From: Paul Brannan <pbrannan@...>
Date: 2005-07-15T01:07:05+09:00
Subject: Re: Hash with default_proc and Marshal

On Wed, Jul 13, 2005 at 08:10:01PM +0900, benjohn@fysh.org wrote:
> I have a hash who's values are hashes. I need the default values in the hashes to
> be unique to each key, rather than shared. I can achieve this using Hash's
> default_proc, instantiating Hash thus:
> 
>     Hash.new {|hash,key| hash[key] = someFunctionGivingANewValue}
> 
> However, I require the Hash to be Marshalable, which it will not be if it has a
> default_proc.

Ruby cannot dump the object, because it cannot dump the proc.  One
solution:

irb(main):001:0> h = Hash.new { }
=> {}
irb(main):002:0> Marshal.dump(h)
TypeError: cannot dump hash with default proc
        from (irb):2:in `dump'
        from (irb):2
irb(main):003:0> module DefaultValueDump; def _dump(limit); Marshal.dump(Hash[*self.keys.zip(self.values)], limit); end; end
=> nil
irb(main):004:0> h.extend(DefaultValueDump)
=> {}
irb(main):005:0> Marshal.dump(h)
=> "\004\010e:\025DefaultValueDumpu:\tHash\t\004\010{\000"

It should be possible to address this using nodewrap, since with
nodewrap it is possible to dump procs.  I may do that in a future
version.

Paul