From: "David A. Black" <dblack@...>
Date: 2004-06-05T20:55:23+09:00
Subject: Re: How to ducktype a Hash?

Hi --

On Sat, 5 Jun 2004, Ara.T.Howard wrote:

> is this totally out of the question?
> 
> create a module that is a ducktype 'flag' for hashables
> 
>    module HashingObject; end
> 
> add it to hash - a bit ugly, but does nothing
> 
>    class Hash; include HashingObject; end
> 
> bless any class you deem good anough with the module

[...]

> now you can simply
> 
>    idx =
>      case obj
>        when HashingObject
>          ...
>        when IndexingObject
>          ...
>        else
>          raise TypeError, obj.class
>      end

I think that could well work in practice, but wouldn't call it duck
typing.  You're not asking it to quack up front; you're checking its
module ancestry -- which, I hasten to add, may be exactly what you
want to do, but isn't the same as what a duck typing approach would
lead you to.

Let me try to flesh this out with a companion example:  

  class UnHashlikeError < Exception
  end

  module Hashlike
    alias :get_by_key :[]
  end

  class Hash           # probably inadvisable, as Paul Brannan has
    include Hashlike   # recently pointed out...
  end

  # ...

  value = begin
            obj.get_by_key(k)
          rescue NoMethodError
            raise UnHashlikeError, "Object isn't hashlike"
          end

or something like that.  The basic idea here is: get your ducks in a
row, so to speak, by manipulating the object universe so that a
certain type or subtype of object understands a certain message, then
at runtime, send that message and branch on the consequences.  


David

-- 
David A. Black
dblack@wobblini.net