URLS

  http://raa.ruby-lang.org/search.rhtml?search=traits
  http://codeforpeople.com/lib/ruby/traits

ABOUT

  traits.rb is set of attr_* like methods on steroids, caffeine, and botox.  it
  encourages better living through meta-programming and uniform access
  priciples.  traits.rb supercedes attributes.rb.  why?  the name is shorter ;-)

VERSION

  0.6.0

HISTORY

  0.6.0
    - fixed bug in where a default trait given as an empty array, eg:

        class C;  has 'a' => [];  end

      was exploded into the empty list when passed to the setter to initialize
      the default value.

  0.5.0
    - general code cleanup

  0.4.0
    - tweaked writer code so multiple values can be passed to setters
    - tweaked method of running blocks to use instance_eval so explicit 'this'
      arg is no longer needed (though it can still be used)

  0.3.0
    added ability of default values to be specified with block for deferred
    context sensitive initialization (see sample/c.rb)

  0.1.0

    completely reworked impl so NO parsing of inspect strings is required -
    it's all straight methods (albeit quite confusing ones) now.  the
    interface is unchanged.

  0.0.0

    initial version


AUTHOR

  ara [dot] t [dot] howard [at] noaa [dot] gov

SAMPLES

  <========< sample/a.rb >========>

  ~ > cat sample/a.rb

    require 'traits'
    #
    # defining a trait is like attr_accessor in the simple case 
    #
    class C
      trait :t
    end
    
    o = C::new
    o.t = 42
    p o.t
    
    #
    # and can be made even shorter
    # 
    
    class B; has :x; end
    
    o = B::new
    o.x = 42
    p o.x 
    

  ~ > ruby sample/a.rb

    42
    42


  <========< sample/b.rb >========>

  ~ > cat sample/b.rb

    require 'traits'
    #
    # multiple traits can be defined at once using a list/array of string/sybmol
    # arguments
    #
    class C
      has :t0, :t1
      has %w( t2 t3 ) 
    end
    
    obj = C::new
    obj.t0 = 4 
    obj.t3 = 2 
    print obj.t0, obj.t3, "\n"

  ~ > ruby sample/b.rb

    42


  <========< sample/c.rb >========>

  ~ > cat sample/c.rb

    require 'traits'
    #
    # a hash argument can be used to specify default values
    #
    class C
      has 'a' => 4, :b => 2 
    end
    
    o = C::new
    print o.a, o.b, "\n"
    
    #
    # and these traits are smartly inherited
    #
    class K < C; end
    
    o = K::new
    o.a = 40
    p( o.a + o.b ) # note that we pick up a default b from C class here since it
                   # has not been set
    
    o.a = 42
    o.b = nil
    p( o.b || o.a ) # but not here since we've explicitly set it to nil
    
    #
    # if a block is specifed as the default the initialization of the default value
    # is deferred until needed which makes for quite natural trait definitions.  the
    # block is passed 'self' so references to the current object can be made. (if
    # this were not done 'self' in the block would be bound to the class!)
    #
    
    class C
      class << self
        has('classname'){ name.upcase }
      end
    
      has('classname'){ self.class.classname.downcase }
    end
    
    class B < C; end
    
    o = C::new
    p C::classname
    p o.classname
    
    o = B::new
    p B::classname
    p o.classname

  ~ > ruby sample/c.rb

    42
    42
    42
    "C"
    "c"
    "B"
    "b"


  <========< sample/d.rb >========>

  ~ > cat sample/d.rb

    require 'traits'
    #
    # all behaviours work within class scope (metal/singleton-class) to define
    # class methods
    #
    class C
      class << self
        traits 'a' => 4, 'b' => 2
      end
    end
    
    print C::a, C::b, "\n"
    
    #
    # singleton methods can even be defined on objects
    #
    
    class << (a = %w[dog cat ostrich])
      has 'category' => 'pets' 
    end
    p a.category
    
    #
    # and modules
    #
    module Mmmm
      class << self; trait 'good' => 'bacon'; end 
    end
    
    p Mmmm.good

  ~ > ruby sample/d.rb

    42
    "pets"
    "bacon"


  <========< sample/e.rb >========>

  ~ > cat sample/e.rb

    require 'traits'
    #
    # shorhands exit to enter 'class << self' in order to define class traits
    #
    class C
      class_trait 'a' => 4
      c_has :b => 2 
    end
    
    print C::a, C::b, "\n"

  ~ > ruby sample/e.rb

    42


  <========< sample/f.rb >========>

  ~ > cat sample/f.rb

    require 'traits'
    #
    # as traits are defined they are remembered and can be accessed 
    #
    class C
      class_trait :first_class_method
      trait :first_instance_method
    end
    
    class C
      class_trait :second_class_method
      trait :second_instance_method
    end
    
