From: "Ara.T.Howard" <ahoward@...>
Date: 2004-06-05T13:43:46+09:00
Subject: Re: How to ducktype a Hash?

On Sat, 5 Jun 2004, Sean O'Dell wrote:

> I need to detect when an object is a hash-like container for other objects,
> one which responds to [] and which can index using any object as a value
> (string, etc.).
>
> My problem is, respond_to?(:[]) returns true for objects which are array-like
> and for strings (both have [] methods).  I need to prevent strings, arrays,
> and any other object which responds to :[] from getting to this one point in
> code, and only allow a hash or hash-like object.  The data type at this
> particular point in the code comes from a variety of data sources, and there
> are lots of ways it could become a string, or an array or theoretically any
> object.  I need to know when it's a hash or hash-like object, and raise an
> exception when it is not.  My biggest problem right now is, when it's a
> string or an array, the code has no way of knowing if the returned object/nil
> is a valid value, or if a "undesirable" object has slipped in and returned
> what it likes to return when you call its [] method.
>
> So, how do I positively identify when an object is a hash or at least behaves
> like a hash?
>
> 	Sean O'Dell

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

   class MyHashLikeObject
     include HashingObject
     def [] k
       ...
     end
     def []= k
       ...
     end
   end

now you can simply

   idx =
     case obj
       when HashingObject
         ...
       when IndexingObject
         ...
       else
         raise TypeError, obj.class
     end

-a
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