URLS
http://rubyforge.org/projects/codeforpeople/
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 supports smart inheritence of class attributes
and a
fistful of hooks for veryifying and munging attr values.
VERSION
0.9.0
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'
# we define instance traits which get their default from the
class
%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/p.rb >========>
~ > cat sample/p.rb
require 'traits'
#
# the TraitInit module provide a simple method for initializing an
object’s
# traits from an options hash
#
class C
include TraitInit
LIST_OF_INTS = lambda{|a| Array === a and a.map{|i| Integer ===
i}.all?}
LIST_OF_STRINGS = lambda{|a| Array === a and a.map{|s| String ===
s}.all?}
trait :li, :validate => LIST_OF_INTS
trait :ls, :validate => LIST_OF_STRINGS
def initialize opts = {}
trait_init opts
end
end
c = C::new "li" => [4, 2], "ls" => %w[4 2]
p c.li.join
p c.ls.join
~ > ruby sample/p.rb
"42"
"42"
CAVEATS
this library is experimental and subject to change - though it has
not for
several versions and much of my code hinges is on it now so you can
expect the
interface to be stable in the near future - the only changes planned
are those
that fix bugs or add features.
LICENSE
same as ruby’s
HISTORY
0.9.0
- luke kaines made quite a few suggestions and bug reports that
enabled this
release including making a few methods indifferent about
string/symbol
args/keys and the introduction of a simple method ‘trait_init’
that can be
used to create keyword based initializers, eg:
require 'traits'
class C
include TraitInit
trait :a, :type => Integer
trait :b, :type => Integer
def initialize opts = {}
trait_init opts
end
end
C::new :a => 4, :b => 2
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
enjoy.
-a
