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YourDSL

A partial internal DSL compiler.

Lexing/Parsing responsibilities are handled by a combination of the Ruby interpreter itself (Ruby already lexes and parses itself) and a recorder to capture undefined Ruby methods, via method_missing, as an Abstract Syntax Tree (AST).

You supply the code generator.

Assumptions.

  • People love libraries with DSLs like Sinatra and ActiveRecord and Babushka and AASM and (name your favourite Ruby library).
  • What they love about these is the small amount of Ruby syntax required to get a whole lot done.
  • They love that they can contextualise their definitions (that make the libraries do things) with natural Ruby syntax such as blocks.
  • To enable these kinds of libraries the use of so-called 'meta-programming' constructs are required (instance_eval, instance_exec).
  • Clever but inexperienced Ruby programmers often make very useful libraries with wonderful APIs but horrible internals

Goals.

  • Decouple the language definition of an internal DSL from its implementation
  • Allow library authors to dream up APIs and implement them without entering Ruby metaprogramming hell (it is a real place).

Example

class Feature < Test::Unit::TestCase
  # Configuring lispy to geenrate an AST for our language

  PROC_KEYWORDS = [:Given, :When, :Then, :And]
  KEYWORDS = [:Scenario, :Tag] + PROC_KEYWORDS

  extend YourDSL
  record_your_dsl :only => KEYWORDS, :retain_blocks_for => PROC_KEYWORDS

  # Using our language
  Scenario "this gets lispyified" do
    Given "something" do
    end

    Then "test something exists" do
      fail "ohai"
    end
  end
end

# Barfing out the AST for the above
require 'rubygems'
require 'awesome_print'
ap Feature.output

# Example 'interpreter' for the AST generated above
# Executes the block on the Given and Then from above sequentially
# Spike on how an acceptance testing DSL could work
Feature.class_eval do
  def test_something
    scenario = Feature.output.expressions.first
    steps = scenario.scope.expressions
    instance_eval &steps[0].proc
    instance_eval &steps[1].proc
  end
end

 OUTPUTS:
 ➜  example git:(no_more_last_last_last) ✗ ruby feature.rb
 {
            :file => "feature.rb",
     :expressions => [
         [0] {
             :symbol => :Scenario,
               :args => "this gets lispyified",
             :lineno => "14",
               :proc => nil,
              :scope => {
                 :expressions => [
                     [0] {
                         :symbol => :Given,
                           :args => "something",
                         :lineno => "15",
                           :proc => #<Proc:[email protected]:15>,
                          :scope => nil
                     },
                     [1] {
                         :symbol => :Then,
                           :args => "test something exists",
                         :lineno => "18",
                           :proc => #<Proc:[email protected]:18>,
                          :scope => nil
                     }
                 ]
             }
         }
     ]
 }
 Loaded suite feature
 Started
 E
 Finished in 0.006305 seconds.

   1) Error:
 test_something(Feature):
 RuntimeError: ohai
     feature.rb:19:in `block (2 levels) in <class:Feature>'
     feature.rb:34:in `instance_eval'
     feature.rb:34:in `test_something'

 1 tests, 0 assertions, 0 failures, 1 errors, 0 skips

Credits

License.

Released under the MIT license (see MIT-LICENSE).