From: "Michael C. Libby" <mcl-ruby-talk@...>
Date: 2004-12-04T20:55:06+09:00
Subject: Re: Hash#eql? and Hash key testing

On Sat, 2004-12-04 at 16:39 +0900, Joel VanderWerf wrote:
> The ri docs for Object#eql? say:
> 
>       The +eql?+ method returns +true+ if _obj_ and _anObject_ have the
>       same value. Used by +Hash+ to test members for equality.
> 
> However, the second sentence doesn't seem to be consistent with the 
> behavior of Hash (and Set):
> 
>    h1 = {1=>2, 3=>4}
>    h2 = {1=>2, 3=>4}
> 
>    p h1.eql?(h2) # ==> true
>
>    h={h1=>true}
> 
>    p h[h1] # ==> true
>    p h[h2] # ==> nil
>
> Based on the docs, I would have expected
> 
>    p h[h2] # ==> true
> 
> Have I misunderstood?

h1 and h2 are not the same object above, though they are equivalent at
the time you test for equality. h1 could get new elements, as could h2.

mcl@saluki:~$ irb
irb(main):001:0> a = {1=>2,3=>4}
=> {1=>2, 3=>4}

irb(main):002:0> b = {1=>2,3=>4}
=> {1=>2, 3=>4}

irb(main):003:0> a == b
=> true

irb(main):004:0> a.__id__ == b.__id__
=> false

irb(main):005:0> c = {a=>true}
=> {{1=>2, 3=>4}=>true}

irb(main):006:0> c[b] = 'foo'
=> "foo"

irb(main):007:0> c
=> {{1=>2, 3=>4}=>"foo", {1=>2, 3=>4}=>true}

irb(main):008:0> a[5]=6
=> 6

irb(main):009:0> c
=> {{1=>2, 3=>4}=>"foo", {5=>6, 1=>2, 3=>4}=>true}

irb(main):010:0> a == b
=> false

With literals it's a different story:

irb(main):011:0> x = 5
=> 5

irb(main):012:0> y = 5
=> 5

irb(main):013:0> x == y
=> true

irb(main):015:0> c[x] = 'bar'
=> "bar"

irb(main):016:0> c
=> {5=>"bar", {1=>2, 3=>4}=>"foo", {5=>6, 1=>2, 3=>4}=>true}

irb(main):017:0> c[y]
=> "bar"

irb(main):018:0> x.__id__ == y.__id__
=> true

-Michael C. Libby, www.andsoforth.com