Adaptive struct

• see also OpenStruct

Sometimes it's convenient to write configuration files in ruby as structure with attributes defined on demands (PHP programmers really like it).

Let's take a look at how this adaptive structure can be defined.

Hash with dynamic methods

Config is nested hash. So it's natural to write method_mising method, with will treat missing method name as key.

class Hash
  def method_missing(method, *args)
    method = method.to_s
    if method.gsub!(/=$/, '')
      self[method.to_sym] = args.first
    else
      self[method.to_sym] ||= {}
    end
  end
end


a = {}
a.bb.cc.dd = 1
a.bb.ff.ee = 2
a.xxx = 'yyy'

require 'yaml'
puts a.to_yaml

It prints

--- 
:bb: 
  :cc: 
    :dd: 1
  :ff: 
    :ee: 2

Cool! But we need more fun.

Nested configs

class Hash
  def method_missing(method, *args, &block)
    method = method.to_s
    if  method.gsub!(/=$/,'') || !args.empty? 
      self[method.to_sym] = args.first
    else
      res = self[method.to_sym] ||= {}
      res.instance_eval(&block) if block
      res
    end
  end
end
a = {}
a.bb.cc { 
  dd.ee 3; 
  ff 4 
}
a.xxx {
  yyy 'y'
  zzz 'z'
}

require 'yaml'
puts a.to_yaml

Output:

--- 
:xxx: 
  :yyy: y
  :zzz: z
:bb: 
  :cc: 
    :dd: 
      :ee: 3
    :ff: 4

We are so good!

But here we stop using equal sign. Why? Code

a.xxx {
  yyy = 'y'
  zzz = 'z'
}

My be we can use binding to fix it? Try it!

At least we can easily get this code working:

a.xxx.ttt.www {|cfg|
  cfg.yyy = 'y'
  cfg.zzz = 'z'
}

-- ArtemVoroztsov - 15 Apr 2008