Writing a DSL in Ruby

1. Writing a DSL in Ruby
Why Ruby makes it soooo easy!

2. DSL!!!!11one

3. domain specific
language TL;DR
➡ Customized to specific problem domain
➡ Evaluated in context of the domain
➡ Helps creating ubiquitous language

4. internal vs. external
„Internal DSLs are particular ways of using a
host language to give the host language the
feel of a particular language.“
„External DSLs have their own custom syntax
and you write a full parser to process them.“

5. internal vs. external
„Internal DSLs are particular ways of using a
host language to give the host language the
feel of a particular language.“
„External DSLs have their own custom syntax
and you write a full parser to process them.“

6. internal vs. external
„Internal DSLs are particular ways of using a
host language to give the host language the
feel of a particular language.“
er
wl
Fo
tin
ar
„External DSLs have their own custom syntax
M
and you write a full parser to process them.“

7. Rails, collection of DSLs
• Routes
• Tag- / Form-Helpers
• Builder
• Associations, Migrations, ...
• Rake / Thor
• Bundler
• [...]

8. all about API design
Ruby makes it easy to create DSL:
• expressive & concise syntax
• open classes
• mixins
• operator overloading
• [...]

11. Gregory Brown Russ Olsen

12. DSL by EXAMPLE

13. Method Params
link_to "Articles", { :controller => "articles" }, :id => "news", :class => "article"

14. Method Params
link_to "Articles", { :controller => "articles" }, :id => "news", :class => "article"
def doit_with(some, parameters)
# does it
end

15. Method Params
link_to "Articles", { :controller => "articles" }, :id => "news", :class => "article"
def doit_with(some, parameters) def doit_with(mandatory, optional=nil, parameters=nil)
# does it # does it
end end

16. Method Params
link_to "Articles", { :controller => "articles" }, :id => "news", :class => "article"
def doit_with(some, parameters) def doit_with(mandatory, optional=nil, parameters=nil)
# does it # does it
end end
def doit_with(options={:named => 'parameter'})
# does it
end

17. Method Params
link_to "Articles", { :controller => "articles" }, :id => "news", :class => "article"
def doit_with(some, parameters) def doit_with(mandatory, optional=nil, parameters=nil)
# does it # does it
end end
def doit_with(options={:named => 'parameter'}) def doit_with(mandatory, *optional_paramters)
# does it # does it
end end

18. Method Params
link_to "Articles", { :controller => "articles" }, :id => "news", :class => "article"
def doit_with(some, parameters) def doit_with(mandatory, optional=nil, parameters=nil)
# does it # does it
end end
def doit_with(options={:named => 'parameter'}) def doit_with(mandatory, *optional_paramters)
# does it # does it
end end
def doit_with(*args)
options = args.pop if Hash === args.last
name = args.first
# [...]
raise ArgumentError, 'Bad combination of parameters' unless args.size == 1
# does it
end

19. Blocks
Twitter.configure do |config|
config.consumer_key = YOUR_CONSUMER_KEY
config.consumer_secret = YOUR_CONSUMER_SECRET
end

20. Blocks
Twitter.configure do |config|
config.consumer_key = YOUR_CONSUMER_KEY
config.consumer_secret = YOUR_CONSUMER_SECRET
end
def doit_with(&explicit_block)
explicit_block.call
end

21. Blocks
Twitter.configure do |config|
config.consumer_key = YOUR_CONSUMER_KEY
config.consumer_secret = YOUR_CONSUMER_SECRET
end
def doit_with(&explicit_block) def doit_with_implicit_block
explicit_block.call yield if block_given?
end end

22. Blocks
Twitter.configure do |config|
config.consumer_key = YOUR_CONSUMER_KEY
config.consumer_secret = YOUR_CONSUMER_SECRET
end
def doit_with(&explicit_block) def doit_with_implicit_block
explicit_block.call yield if block_given?
end end
def doit_with_block_for_configuration
yield self
end

23. Blocks
Twitter.configure do |config|
config.consumer_key = YOUR_CONSUMER_KEY
config.consumer_secret = YOUR_CONSUMER_SECRET
end
def doit_with(&explicit_block) def doit_with_implicit_block
explicit_block.call yield if block_given?
end end
def doit_with_block_for_configuration
yield self
end
def doit_with(&arity_sensitive_block)
arity_sensitive_block.call 'foo', 'bar' if
arity_sensitive_block.arity == 2
end

