From: Florian Frank <flori@...>
Date: 2007-01-23T22:58:55+09:00
Subject: Re: Hash pairs at?

Joel VanderWerf wrote:
> Alex LeDonne wrote:
>> Maybe I'm late to the party, but instead of select_pair, I'd name the
>> method slice, analagous to Array#slice.
> 
> Looks like the rails crowd has beaten us to it:
> 
> http://dev.rubyonrails.org/changeset/5726
> 
> It might be in facets, too.
> 
> Seems like a good name for a good method.
> 

I called it 'extract' and use this:

module HashInclusion
   # Extract a hash out of this hash, that only contains key-value pairs
   # matching _patterns_ and return it. _patterns_ can be for example /^foo/
   # to extract 'foobar' and 'foobaz' or 'foo'/:foo to extract 'foo'.
   def extract(*patterns)
     patterns.map! { |pat| pat.respond_to?(:match) ? pat : pat.to_s }
     result = default_proc ? self.class.new(&default_proc) : 
self.class.new(default)
     each { |k, v| result[k] = v if patterns.any? { |pat| pat === k.to_s } }
     result
   end
end

class ::Hash
   include HashInclusion
end

The duck typing is a bit fishy, but it's possible to add pattern 
objects, that implement #=== and #match this way. Coincidently this is 
the case for Regex instances.

It's also interesting, that it's quite difficult to create an empty 
"copy" of a given hash in Ruby, without duping and clearing it.

-- 
Florian Frank