Ruby DSL coding a bit more docile...

Usage

Basic: Ruby Array as DSL

Let's say that we want to make a DSL for modifying Array objects. Wouldn't it be great if we could just treat the methods of Array as a DSL?

with_array([]) do
  push 1
  push 2
  pop
  push 3
end
#=> [1, 3]

No problem, just define the method with_array like this:

def with_array(arr=[], &block)
  Docile.dsl_eval(arr, &block)
end

Easy!

Next step: Allow helper methods to call DSL methods

What if, in our use of the methods of Array as a DSL, we want to extract helper methods which in turn call DSL methods?

def pop_sum_and_push(n)
  sum = 0
  n.times { sum += pop }
  push sum
end

Docile.dsl_eval([]) do
  push 5
  push 6
  pop_sum_and_push(2)
end
#=> [11]

Without Docile, you may find this sort of code extraction to be more challenging.

Wait! Can't I do that with just instance_eval or instance_exec?

Good question!

In short: No.

Not if you want the code in the block to be able to refer to anything the block would normally have access to from the surrounding context.

Let's be very specific. Docile internally uses instance_exec (see execution.rb#25), adding a small layer to support referencing local variables, instance variables, and methods from the block's context or the target object's context, interchangeably. This is "the hard part", where most folks making a DSL in Ruby throw up their hands.

For example:

class ContextOfBlock
  def example_of_contexts
    @block_instance_var = 1
    block_local_var = 2

    with_array do
      push @block_instance_var
      push block_local_var
      pop
      push block_sees_this_method 
    end
  end
  
  def block_sees_this_method
    3
  end  

  def with_array(&block)
    {
      docile: Docile.dsl_eval([], &block),
      instance_eval: ([].instance_eval(&block) rescue $!),
      instance_exec: ([].instance_exec(&block) rescue $!)
    }  
  end
end

ContextOfBlock.new.example_of_contexts
#=> {
      :docile=>[1, 3],
      :instance_eval=>#<NameError: undefined local variable or method `block_sees_this_method' for [nil]:Array>,
      :instance_exec=>#<NameError: undefined local variable or method `block_sees_this_method' for [nil]:Array>
    }

As you can see, it won't be possible to call methods or access instance variables defined in the block's context using just the raw instance_eval or instance_exec methods. And in fact, Docile goes further, making it easy to maintain this support even in multi-layered DSLs.

Build a Pizza

Mutating (changing) an Array instance is fine, but what usually makes a good DSL is a Builder Pattern.

For example, let's say you want a DSL to specify how you want to build a Pizza:

@sauce_level = :extra

pizza do
  cheese
  pepperoni
  sauce @sauce_level
end
#=> #<Pizza:0x00001009dc398 @cheese=true, @pepperoni=true, @bacon=false, @sauce=:extra>

And let's say we have a PizzaBuilder, which builds a Pizza like this:

Pizza = Struct.new(:cheese, :pepperoni, :bacon, :sauce)

class PizzaBuilder
  def cheese(v=true); @cheese = v; self; end
  def pepperoni(v=true); @pepperoni = v; self; end
  def bacon(v=true); @bacon = v; self; end
  def sauce(v=nil); @sauce = v; self; end
  def build
    Pizza.new(!!@cheese, !!@pepperoni, !!@bacon, @sauce)
  end
end

PizzaBuilder.new.cheese.pepperoni.sauce(:extra).build
#=> #<Pizza:0x00001009dc398 @cheese=true, @pepperoni=true, @bacon=false, @sauce=:extra>

Then implement your DSL like this:

def pizza(&block)
  Docile.dsl_eval(PizzaBuilder.new, &block).build
end

It's just that easy!

Multi-level and Recursive DSLs

Docile is a very easy way to write a multi-level DSL in Ruby, even for a recursive data structure such as a tree:

Person = Struct.new(:name, :mother, :father)

person {
  name 'John Smith'
  mother {
    name 'Mary Smith'
  }
  father {
    name 'Tom Smith'
    mother {
      name 'Jane Smith'
    }
  }
}

#=> #<struct Person name="John Smith",
#                   mother=#<struct Person name="Mary Smith", mother=nil, father=nil>,
#                   father=#<struct Person name="Tom Smith",
#                                          mother=#<struct Person name="Jane Smith", mother=nil, father=nil>,
#                                          father=nil>>

See the full person tree example for details.

Block parameters

Parameters can be passed to the DSL block.

Supposing you want to make some sort of cheap Sinatra knockoff:

@last_request = nil
respond '/path' do |request|
  puts "Request received: #{request}"
  @last_request = request
end

def ride bike
  # Play with your new bike
end

respond '/new_bike' do |bike|
  ride(bike)
end

You'd put together a dispatcher something like this:

require 'singleton'

class DispatchScope
  def a_method_you_can_call_from_inside_the_block
    :useful_huh?
  end
end

class MessageDispatch
  include Singleton

  def initialize
    @responders = {}
  end

  def add_responder path, &block
    @responders[path] = block
  end

  def dispatch path, request
    Docile.dsl_eval(DispatchScope.new, request, &@responders[path])
  end
end

def respond path, &handler
  MessageDispatch.instance.add_responder path, handler
end

def send_request path, request
  MessageDispatch.instance.dispatch path, request
end

Functional-Style Immutable DSL Objects

Sometimes, you want to use an object as a DSL, but it doesn't quite fit the imperative pattern shown above.

Instead of methods like Array#push, which modifies the object at hand, it has methods like String#reverse, which returns a new object without touching the original. Perhaps it's even frozen in order to enforce immutability.

Wouldn't it be great if we could just treat these methods as a DSL as well?

s = "I'm immutable!".freeze

with_immutable_string(s) do
  reverse
  upcase
end
#=> "!ELBATUMMI M'I"

s
#=> "I'm immutable!"

No problem, just define the method with_immutable_string like this:

def with_immutable_string(str="", &block)
  Docile.dsl_eval_immutable(str, &block)
end

All set!

Accessing the block's return value

Sometimes you might want to access the return value of your provided block, as opposed to the DSL object itself. In these cases, use dsl_eval_with_block_return. It behaves exactly like dsl_eval, but returns the output from executing the block, rather than the DSL object.

arr = []
with_array(arr) do
  push "a"
  push "b"
  push "c"
  length
end
#=> 3

arr
#=> ["a", "b", "c"]

def with_array(arr=[], &block)
  Docile.dsl_eval_with_block_return(arr, &block)
end

Features

1. Method lookup falls back from the DSL object to the block's context
2. Local variable lookup falls back from the DSL object to the block's context
3. Instance variables are from the block's context only
4. Nested DSL evaluation, correctly chaining method and variable handling from the inner to the outer DSL scopes
5. Alternatives for both imperative and functional styles of DSL objects

Installation

$ gem install docile

Links

• Source
• Documentation
• Bug Tracker

Status

Works on all ruby versions since 1.8.7, or so Travis CI tells us.

Used by some pretty cool gems to implement their DSLs, notably including SimpleCov. Keep an eye out for new gems using Docile at the Ruby Toolbox.

Release Policy

Docile releases follow Semantic Versioning 2.0.0.

Note on Patches/Pull Requests

• Fork the project.
• Setup your development environment with: gem install bundler; bundle install
• Make your feature addition or bug fix.
• Add tests for it. This is important so I don't break it in a future version unintentionally.
• Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
• Send me a pull request. Bonus points for topic branches.

Copyright & License

Copyright (c) 2012-2018 Marc Siegel.

Licensed under the MIT License, see LICENSE for details.