From: "David A. Black" <dblack@...>
Date: 2008-12-08T02:49:56+09:00
Subject: Re: creating a DSL: adding random methods

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Hi --

On Sun, 7 Dec 2008, dc wrote:

> hi -
>
> I'm trying to create a simple DSL, using method missing.
>
> i want to be able to add different event logic for different
> instances, defined in their own files.
> trivial example:
>
> # eg in fred.rb
> event :say do
>  "hi this is fred!"
> end
>
> # and joe.rb
> event :say do
>  "joe doesnt say much"
> end
>
>
> so i have a class to implement/create the DSL methods:
>
>  def event evt, &block
>    puts "event: #{evt} , #{event}"

Right there is where you seem to be calling event without an argument
(inside the string interpolation). I suspect that's where you're
getting the "Wrong number of arguments" error.

>    str =3D "
>      def #{evt}
>        #{block}
>      end
>    "
>    puts str
>    self.class.class_eval(str)
>  end
>
>
> after loading the individual fred.rb file in
> class Brain
>  def self.load(filename)
>    dsl =3D new
>    dsl.instance_eval(File.read(filename))
>  end
>
> then when the event XX do is encountered, i am getting:
>
> ArgumentError: ./parser.rb:37:in =91event=92: wrong number of arguments (=
0
> for 1)
>
> # this is the method call
> event :say do
>   puts ".code here.."
> end
>
> # calling this
> def event evt, &block
>
>
> i was expecting "say" to be passed as the evt (method name) and block
> to be the code block after do.
> perhaps i need to yield something back to the caller?
>
> appreciate any tips on this!

You can't use a string representation of a block as code. Instead,
you'd want to do something like this:

def event(evt, &block)
   (class << self; self; end).class_eval do
     define_method(evt, &block)
   end
end

so that you can get the local variable evt and the block into the
method definition. (This isn't a complete example but it might point
you in a useful direction.)


David

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