From: Jim Weirich <jim@...>
Date: 2004-06-08T02:30:36+09:00
Subject: Re: How to ducktype a Hash?

I think the problem Sean is dealing with is one of "type discovery" rather
than "type checking".  In other words, Sean is getting a stream of objects
comming from some outside source (my impression was that it was
reconstituted objects from a YAML stream, or something like that).

If the object happens to be a container-like object (such as a Hash or an
object with hash-like qualities), then Sean wishes to do something
specific with those objects.

The problem is with the definition of "hash-like".  Ruby provides a
perfectly fine way of identifying Hashes (e.g. kind_of?(Hash)).  It
doesn't provide a way of identifying things that are almost (but not
quite) hashes.

For example, Sean depends on [] and []= (with certain semantics). Someone
else noted that they check for each_key.  And yet another person
recommended checking fetch and store.  If an object implements [] and []=,
but not each_key, is it hash-like?  If it implements fetch and store, but
not [], is it hash-like?

I think that currently, the concept of hash-like is too vague to be
addressed by the language.  Sean's method of tagged classes that fit his
description is a reasonable compromise in this situation.

Here's another idea ... If Sean want to "do_something" with hash like
modules, then mixin the following module to any class you consider
hash-like ...

   module HashWork
     def do_something
       # Code to do something with hash-like objects
     end
   end

And add the following ...

  module Kernel
    def do_something
      # DO NOTHING!
    end
  end

Then, just invoke "do_something" on all your objects.  The ones that are
hash-like will know what to do...

   obj.do_something.

Now this solution adds a method to existing objects, but I think the main
problem with that is just a name clash issue.  Make the name of
do_something sufficiently unique and you should have no problems.

-- 
-- Jim Weirich     jim@weirichhouse.org    http://onestepback.org
-----------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct,
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)