From: Tim Garnett <tgarnett@...>
Date: 2010-10-21T02:04:56+09:00
Subject: Re: YAML serialization of Hash with Set key not loadable

Thanks all for the help.  Good to know that the issue is fixed in ruby 
1.9.2 Psych library.  I'll have try our switching our Rails app to that 
when we upgrade to 1.9.2.

set seems to get no love sometimes, I remember back in ruby 1.8.6 it 
didn't even hash correctly (i.e. in 1.8.6 which we were stuck on for a 
long time
[1,2,3].to_set == [3,2,1].to_set   =>  true
[1,2,3].to_set.hash == [3,2,1].to_set.hash   =>  false
).  That at least was fixed in ruby 1.8.7/1.9.

My workaround back then was to use sorted uniqued arrays as the key, 
which is what I'll continue to do for the yaml bug too.

---

As to why you might use the set class as a hash key, in ruby any object 
can be used as a hash key (assuming it implements hash consistently) and 
set is a good way to represent a list of unique things where the order 
doesn't matter.

For a made up example I want to cache some computed value in a hash that 
depends on a set of parameters with no order dependency.  Say the sum of 
3 unique values.

require 'set'
require 'pp'
def foo(a,b,c)
  (@cached ||= {})[[a,b,c].to_set] ||= (sleep 10 ; [a,b,c].inject(&:+))
end

foo(1,2,3)
=>6  (slow, 10 seconds)

foo(1,2,3)
=>6  (fast cached)

foo(3,2,1)
=>6  (fast cached)

pp @cached
{#<Set: {1, 2, 3}>=>6}


If you used an array as the cache key instead of a set then the order 
would matter and foo(3,2,1) would have been slow and @cached larger.  My 
actual use case is for something vaguely like this (very expensive 
computation on list of unique elements where order doesn't matter)

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