DSLs As Teaching Tools

Given at Boulder Ruby Group in February 2012. A presentation examining DSLs, what they might be teaching us or especially new programmers, and some common tactics or methodologies for creating them.

Transcript

1. DSLs
   Let Them Teach!
   

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2. WHAT IS A DSL?
   ● DSL = Domain Specific Language
   ● A lingual subset dedicated to a particular
   problem (domain) area
   ● Code that makes it easier to work on specific
   (types of) problems
   ● A specialized tool
   

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3. WHAT IS A DSL? (REDUX)
   talk "DSL" do | topic |
   topic.acronym true , [ "Domain" , "Specific" , "Language" ]
   topic.definition "A lingual subset dedicated to a particular problem (domain) area"
   topic.definition "Code that makes it easier to work on specific (types of) problems"
   topic.definition "A specialized tool"
   end
   

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4. WHAT IS THE GOAL OF A DSL?
   ● To provide a powerful and declarative
   abstraction around a specific set of
   underlying library/code functionality
   ● From @justicefries -- "Provide a means for
   developers to effectively communicate with
   each other and domain experts in a domain."
   

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5. POP CULTURE
   ● Code Academy and Code Year
   ○ Founder Sims - "Programming is the new literacy"
   ○ http://bit.ly/wO7UP3
   ● "Well understood problems lend themselves
   to DSLs"
   ○ http://bit.ly/wCqFKk
   ● "Human beings, as tool makers, can create
   things that amplify our ability"
   ○ Steve Jobs (1980) re: Scientific American study about inter-species
   locomotive efficiency
   ○ http://bit.ly/wE6HyH
   

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6. POP CULTURE
   ● "On the contrary, the best way to teach
   someone to code is within a meaningful
   context, where they are given an existing
   solution and guided how to alter and
   customise it to their preferences or
   requirements, as often required in the “real
   world”."
   ○ Andy Young of Kernel Mag on "Coding For Success"
   ○ http://bit.ly/xGxDAn
   

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7. WHERE OH WHERE
   ● Rake
   ● Sinatra
   ● ActiveRecord
   

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8. RAKE
   ● A DSL for definining command line interface
   tasks in Ruby
   ● A ruby build tool
   

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9. RAKE
   $ cat Rakefile
   namespace :my_task_group do
   task :my_task do
   p "hello"
   end
   end
   $ rake my_task_group:my_task
   hello
   

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10. RAKE
    ● Understand that it's helpful to name
    command line tasks
    ● Understand that it's helpful to namespace
    similar command line tasks
    ● Understand that it's helpful to describe
    command line tasks
    

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11. SINATRA
    ● A DSL for HTTP application servers built on
    Rack
    

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12. SINATRA
    $ cat app.rb
    require "sinatra"
    get "/" { body "Hello World!" }
    $ ruby app.rb &> /dev/null &
    $ curl 0.0.0.0:4567
    Hello World!
    (Who can spot the syntax error?)
    

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13. SINATRA
    $ cat app.rb
    require "sinatra"
    get("/") { body "Hello World!" }
    $ ruby app.rb &> /dev/null &
    $ curl 0.0.0.0:4567
    Hello World!
    

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14. SINATRA
    ● Understand that HTTP servers have a set
    number of methods through which to
    communicate (GET, POST, etc.)
    ● Understand that you have access to certain
    things in an HTTP request
    ● Understand that there are specific ways to
    respond to HTTP requests (response codes)
    ● Understand that there are parts of an HTTP
    response that you can control to have the
    desired effect
    

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15. ACTIVERECORD
    ● ActiveRecord's class methods provide many
    different DSLs
    ● A DSL for object relationships
    ● A DSL for object data validations
    ● A DSL for object callbacks
    

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16. ACTIVERECORD
    class MyModel < ActiveRecord::Base
    belongs_to :my_other_model
    validates_presence_of :my_important_field
    after_save { p "Saved!" }
    end
    

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17. ACTIVERECORD
    ● Understand different ways that data can be
    related
    ● Understand the power behind callbacks and
    how they can help you manage your data
    ● Understand validations and how they can
    help you keep your data standardized
    

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18. OTHERS
    ● What other DSLs do you like?
    

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19. HOW CAN THEY TEACH
    ● DSLs teach by making it easier to
    understand how to accomplish a problem
    though programming
    ● They lower the barrier to entry
    ● Help introduce people to programming
    concepts at a more gradual pace
    ● Provide more output for less input
    

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20. EASY?
    ● Programming is not easy, but DSLs can
    make it easier
    ● People know problem domains, but they
    don't always know how to program
    ● DSLs puts programming in a language that
    people understand inherently
    

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21. BARRIERS TO ENTRY
    ● There are A LOT of them
    ● e.g. Syntax, development environment
    ● Remove some of them and you'll be helping
    someone
    

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22. EASE INTO IT
    ● Help make the landing into programming a
    little softer
    ● A little syntax and maybe a little logic and
    they're off!
    

