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

  0.8.0

HISTORY

  0.8.0
    - traits now supports a whole slew of hooks that can be registered to fire
      pre or post setting an attribute, to cast a value to another type, to
      munge a value destructively, to require only certain types, to require a
      certain ducktype signature, and to validate arguments passed.  check out
      sample/m.rb, sample/n.rb, or sample.o.rb to see it in action.  the
      mechanism is quite flexible allowing method names, lambdas of varying
      arity, and lists of either/or to be passed to any hook.

    - you can find a gem for trais on codeforpeople - but i've still not coded
      up automated updating from codeforpeople to rubyforge so it won't show up
      as a remote gem yet.

  0.7.0
   - patched in the support i had written eariler for 'hooks' to be called
     pre/post setting a trait. plus shortcut to 'validate' traits which simply
     sets up a 'pre' hook which is used as a predicate.  eg:

        class C; trait 'number', 'validate' => proc{|n| Numeric === n}

     pre and post hooks are used in the same way, eg:

        class C
          trait 'a', 
            'pre' => proc{|val| p "#{ val } to set with"},
            'post' => proc{|val| p "#{ val } set"},
        end

     but the really cool thing is that all of these blocks are both passed the
     value in question but also evaluate with 'self' set appropriately.  eg

        class Car
          positive_int = lambda{|n| Fixnum === n and n > 0}
          legal = proc{|s| s < speed_limit}

          trait 'speed_limit', 'validate' => positive_int, 'default' => 42
          trait 'speed', 'validate' => legal
        end

        c = Car::new
        c.speed = 115

      works as you'd expect:

        (eval):14:in `speed=': validation of speed=(115) failed! (ArgumentError)
                from a.rb:13

  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", "first_class_method="], ["second_class_method", "second_class_method="]]
    [["first_instance_method", "first_instance_method="], ["second_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", "derived_class_method="], ["base_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'
    
      %w( color size shape ).each{|t| trait(t){self.class.send t}}
    
      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


  <========< sample/m.rb >========>

  ~ > cat sample/m.rb

    require 'traits'
    #
    # pre and post hooks can be passed a proc or the name of a method, the arity is
    # detected and the proc/method sent either the value, or the name/value pair
    #
    
    class C
      HOOK_A = lambda{|value| puts "HOOK_A : #{ value }"}
      HOOK_B = lambda{|name, value| puts "HOOK_B : #{ name } = #{ value }"}
    
      def hook_a value
        puts "hook_a : #{ value }"
      end
      def hook_b name, value
        puts "hook_b : #{ name } = #{ value }"
      end
    
      trait 'x', 'pre' => HOOK_A, 'post' => 'hook_b'
      trait 'y', 'pre' => HOOK_B, 'post' => 'hook_a'
    end
    
    c = C::new
    c.x = 42
    c.y = 'forty-two'

  ~ > ruby sample/m.rb

    HOOK_A : 42
    hook_b : x = 42
    HOOK_B : y = forty-two
    hook_a : forty-two


  <========< sample/n.rb >========>

  ~ > cat sample/n.rb

    require 'traits'
    #
    # two kinds of in-place modifications are supported : casting and munging. 
    # casting is a hook that requires either a proc or the name of a method that
    # will be used to convert the objects type.  munging is similar execpt the
    # method is called on the object itself.  like all hooks, lists may be provided
    # instead of a single argument
    #
    # you'll notice that the hooks and methods defined here are not strictly needed,
    # but are for illustration purposes only.  note that all hooks operate in the
    # context of self - they have access to instance vars, etc., like instance_eval
    #
    
    class C
      INT = lambda{|i| int i}
      def int i
        Integer i
      end
      trait 'a', 'cast' => 'int'
      trait 'b', 'cast' => INT 
      trait 'c', 'munge' => 'to_i' 
      trait 'd', 'cast' => 'Integer' 
      trait 'e', 'munge' => %w( to_i abs )
    end
    
    c = C::new
    
    c.a = '42'
    p c.a
    c.b = '42'
    p c.b
    c.c = '42'
    p c.c
    c.d = '42'
    p c.d
    c.e = '-42'
    p c.e

  ~ > ruby sample/n.rb

    42
    42
    42
    42
    42


  <========< sample/o.rb >========>

  ~ > cat sample/o.rb

    require 'traits'
    #
    # three kinds of validation hooks are supported, flat out typing, ducktyping,
    # and generic validation.  all hooks are evaluated in the context of self and
    # can consist of lists of strings/symbols and/or lambdas.  any string/symbol is
    # taken to be a method name of the current object.
    #
    
    #
    # type hooks must be an object, which will be compared with '===', or a lambda
    # or method name that returns an object to be compared using '==='.  'case' is
    # an alias for 'type'. 
    #
    class C
      trait 'a', 'types' => [Fixnum, Numeric] 
      trait 'b', 'case' => %r/^4#{ 1 + 1}$/, 'munge' => 'to_s'
      trait 'c', 'type' => lambda{rand > 0.42 ? Fixnum : Numeric}   
    end
    
    c = C::new
    c.a = 42
    p c.a
    c.b = 42
    p c.b
    c.c = 42
    p c.c
    
    #
    # ducktyping is supported as a list of methods an object must support.
    #
    class C
      trait 'a', 'ducktypes' => %w( abs zero? ) 
      trait 'b', 'ducktype' => %w( abs floor ) 
    end
    
    c = C::new
    c.a = 42
    p c.a
    c.b = 42.0
    p c.b
    
    #
    # finally, generic true/false validation is supported
    #
    class C
      def forty_two n
        n.to_i == 42
      end
      trait 'a', 'validate' => lambda{|x| x == 42} 
      trait 'b', 'validate' => 'forty_two'
    end
    
    c = C::new
    c.a = 42
    p c.a
    c.b = 42.0
    p c.b

  ~ > ruby sample/o.rb

    42
    "42"
    42
    42
    42.0
    42
    42.0


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.