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386 lines
10 KiB
Markdown
386 lines
10 KiB
Markdown
# Docile
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[](https://rubygems.org/gems/docile)
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[](https://rubygems.org/gems/docile)
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[](https://gitter.im/ms-ati/docile?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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[](http://rubydoc.info/github/ms-ati/docile)
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[](http://inch-ci.org/github/ms-ati/docile)
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[](https://github.com/ms-ati/docile/actions/workflows/main.yml)
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[](https://codecov.io/github/ms-ati/docile)
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[](https://codeclimate.com/github/ms-ati/docile/maintainability)
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Ruby makes it possible to create very expressive **Domain Specific
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Languages**, or **DSL**'s for short. However, it requires some deep knowledge and
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somewhat hairy meta-programming to get the interface just right.
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"Docile" means *Ready to accept control or instruction; submissive* [[1]]
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Instead of each Ruby project reinventing this wheel, let's make our Ruby DSL
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coding a bit more docile...
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[1]: http://www.google.com/search?q=docile+definition "Google"
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## Usage
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### Basic: Ruby [Array](http://ruby-doc.org/core-3.0.0/Array.html) as DSL
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Let's say that we want to make a DSL for modifying Array objects.
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Wouldn't it be great if we could just treat the methods of Array as a DSL?
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```ruby
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with_array([]) do
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push 1
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push 2
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pop
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push 3
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end
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#=> [1, 3]
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```
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No problem, just define the method `with_array` like this:
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```ruby
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def with_array(arr=[], &block)
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Docile.dsl_eval(arr, &block)
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end
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```
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Easy!
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### Next step: Allow helper methods to call DSL methods
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What if, in our use of the methods of Array as a DSL, we want to extract
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helper methods which in turn call DSL methods?
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```ruby
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def pop_sum_and_push(n)
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sum = 0
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n.times { sum += pop }
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push sum
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end
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Docile.dsl_eval([]) do
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push 5
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push 6
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pop_sum_and_push(2)
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end
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#=> [11]
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```
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Without Docile, you may find this sort of code extraction to be more
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challenging.
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### Wait! Can't I do that with just `instance_eval` or `instance_exec`?
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Good question!
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In short: **No**.
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Not if you want the code in the block to be able to refer to anything
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the block would normally have access to from the surrounding context.
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Let's be very specific. Docile internally uses `instance_exec` (see [execution.rb#26](lib/docile/execution.rb#L26)), 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.
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For example:
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```ruby
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class ContextOfBlock
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def example_of_contexts
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@block_instance_var = 1
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block_local_var = 2
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with_array do
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push @block_instance_var
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push block_local_var
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pop
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push block_sees_this_method
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end
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end
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def block_sees_this_method
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3
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end
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def with_array(&block)
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{
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docile: Docile.dsl_eval([], &block),
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instance_eval: ([].instance_eval(&block) rescue $!),
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instance_exec: ([].instance_exec(&block) rescue $!)
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}
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end
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end
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ContextOfBlock.new.example_of_contexts
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#=> {
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:docile=>[1, 3],
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:instance_eval=>#<NameError: undefined local variable or method `block_sees_this_method' for [nil]:Array>,
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:instance_exec=>#<NameError: undefined local variable or method `block_sees_this_method' for [nil]:Array>
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}
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```
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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.
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### Build a Pizza
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Mutating (changing) an Array instance is fine, but what usually makes a good DSL is a [Builder Pattern][2].
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For example, let's say you want a DSL to specify how you want to build a Pizza:
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```ruby
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@sauce_level = :extra
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pizza do
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cheese
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pepperoni
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sauce @sauce_level
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end
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#=> #<Pizza:0x00001009dc398 @cheese=true, @pepperoni=true, @bacon=false, @sauce=:extra>
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```
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And let's say we have a PizzaBuilder, which builds a Pizza like this:
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```ruby
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Pizza = Struct.new(:cheese, :pepperoni, :bacon, :sauce)
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class PizzaBuilder
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def cheese(v=true); @cheese = v; self; end
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def pepperoni(v=true); @pepperoni = v; self; end
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def bacon(v=true); @bacon = v; self; end
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def sauce(v=nil); @sauce = v; self; end
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def build
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Pizza.new(!!@cheese, !!@pepperoni, !!@bacon, @sauce)
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end
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end
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PizzaBuilder.new.cheese.pepperoni.sauce(:extra).build
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#=> #<Pizza:0x00001009dc398 @cheese=true, @pepperoni=true, @bacon=false, @sauce=:extra>
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```
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Then implement your DSL like this:
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```ruby
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def pizza(&block)
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Docile.dsl_eval(PizzaBuilder.new, &block).build
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end
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```
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It's just that easy!
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[2]: http://stackoverflow.com/questions/328496/when-would-you-use-the-builder-pattern "Builder Pattern"
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### Multi-level and Recursive DSLs
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Docile is a very easy way to write a multi-level DSL in Ruby, even for
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a [recursive data structure such as a tree][4]:
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```ruby
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Person = Struct.new(:name, :mother, :father)
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person {
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name 'John Smith'
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mother {
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name 'Mary Smith'
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}
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father {
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name 'Tom Smith'
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mother {
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name 'Jane Smith'
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}
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}
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}
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#=> #<struct Person name="John Smith",
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# mother=#<struct Person name="Mary Smith", mother=nil, father=nil>,
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# father=#<struct Person name="Tom Smith",
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# mother=#<struct Person name="Jane Smith", mother=nil, father=nil>,
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# father=nil>>
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```
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See the full [person tree example][4] for details.
