From: Kent Dahl <kentda+news@...>
Date: 2003-10-06T23:01:51+09:00
Subject: Re: compare two identically hashes fails

Simon Strandgaard wrote:
> Why does Hash#inspect not output the content of Hash#default ?
> It would make sense, because Ruby already compare Hash#default.

Calling Hash#default may modify the Hash, such as with the proc you use. 
(Try calling h1.default before assert_equal)

There are two values underlying the default, both a value default and a 
proc default. Outputting them both would be ugly and visually 
cluttering, IMHO.

> Why does Ruby not reset Hash#default when you issues a #to_hash ?
> I think it would give a better behaiver?  What does others thinks?

I think it would break the protocol of the to_hash as a type conversion 
method and result in information loss. What if someone calls to_hash 
just to ensure they have a Hash or hash-like object? Why should the 
default value or proc be tossed out?

> 	def test_hash_compare
> 		on_new = proc {|h, k| h[k] = []}
> 		h1 = Hash.new(&on_new)
> 		h1[1] = [["a", 2]] 
> 		h1[2] = []
> 		h2 = {1=>[["a", 2]], 2=>[]}
> 		h1.default = nil   # this is annoying!!
> 		h2.default = nil   # this is annoying!!

That last line is not necessary, since the literal hash has nil as 
default value anyway. So it is only half as annoying. :-)

> 		assert_equal(h1, h2)
> 	end 
> 
> If one instead could write it like following:
> 
> 	def test_hash_compare
> 		on_new = proc {|h, k| h[k] = []}
> 		h1 = Hash.new(&on_new)
> 		h1[1] = [["a", 2]] 
> 		h1[2] = []

Just a tangential nitpick, but shouldn't the above line only be
                 h1[2]
if you're trying to exercise the on_new proc in the test?

> 		h2 = {1=>[["a", 2]], 2=>[]}
> 		assert_equal(h1.to_hash, h2.to_hash)
> 	end 

What I realized I missed upon seeing your question, was a 
Hash#default_proc= method. It seems a bit restrictive to only be able to 
set it on object construction. Does anyone know if there is a rationale 
behind this decision?

-- 
(\[ Kent Dahl ]/)_    _~_    _____[ http://www.pvv.org/~kentda/ ]_____/~
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( \__\_�|�_/__/ ) _)Industrial economics and technological management(
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