From: tilde <tilde@...>
Date: 2010-09-03T17:50:08+09:00
Subject: Re: case (variable) when type1 ... when type2 ... end

On 09/03/10 09:49, tilde wrote:
> 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!

Also (at least in 1.9.2) the alias is unnecessary, if a proc is provided 
within a "when" statment, the var is automatically passed as proc's 
argument ;)