Better Composition With Traits

This talk explains the concept of traits, specifically the model described in the 2006 paper Traits: A Mechanism for Fine-grained Reuse by Ducasse et al.

It looks at differing trait implementations, which specific everyday design problems traits solve and how they differ from Ruby modules as a method for code reuse.

Finally it explores how some features introduced in Ruby 2 allow us to implement Ducasse's traits in Ruby and how to compose a class without touching the class hierarchy at all.

https://github.com/joecorcoran/fabrik
https://github.com/joecorcoran/talks/tree/master/traits

Transcript

1. Better Composition With Traits Joe Corcoran · corcoran.io · @josephcorcoran
2. class Bike def wheels; 2 end end
3. class Bike def wheels; 2 end end class Track < Bike def gearing; :fixed end def brakes; [] end end
4. class Bike def wheels; 2 end end class Track < Bike def gearing; :fixed end def brakes; [] end end track = Track.new track.wheels # => 2 track.gearing # => :fixed track.brakes # => []
5. class Track < Bike def gearing; :fixed end def brakes; [] end end class Fixie < Track def brakes; [:front, :back] end end
6. class Track < Bike def gearing; :fixed end def brakes; [] end end class Fixie < Track def brakes; [:front, :back] end end fixie = Fixie.new fixie.wheels # => 2 fixie.gearing # => :fixed fixie.brakes # => [:front, :back]
7. Composition
8. Prefer composition over inheritance. — Somebody
9. module Bike def wheels; 2 end end module Track def gearing; :fixed end def brakes; [] end end class Fixie include Bike include Track def brakes; [:front, :back] end end
10. But
11. module Bike def wheels; 2 end def brakes; [:front, :back] end end module Track def gearing; :fixed end def brakes; [] end end class Fixie include Bike include Track end fixie = Fixie.new fixie.brakes # => ?
12. module Bike def wheels; 2 end def brakes; [:front, :back] end end module Track def gearing; :fixed end def brakes; [] end end class Fixie include Bike include Track end fixie = Fixie.new fixie.brakes # => []
13. Fixie.ancestors # => [Fixie, Track, Bike, Object, ...]
14. Problems
15. Problems • Implied design
16. Problems • Implied design • Maintenance is harder
17. Problems • Implied design • Maintenance is harder • We need tests to cover our basic design
18. The diamond problem
20. Multiple inheritance is good, but there is no good way to do it. — Steve Cook
21. Traits
22. Traits: A Mechanism for Fine-grained Reuse Ducasse, Nierstrasz, Schärli, Wuyts and Black Software Composition Group University of Berne, 2006
23. Traits • Finite method dictionaries
24. Traits • Finite method dictionaries • Composable
25. Traits • Finite method dictionaries • Composable • Can be summed with other traits
29. Resolving conflicts
30. Resolving conflicts • Override the conflicting method
31. Resolving conflicts • Override the conflicting method • Exclude methods
32. Resolving conflicts • Override the conflicting method • Exclude methods • Alias methods
33. Flattening
34. Flattening • A well-defined class...
35. Flattening • A well-defined class has no conflicting methods after composition
36. Flattening • A well-defined class has no conflicting methods after composition • No need for traits!
37. Scala
38. trait Bike { def wheels() : Int = { return 2 } def brakes() : Vector[Symbol] = { return Vector('front, 'back) } } trait Track { def gearing() : Symbol = { return 'fixed } def brakes() : Vector[Symbol] = { return Vector() } } class Fixie extends Bike with Track {}
39. trait Bike { def wheels() : Int = { return 2 } def brakes() : Vector[Symbol] = { return Vector('front, 'back) } } trait Track { def gearing() : Symbol = { return 'fixed } def brakes() : Vector[Symbol] = { return Vector() } } class Fixie extends Bike with Track {} // => error: class Fixie inherits conflicting members
40. trait Bike { def wheels() : Int = { return 2 } def brakes() : Vector[Symbol] = { return Vector('front, 'back) } } trait Track { def gearing() : Symbol = { return 'fixed } def brakes() : Vector[Symbol] = { return Vector() } } class Fixie extends Bike with Track { override def brakes() : Vector[Symbol] = { return Vector('front, 'back) } }
41. module Bike def wheels; 2 end def brakes; [:front, :back] end end module Track def gearing; :fixed end def brakes; [] end end class Fixie include Bike include Track end
42. module Bike def wheels; 2 end def brakes; [:front, :back] end end module Track def gearing; :fixed end def brakes; [] end end class Fixie compose Bike, Track.methods(exclude: :brakes) end
43. source 'https://rubygems.org' gem 'fabrik'
44. class Bike extend Fabrik::Trait provides do def wheels; 2 end def brakes; [:front, :back] end end end class Track extend Fabrik::Trait provides do def gearing; :fixed end def brakes; [] end end end class Fixie extend Fabrik::Composer compose Bike, Track[exclude: :brakes] end
45. class Fixie extend Fabrik::Composer compose Bike, Track[aliases: { brakes: :stopping_machines }] end
46. class Fixie extend Fabrik::Composer def brakes; [:front] end compose Bike, Track end
47. class Bike extend Fabrik::Trait provides do def wheels; 2 end def brakes; [:front, :back] end end end
48. module Foo def bar; :baz end end bar = Foo.instance_method(:bar) # => #<UnboundMethod: Foo#bar>
49. module Foo def bar; :baz end end bar = Foo.instance_method(:bar) class Qux; end Qux.send(:define_method, :quux, bar) Qux.new.quux # => :baz
50. module Foo def bar; :baz end end bar = Foo.instance_method(:bar) class Qux; end Qux.send(:define_method, :quux, bar) # => TypeError: bind argument must be a subclass of Foo
52. module Foo def bar; :baz end end b1 = Foo.instance_method(:bar) b2 = Foo.instance_method(:bar) b1 == b2 # => true
53. module Foo def bar; :baz end end class Qux extend Fabrik::Trait provides_from Foo, :bar end
54. module Paintwork def paint! [:red, :green, :blue].sample end end class Bike extend Fabrik::Trait provides_from Paintwork, :paint! end class Track extend Fabrik::Trait provides_from Paintwork, :paint! end class Fixie extend Fabrik::Composer compose Bike, Track end fixie = Fixie.new fixie.paint! # => :green
55. Links • Traits: A Mechanism for Fine-grained Reuse (PDF) • github.com/joecorcoran/fabrik • github.com/joecorcoran/talks/tree/master/traits • corcoran.io • @josephcorcoran