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docile/README.md
2014-03-31 12:36:52 -04:00

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# Docile
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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...
[1]: http://www.google.com/search?q=docile+definition "Google"
## Usage
### Basic
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?
```ruby
with_array([]) do
push 1
push 2
pop
push 3
end
#=> [1, 3]
```
No problem, just define the method `with_array` like this:
```ruby
def with_array(arr=[], &block)
Docile.dsl_eval(arr, &block)
end
```
Easy!
### Advanced
Mutating (changing) an Array instance is fine, but what usually makes a good DSL is a [Builder Pattern][2].
For example, let's say you want a DSL to specify how you want to build a Pizza:
```ruby
@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:
```ruby
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:
``` ruby
def pizza(&block)
Docile.dsl_eval(PizzaBuilder.new, &block).build
end
```
It's just that easy!
[2]: http://stackoverflow.com/questions/328496/when-would-you-use-the-builder-pattern "Builder Pattern"
### Block parameters
Parameters can be passed to the DSL block.
Supposing you want to make some sort of cheap [Sinatra][3] knockoff:
```ruby
@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:
```ruby
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
```
[3]: http://www.sinatrarb.com "Sinatra"
### Functional-Style DSL Objects
Sometimes, you want to use an object as a DSL, but it doesn't quite fit the
[imperative](http://en.wikipedia.org/wiki/Imperative_programming) pattern shown
above.
Instead of methods like
[Array#push](http://www.ruby-doc.org/core-2.0/Array.html#method-i-push), which
modifies the object at hand, it has methods like
[String#reverse](http://www.ruby-doc.org/core-2.0/String.html#method-i-reverse),
which returns a new object without touching the original. Perhaps it's even
[frozen](http://www.ruby-doc.org/core-2.0/Object.html#method-i-freeze) in
order to enforce [immutability](http://en.wikipedia.org/wiki/Immutable_object).
Wouldn't it be great if we could just treat these methods as a DSL as well?
```ruby
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:
```ruby
def with_immutable_string(str="", &block)
Docile.dsl_eval_immutable(str, &block)
end
```
All set!
## 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
``` bash
$ gem install docile
```
## Links
* [Source](https://github.com/ms-ati/docile)
* [Documentation](http://rubydoc.info/gems/docile)
* [Bug Tracker](https://github.com/ms-ati/docile/issues)
## Status
Works on [all ruby versions since 1.8.7](https://github.com/ms-ati/docile/blob/master/.travis.yml), or so Travis CI [tells us](https://travis-ci.org/ms-ati/docile).
Used by some pretty cool gems to implement their DSLs, notably including [SimpleCov](https://github.com/colszowka/simplecov). Keep an eye out for new gems using Docile at the [Ruby Toolbox](https://www.ruby-toolbox.com/projects/docile).
## 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) 2012-2014 Marc Siegel.
Licensed under the [MIT License](http://choosealicense.com/licenses/mit/), see [LICENSE](LICENSE) for details.