From: Alexey Verkhovsky <alex@...>
Date: 2005-02-16T06:54:32+09:00
Subject: Re: Ruby as a domain specific language?

* cnmaclean@hotmail.com <cnmaclean@hotmail.com> [2005-02-16 05:49:56 +0900]:

>>In our current (proprietary) language, it's something like this:
>>
>>Define File 1234
>>	Name = "My File"
>>	Parent = 5678
>>	DataItem = "DataItem 1", Decimal, 10 bytes
>>	DataItem = "DataItem 2", Hex, 10 bytes
>>    
>>
Actually, quick and dirty domain language in Ruby is very easy. E.g., 
you can write in less than a day some Ruby that will take this:

class File1234Parser < FileParser::Base
  define_file 1234,
    :name => 'My File',
    :Parent => 5678 {
      decimal "DataItem 1", 10.bytes
      hex "DataItem 2", 10.bytes
  }
end

File1234Parser.new(open('~/files/foo.1234')).parse
... or whatever...

This would work by invoking a method FileParser::Base.define_file, that 
would take whatever arguments you give it, make some Ruby code out of it 
and then

self.module_eval <<-EOL
  def parse
    <code generated from arguments>
  end
  ...
EOL

For things like parsing and/or generating a flat, fixed-width file, this 
can be done in a day (I did it). I also used the same approach once to 
generate a test oracle for a fairly complicated billing application. The 
"price definition files" there looks like:

class GprsPromotion << StandardPricePlan
  market_rates(:DEFAULT_PREPAID_RATES)
  rate :voice => flat_rate(1.00.ntd.per_minute, :rounding_factor => 
15.seconds, :round => :up)
  rate :sms => event_rate(2.00.ntd.per_event)
  # etc
end

You can show it to someone who doesn't know Ruby and s/he will 
understand it.

It is also very easy to add special behaviors, because you can

define_file :Foo
alias :generic_parse :parse
def parse
  generic_parse
  do.some { ruby.wizardry } if necessary?
end

The dark side is that it all has to be valid Ruby syntax, and that is a 
bit more commas and braces than in the perfect world.

-- 
Best regards,

Alexey Verkhovsky

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