Writing your own DSL
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This document discusses how to write your own domain-specific language (DSL). It begins by defining what a DSL is - a language designed for a particular domain that allows for more expressive communication within that domain. It then covers the key aspects of writing a DSL, including parsing the DSL into an abstract syntax tree using a parser generator like DSL::Maker, validating the parsed structure follows the rules of the language, and producing output from the validated structure. An example DSL for defining vehicle data is used to demonstrate these concepts. The document concludes by noting that production from a parsed DSL structure is left to the developer and encourages an outside-in, multi-pass approach to transformation.
![#!/usr/bin/env ruby
require ‘vehicle/dsl’
filename = ARGV.shift ||
raise “No filename provided.”
vehicles = Vehicle::DSL.parse_dsl(
IO.read(filename),
)
# Do something here with vehicles
[
Car[
:make => ‘Accord’,
:year => 1990,
:engine => Engine[
:hemi => true,
],
],
Car[
:make => Civic,
:year => 2014,
:engine => nil,
],
]](https://image.slidesharecdn.com/dsl-makercrb2015-08-150818125914-lva1-app6891/85/Writing-your-own-DSL-31-320.jpg)
![#!/usr/bin/env ruby
require ‘vehicle/dsl’
filename = ARGV.shift ||
raise “No filename provided.”
vehicles = Vehicle::DSL.parse_dsl(
IO.read(filename),
)
# Do something here with vehicles
[
. . .
Truck[
:make => ‘F-150’,
:year => 1999,
:engine => nil
],
. . .
]](https://image.slidesharecdn.com/dsl-makercrb2015-08-150818125914-lva1-app6891/85/Writing-your-own-DSL-33-320.jpg)
Writing your own DSL
- 1. Writing your own DSL Yes, it is that easy!
- 3. Who am I? ● Rob Kinyon ○ @rkinyon ○ rob.kinyon@gmail.com ● Devops lead for many years ● Developer in Ruby, Python, Perl, JS, and others.
- 4. What is a DSL? ● Domain-Specific Language
- 5. What is a DSL? ● Domain-Specific Language ● Language - A vehicle for communication
- 6. What is a DSL? ● Domain-Specific Language ● Language - A vehicle for communication ● Domain - A restrained set of concepts
- 7. What is a DSL? ● Domain-Specific Language ● Language - A vehicle for communication ● Domain - A restrained set of concepts ● Specific - Limited to.
- 8. What is a DSL? ● Domain-Specific Language ● Language - A vehicle for communication ● Domain - A restrained set of concepts ● Specific - Limited to. ○ No, really. :)
- 9. Language and Communication ● Communicate in one direction ○ Author -> Executor
- 10. Language and Communication ● Communicate in two directions ○ Author -> Executor ○ Author -> Maintainer
- 11. Language and Communication ● Communicate in three directions ○ Author -> Executor ○ Author -> Maintainer ○ Specifier -> Author
- 12. Language and Communication ● Communicate in four directions ○ Author -> Executor ○ Author -> Maintainer ○ Specifier -> Author ○ Author -> Verifier
- 13. Language and Communication ● Communicate in MANY directions ○ Author -> Executor ○ Author -> Maintainer ○ Specifier -> Author ○ Author -> Verifier ○ Author -> Teammate(s) ○ Developer -> Sysadmin/Devops ○ … -> …
- 14. Language and Communication ● Communicate in MANY directions ○ Author -> Executor ○ Author -> Maintainer ○ Specifier -> Author ○ Author -> Verifier ○ Author -> Teammate(s) ○ Developer -> Sysadmin/Devops ○ … -> … The ONLY computer
- 15. Language and Communication ● Communicate in MANY directions ○ Author -> Executor ○ Author -> Maintainer ○ Specifier -> Author ○ Author -> Verifier ○ Author -> Teammate(s) ○ Developer -> Sysadmin/Devops ○ … -> … All humans
- 16. Language and Communication ● Communicate in MANY directions ○ Author -> Executor ○ Author <-> Maintainer ○ Specifier <-> Author ○ Author <-> Verifier ○ Author <-> Teammate(s) ○ Developer <-> Sysadmin/Devops ○ … <-> … All human communication is two-way
- 17. Domain-Specific ● Eskimos supposedly have 50+ words for “snow” ○ Depends on how you count it ● Saami has 1000+ words dealing with reindeer ○ snarri - a reindeer with short, branched horns ○ busat - a bull with a single, large testicle
- 18. Domain-specific : Busat Busat - The quality of having appropriately-specific expressiveness for the domain.
- 19. DSLs you already use ● SQL ○ set manipulation DSL ● CSS ○ tree-visitor-defining DSL for setting metadata ● HAML ○ HTML-definition DSL ● Bash ○ A crappy way of issue shell commands with logic
- 20. Places for a DSL ● Packaging and orchestration ○ most devops/operations activities ● Configuration file generation ○ web servers ○ monitoring ○ datastores ● Configuration value management across environments ● Anything extremely complicated (such as SQL) ● Anything repetitive (such as CSS)
- 21. Reasons for a DSL ● Let the important things shine ● General-purpose is overly-verbose ● Bugs hide in boilerplate ● Non-developers can read and comprehend ○ And maybe even propose changes through PRs?