    #
    # readers and writers are remembered separatedly
    #
    p C::class_reader_traits 
    p C::instance_writer_traits 
    
    #
    # and can be gotten together at class or instance level
    #
    p C::class_traits
    p C::traits

  ~ > ruby sample/f.rb

    ["first_class_method", "second_class_method"]
    ["first_instance_method=", "second_instance_method="]
    [["first_class_method", "second_class_method"], ["first_class_method=", "second_class_method="]]
    [["first_instance_method", "second_instance_method"], ["first_instance_method=", "second_instance_method="]]


  <========< sample/g.rb >========>

  ~ > cat sample/g.rb

    require 'traits'
    #
    # another neat feature is that they are remembered per hierarchy 
    #
    class C
      class_traits :base_class_method
      trait :base_instance_method
    end
    
    class K < C
      class_traits :derived_class_method
      trait :derived_instance_method
    end
    
    p C::class_traits 
    p K::class_traits 

  ~ > ruby sample/g.rb

    [["base_class_method"], ["base_class_method="]]
    [["derived_class_method", "base_class_method"], ["derived_class_method=", "base_class_method="]]


  <========< sample/h.rb >========>

  ~ > cat sample/h.rb

    require 'traits'
    #
    # a depth first search path is used to find defaults 
    #
    class C
      has 'a' => 42 
    end
    class K < C; end
    
    k = K::new
    p k.a 
    
    #
    # once assigned this is short-circuited
    #
    k.a = 'forty-two'
    p k.a 

  ~ > ruby sample/h.rb

    42
    "forty-two"


  <========< sample/i.rb >========>

  ~ > cat sample/i.rb

    require 'traits'
    #
    # getters and setters can be defined separately 
    #
    class C
      has_r :r
    end
    class D
      has_w :w
    end
    
    #
    # defining a reader trait still defines __public__ query and __private__ writer
    # methods
    #
    class C
      def using_private_writer_and_query
        p r?
        self.r = 42
        p r
      end
    end
    C::new.using_private_writer_and_query
    
    #
    # defining a writer trait still defines __private__ query and __private__ reader
    # methods
    #
    class D
      def using_private_reader
        p w?
        self.w = 'forty-two' 
        p w
      end
    end
    D::new.using_private_reader

  ~ > ruby sample/i.rb

    false
    42
    false
    "forty-two"


  <========< sample/j.rb >========>

  ~ > cat sample/j.rb

    require 'traits'
    #
    # getters delegate to setters iff called with arguments 
    #
    class AbstractWidget
      class_trait 'color' => 'pinky-green'
      class_trait 'size' => 42
      class_trait 'shape' => 'square'
    
      trait 'color'
      trait 'size'
      trait 'shape'
    
      def initialize
        color self.class.color
        size self.class.size
        shape self.class.shape
      end
      def inspect
        "color <#{ color }> size <#{ size }> shape <#{ shape }>"
      end
    end
    
    class BlueWidget < AbstractWidget
      color 'blue'
      size 420
    end
    
    p BlueWidget::new

  ~ > ruby sample/j.rb

    color <blue> size <420> shape <square>


  <========< sample/k.rb >========>

  ~ > cat sample/k.rb

    require 'traits'
    #
    # the rememberance of traits can make generic intializers pretty slick 
    #
    class C 
      #
      # define class traits with defaults
      #
      class_traits(
        'a' => 40,
        'b' => 1,
        'c' => 0
      ) 
    
      #
      # define instance traits whose defaults come from readable class ones
      #
      class_rtraits.each{|ct| instance_trait ct => send(ct)}
    
      #
      # any option we respond_to? clobbers defaults
      #
      def initialize opts = {}
        opts.each{|k,v| send(k,v) if respond_to? k}
      end
    
      #
      # show anything we can read
      #
      def inspect
        self.class.rtraits.inject(0){|n,t| n += send(t)}
      end
    end
    
    c = C::new 'c' => 1
    p c

  ~ > ruby sample/k.rb

    42


  <========< sample/l.rb >========>

  ~ > cat sample/l.rb

    require 'traits'
    #
    # even defining single methods on object behaves
    #
    a = [] 
    
    class << a
      trait 'singleton_class' => class << self;self;end
    
      class << self 
        class_trait 'x' => 42
      end
    end
    
    p a.singleton_class.x

  ~ > ruby sample/l.rb

    42


CAVEATS

  this library is experimental and subject to (eg. will) change - though it has
  not for several version and much of my code hinges is on it now so you can
  expect it to be stable-ish in the future - the only changes would be ones to
  fix bugs.