From: Sean O'Dell <sean@...>
Date: 2004-06-13T09:55:53+09:00
Subject: Re: How to ducktype a Hash?

On Saturday 12 June 2004 16:48, Gavin Sinclair wrote:
> On Sunday, June 13, 2004, 3:30:38 AM, Sean wrote:
> > On Saturday 12 June 2004 04:23, Rene van Bevern wrote:
> >> Sean O'Dell wrote:
> >> > So, how do I positively identify when an object is a hash or at least
> >> > behaves like a hash?
> >>
> >> Use the method #to_hash instead. All hash-like objects should be able to
> >> give a hash-representation of themselves.
> >
> > Not necessarily.  A database abstraction might act like a hash, but
> > wouldn't dump entire tables, which could contain tens of thousands of
> > records, as one big Ruby hash.  That isn't positive identification.
>
> Such an abstraction could give a lazy-loading Hash, as in
>
>   class DBAbstraction
>     def to_hash
>       Hash.new { |hash, key|
>         hash[key] = expensive_lookup(key)
>       }
>     end
>   end
>
> So when you call #to_hash in the first place, it's empty.  Each key
> you call results in a calculation.  All unbeknownst to the user of the
> hash.

I don't see how this works.  When you call to_hash, no arguments are provided, 
so how could you use the key value there?  I've also never seen the block 
parameter to Hash.new.  Not that this changes anything regarding positive 
identification of a class' purpose, but it's very interesting.  Is this all 
new?  My Pragmatic Programmer's Guide must already be far out of date.

	Sean O'Dell