From: tilde <tilde@...>
Date: 2010-09-03T16:50:27+09:00
Subject: Re: case (variable) when type1 ... when type2 ... end

On 09/03/10 09:07, Robert Klemme wrote:
> On Fri, Sep 3, 2010 at 7:25 AM, tilde<tilde@autistici.org>  wrote:
>> On 08/30/10 10:33, Jean-Julien Fleck wrote:
>>>>
>>>> how can I write case statement to check of specific type the variable
>>>> has?
>>>> Something like:
>>>
>>> It simpler:
>>>
>>>> case var
>>>> when :Fixnum
>>>>   puts "#{var} is Fixnum"
>>>> when :String
>>>>   puts "#{var} is String"
>>>> end
>>>
>>> case var
>>> when Fixnum
>>>   puts "#{var} is Fixnum"
>>> when String
>>>   puts "#{var} is String"
>>> end
>>>
>>> (no ":")
>>>
>>> Cheers,
>>>
>>
>> Hi, could you explain me why this works? It's a special behaviour of the
>> "case" statment?
>
> "case" uses operator "===" to check conditions.  If you do "case x
> when y" then "y === x" is invoked to check the condition (note the
> order of x and y).  You can even cook your own
>
> positive = lambda {|x| x>= 0}
> class<<  positive
>    alias :=== :call
> end
>
> (-4..4).each do |i|
>    case i
>    when positive
>      puts "#{i} pos"
>    else
>      puts "#{i} neg"
>    end
> end
>
> btw, there is also another form of case
>
> case # no value here!
> when i>= 0
>    puts "#{i} pos"
> else
>    puts "#{i} neg"
> end
>
> Kind regards
>
> robert
>
Clear yet concise explaination, thank you!