DSLs in Ruby

Dave Burt

What Is A DSL?

• Not a general-purpose language
• No inscrutable, arcane syntax or geek-speak
• Not verbose for domain problems

Rubix Cube Moves

RL’FB’UD’RL’

HR Management

with Employee("123-45-6789") do
  dock_salary 1000
  warn_about :misconduct
end

Dungeon

class Dragon < Creature
  life 1340     # tough scales
  strength 451  # bristling veins
  charisma 1020 # toothy smile
  weapon 939    # fire breath
end

Database connection

development:
  adapter: mssql
  host: localhost
  database: northwind
  authentication: integrated

production:
  development
  

Database table definition

create_table :updates {
  foreign_key :user
  foreign_key :group

  text   :body
  string :type

  timestamps!
}

HTML templating

html {
  head {
    title action_name
    stylesheet_link_tag 'scaffold'
  }

  body {
    p flash[:notice], :style => "color: green" 
    self << @content_for_layout
  }
}

Interdependendent tasks

  task :all => ['hello']

  file 'main.o' => ["main.c", "greet.h"] do
    sh "cc -c -o main.o main.c"
  end

  file 'greet.o' => ['greet.c'] do
    sh "cc -c -o greet.o greet.c"
  end

  file "hello" => ["main.o", "greet.o"] do
    sh "cc -o hello main.o greet.o"
  end
  

Invoicing

new_invoice do
  address 'Test Client, Street, ZIP'
  code '2006-45'
  date '3rd Oct'

  at_rate 50 do
    spent 3.hours_on('Morning meeting')
    spent 0.5.hours_on('Making the coffee for the boss').on('3rd Sept')
  end 
end

Workflow

route "Problems" do
  step "Problem Resolution"
end

route "EMail Order" do
  step "Validate Customer"
  step "Assemble Order"
  step "Charge Credit Card" do
    on "invalid", :route => "Problems", :step => "Problem Resolution"
  end
  step "Ship"
end

Conversation

>> I.say "I love the Ruby language”
You said, ‘I love the Ruby language’

>> You.cannot :learn, “Ruby”
Didn’t your mother tell you never say never? Of course you can learn Ruby

Types of DSL

• Brand new language
• Build on existing language

How to make a DSL

1. Write code in the language
2. Make it legal

Let's make one

Write code in the language

slideshow {
  slide "How to make a DSL" {
    list {
      item "Write code in the language"
      item "Make it legal"
    }
  }
}

How will our language be used?

What do we want it to do?

Generate XHTML like this slideshow here

How will we make that happen?

Running the program using the Ruby interpreter will dump the XHTML on standard output

References

• Jim Weirich 2005, Speaking the Lingo: Creating Domain Specific Languages in Ruby
• why the lucky stiff, Dwemthy's Array

Please Ask Questions Now

Thanks for listening