From: jmichel@... (Jean Michel)
Date: 2001-01-08T06:40:02+09:00
Subject: [ruby-talk:8787] Re: bug??

In article <200101062231.RAA02774@yashi.com>,
Yasushi Shoji  <yashi@yashi.com> wrote:
...
>Hash#default and its accessors were added to prevent error with code
>like:
>
>hash = {}
>...
>hash["foo"] += 1
>
>after the change so that nil + 1 yields error. [ruby-list:26602]
>
>so having two ways (default and key_missing) to fix the same thing
>isn't good IMHO.

The problem  is that  the chosen implementation  of Hash#default  is not
good, I think. It would be perfect if

   a.default=""   (or    a.default=[])

was equivalent to

   def a.key_missing(f)
     a[f]=""
   end

but it  is not, because  the last initializes  entries to a  *new* empty
string each time, while  the other does not. This is  the trouble I had:
after

  a.default=""
  a[1]+="a"
  a[2]+="b"
 
a  is  {1=>"ab",  2=>"ab"},  not  {1=>"a",  2=>"b"}  as  it  should  be.
Internally Hash should  call dup on a.default when creating  a new entry
to get the desired behaviour.

 Best regards,
 Jean MICHEL