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[4]: https://gist.github.com/ms-ati/2bb17bdf10a430faba98
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### Block parameters
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Parameters can be passed to the DSL block.
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Supposing you want to make some sort of cheap [Sinatra][3] knockoff:
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```ruby
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@last_request = nil
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respond '/path' do |request|
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puts "Request received: #{request}"
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@last_request = request
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end
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def ride bike
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# Play with your new bike
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end
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respond '/new_bike' do |bike|
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ride(bike)
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end
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```
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You'd put together a dispatcher something like this:
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```ruby
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require 'singleton'
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class DispatchScope
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def a_method_you_can_call_from_inside_the_block
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:useful_huh?
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end
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end
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class MessageDispatch
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include Singleton
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def initialize
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@responders = {}
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end
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def add_responder path, &block
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@responders[path] = block
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end
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def dispatch path, request
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Docile.dsl_eval(DispatchScope.new, request, &@responders[path])
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end
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end
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def respond path, &handler
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MessageDispatch.instance.add_responder path, handler
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end
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def send_request path, request
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MessageDispatch.instance.dispatch path, request
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end
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```
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[3]: http://www.sinatrarb.com "Sinatra"
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### Functional-Style Immutable DSL Objects
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Sometimes, you want to use an object as a DSL, but it doesn't quite fit the
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[imperative](http://en.wikipedia.org/wiki/Imperative_programming) pattern shown
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above.
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Instead of methods like
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[Array#push](http://www.ruby-doc.org/core-3.0.0/Array.html#method-i-push), which
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modifies the object at hand, it has methods like
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[String#reverse](http://www.ruby-doc.org/core-3.0.0/String.html#method-i-reverse),
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which returns a new object without touching the original. Perhaps it's even
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[frozen](http://www.ruby-doc.org/core-3.0.0/Object.html#method-i-freeze) in
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order to enforce [immutability](http://en.wikipedia.org/wiki/Immutable_object).
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Wouldn't it be great if we could just treat these methods as a DSL as well?
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```ruby
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s = "I'm immutable!".freeze
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with_immutable_string(s) do
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reverse
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upcase
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end
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#=> "!ELBATUMMI M'I"
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s
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#=> "I'm immutable!"
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```
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No problem, just define the method `with_immutable_string` like this:
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```ruby
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def with_immutable_string(str="", &block)
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Docile.dsl_eval_immutable(str, &block)
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end
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```
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All set!
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### Accessing the block's return value
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Sometimes you might want to access the return value of your provided block,
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as opposed to the DSL object itself. In these cases, use
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`dsl_eval_with_block_return`. It behaves exactly like `dsl_eval`, but returns
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the output from executing the block, rather than the DSL object.
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```ruby
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arr = []
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with_array(arr) do
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push "a"
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push "b"
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push "c"
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length
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end
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#=> 3
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arr
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#=> ["a", "b", "c"]
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```
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```ruby
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def with_array(arr=[], &block)
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Docile.dsl_eval_with_block_return(arr, &block)
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end
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```
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## Features
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1. Method lookup falls back from the DSL object to the block's context
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2. Local variable lookup falls back from the DSL object to the block's
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context
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3. Instance variables are from the block's context only
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4. Nested DSL evaluation, correctly chaining method and variable handling
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from the inner to the outer DSL scopes
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5. Alternatives for both imperative and functional styles of DSL objects
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## Installation
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``` bash
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$ gem install docile
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```
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## Links
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* [Source](https://github.com/ms-ati/docile)
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* [Documentation](http://rubydoc.info/gems/docile)
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* [Bug Tracker](https://github.com/ms-ati/docile/issues)
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## Status
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Works on [all currently supported ruby versions](https://github.com/ms-ati/docile/blob/master/.github/workflows/main.yml),
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or so [Github Actions](https://github.com/ms-ati/docile/actions)
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tells us.
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Used by some pretty cool gems to implement their DSLs, notably including
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[SimpleCov](https://github.com/colszowka/simplecov). Keep an eye out for new
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gems using Docile at the
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[Ruby Toolbox](https://www.ruby-toolbox.com/projects/docile).
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## Release Policy
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Docile releases follow
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[Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html).
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## Note on Patches/Pull Requests
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* Fork the project.
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* Setup your development environment with:
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`gem install bundler; bundle install`
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* Make your feature addition or bug fix.
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* Add tests for it. This is important so I don't break it in a future version
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unintentionally.
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* Commit, do not mess with rakefile, version, or history.
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(if you want to have your own version, that is fine but bump version in a
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commit by itself I can ignore when I pull)
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* Send me a pull request. Bonus points for topic branches.
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## Copyright & License
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Copyright (c) 2012-2021 Marc Siegel.
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Licensed under the [MIT License](http://choosealicense.com/licenses/mit/),
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see [LICENSE](LICENSE) for details.
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