From: Sam Roberts <sroberts@...>
Date: 2004-01-28T02:25:30+09:00
Subject: Re: newbee question about "missing" hash methods +, += and <<

Quoteing hal9000@hypermetrics.com, on Wed, Jan 28, 2004 at 01:34:36AM +0900:
> ts wrote:
> > Array#+ always add elements, with Hash#+ keys will be added only if the
> > key don't exist otherwise the old value is replaced.
> 
> I have seen this discussion before.
> 
> It doesn't bother me much that keys are replaced. That is the nature of
> a Hash, that keys are unique.
> 
> What bothers me more: If h2 and h3 have common keys, then
>    h1 = h2 + h3
> is not the same as
>    h1 = h3 + h2
> 
> In other words, I expect + to be commutative always. But probably
> someone will now show me a case where it is not in Ruby.  :)

% irb
irb(main):001:0> "a" + "b"
=> "ab"
irb(main):002:0> "b" + "a"
=> "ba"
irb(main):003:0> "b" * 3
=> "bbb"
irb(main):004:0> 3 * "b"
TypeError: String can't be coerced into Fixnum
 from (irb):4:in `*'
 from (irb):4


I've heard this used as an argument that string classes in C++ should
not have "+" - that it doesn't behave like addition should... (I know
you're not among that group!). Those arguing this point lost, and now
most people expect any reasonable implementation of a string object to
respond to "+".

On the other hand, it isn't as instantly obvious that you can lose data
when you do "addition" on hashes, so maybe it protects us.

Me, I vote it should be added as a standard method to Hash. It's useful
to those who understand how it works, and it doesn't have any other
obvious meaning. I.e., it would be hard for somebody to come back and
say "that surprised me, I thought the "+" method would do ????".

And for those who don't find it useful... don't use it!

My 2 cents ($CA) ...

Sam