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 === x and x > 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
<========< 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
"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
<========< 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
42.0
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.
enjoy.
-a
--
email :: ara [dot] t [dot] howard [at] noaa [dot] gov
phone :: 303.497.6469
anything that contradicts experience and logic should be abandoned.
-- h.h. the 14th dalai lama
===============================================================================