From: "David A. Black" <dblack@...>
Date: 2008-09-09T20:53:30+09:00
Subject: Re: Why is Ruby dynamic?

Hi --

On Tue, 9 Sep 2008, Phlip wrote:

> David Masover wrote:
>
>> A nitpick, but... string evals considered harmful!
>
>>>        eval '
>
>> I always loved define_method.
>
> I recently tried define_method and it didn't work, so I fell back to eval. 
> (Note to the newbs: Try things the most progressive way possible, such as a 
> fixed version of Dave's version of my example, before trying a hack...)
>
> Here's the lines, from inspect_sql:
>
>  def self._decorate_explainers(explains) #:nodoc:
>    def explains.find_statements(regexp)
>      return AssertEfficientSql._decorate_explainers(select{|explanation|
>        explanation[:statement] =~ regexp
>      })
>    end
>
>    (_multiples + _singulars).each do |rollup, explainer|
>      eval "def explains.#{rollup}; map{|q|q[:#{rollup}]}.flatten.uniq; end"
>    end
>
>    return explains
>  end
>
> For the eval line, I first tried explains.define_method, and it didn't work. 
> Instead of researching why, or finding a kindler gentler system, I whipped 
> out eval and put a bullet thru the problem.

define_method is a private instance method of Module, so a typical use
follow something like this path:

   c.class_eval { define_method(m) {|x| puts x } }

> It's the shortest possible eval, anyway!
>
> Also I think you can't say 'def foo; def bar; end; end' - the def is in 
> method-space, not class-space.

You can nest def's, but the nested one (once the outer one is
executed) becomes an outer one:

   class C
     def x
       def y
         puts "Hi from y"
       end
     end
   end

   c = C.new
   c.x
   c.y        # Hi from y

   d = C.new
   d.y        # Hi from y


David

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