From: Gavin Sinclair <gsinclair@...>
Date: 2004-06-13T11:19:56+09:00
Subject: Re: How to ducktype a Hash?

On Sunday, June 13, 2004, 10:55:53 AM, Sean wrote:

>> > 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  My Pragmatic Programmer's Guide must already be far out of date.

Try 'ri Hash.new'.  All will be explained.

Your PragProgGuide certainly is out of date (so is everyone else's,
BTW :).  It covers Ruby 1.6; Hash.new(&block) was introduced in
1.7/1.8.

Note that the above implementation of #to_hash would return a *new*
hash every time, thus losing the benefit of any precalculated values.
But you clearly get the point.  And yes, it is interesting: a very
nice example of closures.  It would perhaps be nice if all hash-like
classes offered this.

Gavin