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=== operator


Hi,

>> {} === Hash
=> false
>> case {}
>> when Hash then true
>> end
=> true
>> Hash === {}

=> true

Why is the first statement false when the second is true? I understand
case uses === to compare the objects? Perhaps it switches them round as
in the last statement?

Cheers,

Jon

--
Posted via http://www.ruby-forum.com/.

Hi --

On Thu, 7 Jun 2007, Jon Leighton wrote:
> Hi,

>>> {} === Hash
> => false
>>> case {}
>>> when Hash then true
>>> end
> => true
>>> Hash === {}
> => true

> Why is the first statement false when the second is true? I understand
> case uses === to compare the objects? Perhaps it switches them round as
> in the last statement?

Yes; === is called on the when expression(s), with the case object as
argument.

David

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

On 6/6/07, Jon Leighton <tur@turnipspatch.com> wrote:

> Hi,

Maybe a more explicit notation helps

> >> {} === Hash
{}.send(:===, Hash)
> => false
> >> case {}
> >> when Hash then true

when Hash.send(:===, {}) then true
> >> end
> => true
> >> Hash === {}
> => true

> Why is the first statement false when the second is true? I understand
> case uses === to compare the objects? Perhaps it switches them round as
> in the last statement?

No the LHS is the receiver and the RHS is the first argument of the message.
{}.send(:===, Hash) the === instance_method of Hash is called which is
inherited from Object and basically means equality, hence false.

Hash.send(:===, {}) the class method of Hash is called which is
inherited from Class and means is_a? hence true.

HTH
Robert

--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw

On 6/6/07, dbl@wobblini.net <dbl@wobblini.net> wrote:
> Hi --
<snip>
> Yes; === is called on the when expression(s), with the case object as
> argument.

Well you are right of course, I said now because I did not understand
what OP meant :(

Robert

Am Donnerstag, 07. Jun 2007, 05:32:38 +0900 schrieb Robert Dober:

> On 6/6/07, dbl@wobblini.net <dbl@wobblini.net> wrote:
> <snip>
> >Yes; === is called on the when expression(s), with the case object as
> >argument.
> Well you are right of course, I said now because I did not understand
> what OP meant :(

Some may say it's an unexpected behaviour. Others will find
themselves detecting it as a welcome surprise. This is Ruby
at its best.

Bertram

--
Bertram Scharpf
Stuttgart, Deutschland/Germany
http://www.bertram-scharpf.de

Hi --

On Thu, 7 Jun 2007, Bertram Scharpf wrote:
> Am Donnerstag, 07. Jun 2007, 05:32:38 +0900 schrieb Robert Dober:
>> On 6/6/07, dbl@wobblini.net <dbl@wobblini.net> wrote:
>> <snip>
>>> Yes; === is called on the when expression(s), with the case object as
>>> argument.
>> Well you are right of course, I said now because I did not understand
>> what OP meant :(

> Some may say it's an unexpected behaviour. Others will find
> themselves detecting it as a welcome surprise. This is Ruby
> at its best.

I always thought it was just the logical way to do the case statement.
Since you're testing something about the case object, you may not know
what it is or what its === method does:

   case x     # what is x?
   when 1     ...
   when "yes" ...
   when C
   when nil   ...
   end

So I don't think it would make sense to have a case construct where
=== was called on x.

David

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

On 07.06.2007 13:02, dbl@wobblini.net wrote:

Another reason why that would be an odd way to do it: *all* tests then
would have to be implemented in x's class - now how much sense would
that make to do that?  Just think of the type test (i.e. using class
objects in when clause)...

Kind regards

        robert

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