From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2010-12-20T17:59:13+09:00
Subject: Re: How to implement such a Domain-Specific-Language (DSL)?

On Mon, Dec 20, 2010 at 8:22 AM, Zd Yu <zdyu2000@gmail.com> wrote:
> I need to use Ruby to process performance data and calculate performance
> metrics. What I have is a Hash that contains event_name => event_value
> mappings. What I want to do is calculating metrics in a elegant style.
>
> for example:
>
> events = Hash.new
>
> # reading the raw data.
> # code omitted here.
>
> # now I get the basic events, like
> # events["event_A"] = 123.456
> # events["event_B"] = 456.789
>
> # the normal way looks like:
> events["metric_A"] = events["event_A"] * 64 / events["event_B"]
>
> # but what I am wondering if I can do it in an elegant way, like:
> metric_A = event_A * 64 / event_B
>
> Can anybody give some hints?

If the only requirement is to have event_A evaluate to
events["event_A"] you can do that easily with method_missing. This can
give you some ideas:

class EventData
  def initialize events
    @events = events
  end

  def method_missing meth, *args, &blk
    super unless /\Aevent_/ =~ meth.to_s
    @events.send(:[], meth.to_s)
  end
end

data = EventData.new("event_A" => 123.456, "event_B" => 456.789)
metric_A = data.instance_eval { event_A * 64 / event_B }

# => 17.2972291364284

You can have a file with only the block for instance_eval for your
users to type, with just an extra little bit of work.

Jesus.