I am creating a DSL using Ruby. A cut down version is below:
class _Box
attr_accessor :width
def initialize (&block)
self.width = 20 # some default value
instance_eval(&block) if block_given?
end
end
# Helper function to avoid the DSL user having to use .new
def Box(&block)
_Box.new(&block)
end
I want to do something like:
Box {width = 10}
but the only way I can get it to work is
Box {self.width = 10}
Having to prefix the ivar with self. makes the DSL look clunky and wouldn't
be acceptable to my users.
Is there anyway to avoid the self.? Using {@width = 10} will work but
prevents some necessary validation from taking place.
I have an alternative where width is a method:
class _Box
def width (val = :no_param)
if val != :no_param
@width = val
end
@width
end
end
so with this I can write
Box {width 10}
which is acceptable from the DSL view point, but makes the getter operations
more expensive. In the real version a getter operation takes about 7
statements to allow for evaluation of block if the ivar had been set to a
Proc object and inheritance of values from its parent object (in a visual
hierarchy). The setter operation is often only done when the object is
created but the getter is done very frequently so I want to move down the
route of separate setters and getters rather than combining them in the one
method as an optimization.
here is one simple way:
harp:~ > cat a.rb
class OpenStruct
alias_method "__eval__", "instance_eval"
alias_method "__set__", "instance_variable_set"
alias_method "__get__", "instance_variable_get"
instance_methods.each{ |m| undef_method m unless m =~ /^__/ }
def initialize(&block) __eval__ &block end
def method_missing m, *a, &b
m = m.to_s.delete "=?"
a.size == 0 ? __get__("@#{ m }") : __set__("@#{ m }", a.shift)
end
end
module Initializable
def initialize &block
os = OpenStruct.new &block
attributes.each{|a| instance_variable_set "@#{ a }", os.__send__(a)}
end
def to_s
require "yaml"
attributes.inject({}){|h,a| h.update a => send(a)}.to_yaml
end
def attributes
self.class::ATTRIBUTES
end
end
class Box
ATTRIBUTES = %w[ width height ]
ATTRIBUTES.each{|a| attr a}
include Initializable
end
def Box(*a, &b) Box.new(*a, &b) end
puts Box {
width 42
height 42
}
harp:~ > ruby a.rb
---
height: 42
width: 42
the idea is to initialize another object, the OpenStruct in this case, and then
relay the properties. this object can have any 'slow' but convenient behaviour
your desire, leaving the Box class to have normal 'fast' attributes.
hth.
-a