24. Instance Eval
Twitter.configure do |config|
config.consumer_key = YOUR_CONSUMER_KEY
config.consumer_secret = YOUR_CONSUMER_SECRET
end

25. Instance Eval
Twitter.configure do |config|
consumer_key = YOUR_CONSUMER_KEY
config.consumer_key = YOUR_CONSUMER_KEY
consumer_secret = YOUR_CONSUMER_SECRET
config.consumer_secret = YOUR_CONSUMER_SECRET
end

26. Instance Eval
Twitter.configure do |config|
consumer_key = YOUR_CONSUMER_KEY
config.consumer_key = YOUR_CONSUMER_KEY
consumer_secret = YOUR_CONSUMER_SECRET
config.consumer_secret = YOUR_CONSUMER_SECRET
end
def doit_with(&instance_eval_block)
if instance_eval_block.arity == 0
# could also be class_eval, module_eval
self.instance_eval(&instance_eval_block)
else
instance_eval_block.call self
end
end

27. Method Missing
Client.find_by_firstname_and_lastname_and_gender('uschi', 'mueller', :female)

28. Method Missing
Client.find_by_firstname_and_lastname_and_gender('uschi', 'mueller', :female)
METHOD_PATTERN = /^find_by_/
def method_missing(method, *args, &block)
if method.to_s =~ METHOD_PATTERN
# finder magic
else
super
end
end

29. Method Missing
Client.find_by_firstname_and_lastname_and_gender('uschi', 'mueller', :female)
METHOD_PATTERN = /^find_by_/
METHOD_PATTERN = /^find_by_/
def method_missing(method, *args, &block)
if method.to_s =~ METHOD_PATTERN
def method_missing(method, *args, &block)
# finder magic
if method.to_s =~ METHOD_PATTERN
else
# finder magic
super
else
end
end super
end
end respond_to?(method)
def
return true if method =~ METHOD_PATTERN
super
end

30. Code Generation
client.firstname = 'uschi'
puts client.lastname

31. Code Generation
client.firstname = 'uschi'
puts client.lastname
def generate(name)
self.class.send(:define_method, name){puts name}
eval("def #{name}() puts '#{name}' end")
def some_method(name)
puts name
end
# [...]
end

32. Makros
class Client < ActiveRecord::Base
has_one :address
has_many :orders
belongs_to :role
end

33. Makros
class Client < ActiveRecord::Base
has_one :address
has_many :orders
belongs_to :role
end
class << self
def has_something
# macro definition
end
end

34. Hooks
$ ruby simple_test.rb

35. Hooks
$ ruby simple_test.rb
at_exit do
# shut down code
end

36. Hooks
$ ruby simple_test.rb
at_exit do
# shut down code
end
inherited, included, method_missing, method_added,
method_removed, trace_var, set_trace_func, at_exit ...

37. Hooks
$ ruby simple_test.rb
at_exit do
# shut down code
end
set_trace_func proc { |event, file, line, id, binding, classname|
inherited, included, method_missing, method_added,
printf "%8s %s:%-2d trace_var, set_trace_func, line, id,...
method_removed, %10s %8sn", event, file, at_exit classname
}

38. Core Extensions
'text'.blank?

39. Core Extensions
'text'.blank?
class Object
def blank?
nil? || (respond_to?(:empty?) && empty?)
end
end

40. Core Extensions
'text'.blank?
class Object
def blank?
nil? || (respond_to?(:empty?) && empty?)
end unless method_defined?(:blank?)
end

41. Core Extensions
'text'.blank?
module MyGem
class Object
module CoreExtensions
module blank?
def Object
defnil? || (respond_to?(:empty?) && empty?)
blank?
end unless method_defined?(:blank?)
respond_to?(:empty?) ? empty? : !self
end
end
end
end
end
Object.send :include, MyGem::CoreExtensions::Object

42. Ruby techniques
✓Method Params ✓Makros
✓Blocks ✓Code generation
✓Instance Eval ✓Hooks
✓Method Missing ✓Core Extensions

43. External DSL
Ruby Tools:
• Switch Cases
• Regular Expressions
Ruby Parser:
• Treetop
• RACC

44. Links
★ Eloquent Ruby
★ Ruby Best Practices
★ Domain Specific Languages
★ Rebuil
★ Treetop

45. Over and Out

46. Over and Out