From: Robert Klemme <shortcutter@...>
Date: 2011-11-07T20:56:54+09:00
Subject: Re: Tricky DSL, how to do it?

On Sun, Nov 6, 2011 at 11:35 PM, Intransition <transfire@gmail.com> wrote:
> I'd want to write a DSL such that a surface method_missing catches undefined
> methods and records the blocks that they define e.g.
>   foo do
>     puts "foo"
>   end
> So I would end up with:
>   { :foo=>#<Proc ...> }
> Presently I have something like this:
>
>   #
>   class Evaluator < BasicObject
>     def initialize(&block)
>       @__config__ = {}
>       instance_eval(&block) if block
>     end
>     def __config__
>       @__config__
>     end
>     def method_missing(sym, *args, &block)
>       @__config__[sym] = block
>     end
>   end
> However when I call on a block I want it to evaluate as if in the defining
> context (in this case toplevel), not inside the "DSL" class that evaluated
> via method_missing.
>   e = Evaluator.new do
>     foo do
>       puts "foo"
>     end
>   end
>
>   e.__config__[:foo].call
> Instead of what I want, I get a method missing error for #puts.
> Any ideas?

You can't have both (evaluate with self = Evaluator to trigger
Evaluator's method missing and invoking the block in the defining
context) at the same time.

Question is, what happens if a missing method is invoked?  In your
implementation the block is returned.   I would at least change that
to return nil to avoid nasty side effects.  But generally execution
cannot reasonably continue.  What about catching the exception and
recording it?

12:54:59 ~$ ./x.rb
{:foo=>#<Proc:0x101b3554@./x.rb:24>}
12:55:02 ~$ cat -n x.rb
     1  #!/bin/env ruby19
     2
     3  class Evaluator < BasicObject
     4    def initialize(&block)
     5      @__config__ = {}
     6
     7      if block
     8        begin
     9          block.call
    10        rescue ::NoMethodError => e
    11          @__config__[e.name] = block
    12          nil
    13        end
    14      end
    15    end
    16
    17    def __config__
    18      @__config__
    19    end
    20
    21  end
    22
    23
    24  e = Evaluator.new do
    25    foo do
    26      puts "foo"
    27    end
    28  end
    29
    30  p e.__config__
    31
12:55:03 ~$

Kind regards

robert

-- 
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