From: Robert Klemme <shortcutter@...>
Date: 2010-10-07T00:13:46+09:00
Subject: Re: "map" a deeply nested structure: Object#deep_map

On Wed, Oct 6, 2010 at 4:43 PM, Guido De Rosa <guidoderosa@gmail.com> wrote:
> Hi,
>
> most of you probably know/use Ruby Facets
>
> http://rubyworks.github.com/facets/
>
> I've recently posted a question on Facets discussion group:
>
> http://groups.google.com/group/facets-universal/browse_thread/thread/683215a35f06af36
>
> and I also tried my implementation (which apparently works fine):
>
> http://snurl.com/19n6qh
>
>    class Object
>
>      def deep_map(&block)
>        if self.respond_to? :each_pair
>          out = {}
>          self.each_pair do |k, v|
>            out[k] = v.deep_map(&block)

What about keys?  Hash#map allows to map keys and values

irb(main):001:0> h={1=>2,3=>4}
=> {1=>2, 3=>4}
irb(main):002:0> h.map {|k,v| [k*10,v*100]}
=> [[10, 200], [30, 400]]

What, if the key is an Array or Hash?  Wouldn't this be more appropriate?

           out[k.deep_map(&block)] = v.deep_map(&block)

OTOH then you cannot map key and value together.

>          end
>          return out
>        elsif self.respond_to? :each
>          out = []
>          self.each do |x|
>            out << x.deep_map(&block)
>          end
>          return out
>        else
>          return block.call(self)
>        end
>      end
>
>    end
>
>
> Is there room for improvements? Facets author suggested to start a
> discussion here too.

You could remove lines "out = ..." and replace them by

out = self.class.new

> So: what is, in your opinion, the best way to map a deeply nested
> structure made up of Arrays, Hashes, Array of Hashes etc. etc. ?

I wonder how many use cases for this there are actually.  In your
example you can uniformly treat each object since you want the
object_id.  That would work for a few other methods as well (e.g.
#inspect, #to_s).  In other cases you would have to discriminate
treatment of leave values in your block, e.g.

x.deep_map |y|
  case y
  when String
    ...
  when Fixnum
   ...
  else
   ...
end

That list could become lengthy.  Basically you implemented tree
traversal with double dispatch - only that the double dispatch must be
done manually in the block. :-)

Kind regards

robert


-- 
remember.guy do |as, often| as.you_can - without end
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