From: Brian Candler <b.candler@...>
Date: 2010-09-23T04:17:58+09:00
Subject: Re: Ruby-based data language

Thomas Sawyer wrote:
> On Sep 22, 7:20�am, Brian Candler <b.cand...@pobox.com> wrote:
>> I'd have said most packages use YAML. If you can provide examples which
>> use method_missing and block-based structure just for conveying static
>> configuration information, I'd be interested to see them. But then
>> that's what you asked for in the first place :-)
> 
> I think a Gemfile is a pretty good example

Do you mean a Gemfile from Bundler? e.g.

  source "http://rubygems.org"

  gem "rails", "3.0.0.rc"
  gem "rack-cache"
  gem "nokogiri", "~> 1.4.2"

Some attributes have one value, some attributes have more than one 
value, some attributes can be repeated. I guess a generic parser API for 
that would be a call to callback(key, *args), just like method_missing, 
or it could build a linear data structure like

  [[:source,"http://rubygems.org"],
   [:gem,"rack-cache"],
   [:gem,"nokogiri","~> 1.4.2"]]

I'm not sure what output you'd want from nested blocks.

> I would expect to get an object that gave me access to the data. What
> kind of object depends on the parser.

If the parser is generic (not application-specific) then I guess you'd 
just get the structure I've shown above. If you want to build that into 
application-specific objects, then you can do so. For example, for each 
"gem" line you might want to build a Gem object and append it to an 
Array of gems.

> I get what you are saying. But even JSON goes through a parser in
> Javascript

It *can* go through a parser for security reasons, but the result from 
parsing it is exactly the same as if you read it in as a Javascript 
literal. And there are cases where you intentionally interpret it as 
Javascript code, e.g. JSONP.

> when someone thinks "Ruby-
> syntax data/config format" they are thinking builder-style, not hash
> literals.

Perhaps. I think more common would be a Rails-style configuration or a 
gemspec:

Gem::Specification.new do |s|
  s.name = %q{snailgun}
  s.version = "1.0.6"
  ... etc
end

Of course, the advantage of having it as real executable code is that 
you can use Ruby to assemble data constructs, and conditional inclusion.

  s.files = [ ...lots of items... ]
  s.files.concat [ ...more items... ]

  if s.respond_to? :specification_version then
    s.specification_version = 2
  end
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