- 22. Reasons for a DSL DSL is to Ruby as Ruby is to Java
- 23. Writing a DSL ● Three passes ○ Parsing ○ Validation ○ Production
- 24. Writing a DSL - Parsing ● DSL::Maker for parsing
- 25. Car = Struct.new(:make, :year, :engine) Engine = Struct.new(:hemi) class VehicleDSL < DSL::Maker add_entrypoint(:car, { :make => String, :year => Integer, :engine => generate_dsl({ :hemi => Boolean, }) do Engine.new(hemi) end, }) do |*args| default(:make, args, 0) Car.new(make, model, engine) end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 26. Car = Struct.new(:make, :year, :engine) Engine = Struct.new(:hemi) class VehicleDSL < DSL::Maker add_entrypoint(:car, { :make => String, :year => Integer, :engine => generate_dsl({ :hemi => Boolean, }) do Engine.new(hemi) end, }) do |*args| default(:make, args, 0) Car.new(make, model, engine) end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 27. Car = Struct.new(:make, :year, :engine) Engine = Struct.new(:hemi) class VehicleDSL < DSL::Maker add_entrypoint(:car, { :make => String, :year => Integer, :engine => generate_dsl({ :hemi => Boolean, }) do Engine.new(hemi) end, }) do |*args| default(:make, args, 0) Car.new(make, model, engine) end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 28. Car = Struct.new(:make, :year, :engine) Engine = Struct.new(:hemi) class VehicleDSL < DSL::Maker add_entrypoint(:car, { :make => String, :year => Integer, :engine => generate_dsl({ :hemi => Boolean, }) do Engine.new(hemi) end, }) do |*args| default(:make, args, 0) Car.new(make, model, engine) end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 29. Car = Struct.new(:make, :year, :engine) Engine = Struct.new(:hemi) class VehicleDSL < DSL::Maker add_entrypoint(:car, { :make => String, :year => Integer, :engine => generate_dsl({ :hemi => Boolean, }) do Engine.new(hemi) end, }) do |*args| default(:make, args, 0) Car.new(make, model, engine) end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 30. Car = Struct.new(:make, :year, :engine) Engine = Struct.new(:hemi) class VehicleDSL < DSL::Maker add_entrypoint(:car, { :make => String, :year => Integer, :engine => generate_dsl({ :hemi => Boolean, }) do Engine.new(hemi) end, }) do |*args| default(:make, args, 0) Car.new(make, model, engine) end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 31. #!/usr/bin/env ruby require ‘vehicle/dsl’ filename = ARGV.shift || raise “No filename provided.” vehicles = Vehicle::DSL.parse_dsl( IO.read(filename), ) # Do something here with vehicles [ Car[ :make => ‘Accord’, :year => 1990, :engine => Engine[ :hemi => true, ], ], Car[ :make => Civic, :year => 2014, :engine => nil, ], ]
- 32. . . . truck ‘F-150’ { year 1999 } . . . . . . Truck = Struct.new(:make, :year, :engine) . . . class VehicleDSL < DSL::Maker . . . add_entrypoint(:truck, { :make => String, :year => Integer, :engine => . . ., }) do |*args| default(:make, args, 0) Truck.new(make, model, nil) end end
- 33. #!/usr/bin/env ruby require ‘vehicle/dsl’ filename = ARGV.shift || raise “No filename provided.” vehicles = Vehicle::DSL.parse_dsl( IO.read(filename), ) # Do something here with vehicles [ . . . Truck[ :make => ‘F-150’, :year => 1999, :engine => nil ], . . . ]
- 34. Writing a DSL - Validation ● DSL::Maker for parsing ● DSL::Maker for validation
- 35. . . . class VehicleDSL < DSL::Maker . . . add_validation(:car) do |car| unless car.engine return “Cars must have an engine” end end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 36. . . . class VehicleDSL < DSL::Maker . . . add_validation(:car) do |car| unless car.engine return “Cars must have an engine” end end end car { make ‘Accord’ year 1990 engine { hemi Yes } } car ‘Civic’ { year 2014 }
- 37. #!/usr/bin/env ruby require ‘vehicle/dsl’ filename = ARGV.shift || raise “No filename provided.” # This raises the error vehicles = Vehicle::DSL.parse_dsl( IO.read(filename), ) # Do something here with vehicles Error: Cars must have an engine
- 38. Writing a DSL - Production ● DSL::Maker for parsing ● DSL::Maker for validation ● You’re on your own for production
- 39. Writing a DSL - Production ● Work from outside in. ○ Parsing is done inside-out. ● Transform in a series of passes. ○ Expand everything (it’s just data) ● Don’t do anything irrevocable until the end ○ Work in temp directories, stage everything
- 40. Conclusion ● DSL::Maker 0.1.0 is available right now ● Patches welcome ○ 100% test coverage ● I’m blogging about this at http: //streamlined-book.blogspot.com ○ First post on the topic
- 41. Questions?