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23. MORE FOR LESS
    ● By writing only a tidbit of expressive and
    declarative code, a user can have built
    something very powerful
    

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24. THE POINT
    ● DSLs can be teaching tools unintentionally
    ● Writing a ton of DSLs around the problems
    WE solve isn't necessarily helpful
    ● If people can be taught how to use DSLs
    then they're one step closer to being a
    programmer
    

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25. WHAT I REALIZED
    ● DSLs are already a powerful teaching tool
    ● They teach programming through use
    assuming you know, or are willing to learn,
    the domain
    ● What's hard about them are the barriers to
    entry
    

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26. HOW TO DIY
    ● 3 common methodologies
    ○ Naked
    ○ Proc passing
    ○ Class methods
    ● Think: Object Builder
    

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27. NAKED DSL
    ● Rake
    ● Add methods globally
    ○ http://bit.ly/wk51Xa
    

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28. NAKED DSL (RAKE)
    $ cat lib/rake/dsl_definition.rb
    module Rake
    module DSL
    ... ALL the code ...
    end
    end
    self.extend Rake::DSL
    

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29. NAKED DSL
    ● Eval in context
    ● Rackup
    ○ Rack::Builder
    ○ http://bit.ly/ylRd0z
    ● Capistrano
    ○ Capistrano::Configuration::Loading
    ○ http://bit.ly/xLwveX
    

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30. NAKED DSL (RACKUP)
    $ cat lib/rack/builder.rb
    module Rack
    class Builder
    def self.parse_file( config , opts = Server::Options.new )
    ... other code ...
    # cfgfile is the result of a File::read
    app = eval \
    "Rack::Builder.new {\n" + cfgfile + "\n}.to_app",
    TOPLEVEL_BINDING,
    config
    ... other code ...
    end
    end
    end
    

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31. NAKED DSL (CAPISTRANO)
    $ cat lib/capistrano/configuration/loading.rb
    module Capistrano; class Configuration
    module Loading; module ClassMethods
    def load( *params , &block )
    ... other code ...
    # options[ :string ] is the result of a File::read
    instance_eval options[:string] , options[:name] || ""
    ... other code ...
    end
    end; end
    end; end
    

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32. PROC PASSING DSL
    ● Rails routes
    ○ Rails 2.3.X -- `map` object yielded to block directly
    ■ http://bit.ly/yU27AL
    ○ Rails 3.X.X -- proc `instance_exec`-ed behind the
    scenes on to `map` object
    ■ http://bit.ly/xGgvVg
    ■ http://bit.ly/zJkxd3
    

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33. PROC PASSING DSL (RAILS 2.3.X)
    $ cat actionpack/lib/action_controller/routing/route_set.rb
    module ActionController; module Routing
    class RouteSet
    class Mapper; ... other code ... end
    def draw
    yield Mapper.new( self )
    ... other code ...
    end
    end
    end; end
    

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34. PROC PASSING DSL (RAILS 3.X.X)
    $ cat actionpack/lib/action_dispatch/routing/route_set.rb
    module ActionDispatch; module Routing
    class RouteSet
    def draw( &block )
    eval_block block
    end
    def eval_block( block )
    Mapping.new( self ).instance_exec &block
    end
    end
    end; end
    

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35. CLASS METHODS
    ● ActiveRecord
    ○ http://bit.ly/xr34Qk
    

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36. CLASS METHODS (AR)
    $ cat activerecord/lib/active_record/associations.rb
    module ActiveRecord; module Associations
    module ClassMethods
    def belongs_to( name , options = {} )
    Builder::BelongsTo.new( self , name , options )
    end
    end
    end; end
    

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37. TIPS
    ● `Class.new`
    ● `const_get` and `const_set`
    ● `class_variable_get` and
    `class_variable_set` as well as their instance
    variable counterparts
    ● `Forwardable` and `def_delegators`
    ● Defining things directly on the included class
    with `Module::included`
    

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38. PITFALLS
    ● Definition vs. execution scope
    ● What is `self`?
    

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39. IN CONCLUSION
    ● Create DSLs for non-programmers
    ● Remove barriers to entry
    ● When you're writing a DSL, think about
    building objects
    

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40. THE END
    Questions?
    Thank you!
    Links: http://bit.ly/cookrn_boulder_ruby_feb_12
    Ryan Cook
    cookrn@gmail.com
    

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