From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-07-28T02:55:05+09:00
Subject: Re: Symbols are your friends

Caleb Clausen wrote:
> On 7/27/06, Daniel Schierbeck <daniel.schierbeck@gmail.com> wrote:
>> Yes, it's time for another Symbol trick!
>>
>>    class Symbol
>>      def ===(obj)
>>        obj.respond_to? "to_#{self}"
>>      end
>>    end
> 
> This idea has been discussed on this list before; I can't find the
> thread at the moment.
> This seems like a really clever idea, and is fine for just playing
> around... but it's not advisable for general use. You're changing the
> semantics of Symbol#===, which will break things like this:
> 
> case method_name
> when :reverse
>  #....
> when :each
>  #...
> else #.....
> end

Yes, that's true -- I only ever intended it to be a trick.

> (You're prepending "to_" to your method names before checking; I don't
> know why. This way seems more general....)

I was using it to demonstrate how you could duck type check with a case 
statement, so focusing on the "type" made sense. I see this all the time:

   case obj
     when Integer then foo
     when String then bar
   end

and it strikes me as not being very duckish (quacky?) Using if 
statements can be annoying:

   if obj.respond_to? :to_str
     foo
   elsif obj.respond_to? :to_int
     bar
   end

Maybe this would work?

   class Symbol
     def ===(obj)
       return true if self == obj
       obj.respond_to? "to_#{self}"
     end
   end

though something like this may cause trouble:

   obj = :int
   case obj
     when :int then "integer"
     when :str then "string"
   end


Cheers,
Daniel