From: Trans <transfire@...>
Date: 2007-11-14T20:45:34+09:00
Subject: Re: "once" meta-method without "module_eval" (Pickaxe)



On Nov 14, 1:23 am, Greg Weeks <greg.we...@arm.com> wrote:
> My impression is that Ruby meta-programming used to lean heavily on
> evaluating strings as code, but is moving away from that.  (Good!)  In
> that spirit, it would be nice to replace this example from Chapter 24,
> Classes and Objects:
>
> class Date
>   class <<self
>     def once(*ids) # :nodoc:
>       for id in ids
>         module_eval <<-"end;"
>           alias_method :__#{id.to_i}__, :#{id.to_s}
>           private :__#{id.to_i}__
>           def #{id.to_s}(*args, &block)
>             (@__#{id.to_i}__ ||= [__#{id.to_i}__(*args, &block)])[0]
>           end
>         end;
>       end
>     end
>   end
>   ...
>   once :as_string, :as_YMD
> end
>
> "once" replaces a specified method with a method that caches the return
> value on the first call.
>
> The example above served to illustrate the use of class instance
> variables.  However, the replacement that comes to mind does not:
>
>   class <<self
>     def once(*ids)
>       values = {}
>       for id in ids
>         new_id = "__#{id.to_i}__".to_sym
>         alias_method new_id, id
>         private new_id
>         define_method (id) do |*args|
>           (values[id] ||= [ method(new_id).call(*args) ])[0]
>         end
>       end
>     end
>   end
>
> Is there some way to avoid evals and still use class instance variables?

instance_variable_get/set

> Is there some way to avoid evals and still handle the "&block" argument?

Wait for 1.9.

> Is there a natural alternative to the local hash variable?

instance_variable_get/set if you want to do it like the original, but
your local might be better.

> Am I right to think that eval-ing strings is retro?

I'm not sure there's anything really wrong with eval. Often the code
looks cleaner compared to these long winded meta-methods.

T.