From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-08-20T00:11:23+09:00
Subject: Re: Case Expressions and Classes

On Fri, 19 Aug 2005, Daniel Schierbeck wrote:

> Could anyone explain to me why this isn't working?
>
> 	# returns 'no'
> 	case :foo.class
> 	  when Symbol then
> 	    puts 'yes'
> 	  else
> 	    puts 'no'
> 	end
>
> .... when this does
>
> 	# returns 'yes'
> 	case :foo.class.object_id
> 	  when Symbol.object_id then
> 	    puts 'yes'
> 	  else
> 	   puts 'no'
> 	end
>
> I'm puzzled...
>
>
> Thank you in advance,
> Daniel Schierbeck

case statements use the '===' operator.  the '===' for classes is the same as
'is_a?' for classes.  so writing

with case statements the obj of the statement is used as the argument for the
'===' operator of each case, so

   case obj
     when Foo
       ...
     when Bar
   end

is the same as

   if Foo === obj
     ...
   elsif Bar === obj
     ...
   end

and

   case string
     when %r/foo/
       ...
     when %r/bar/
   end

is the same as

   if %r/foo/ === string
     ...
   elsif %/bar/ === string
     ...
   end

so what you've written:

   case Symbol
     when Symbol
       'yes'
     else
       'no'
   end

is the same as

   if Symbol === Symbol
     'yes'
   else
     'no'
   end

so, '===', when applied to a class, tests if an object is an __instance__ of
that class.  so

     String === 'a string' #=> true
     Array === [42]        #=> true

therefore, you are asking if Symbol is an instance of a Symbol - which it is
not.  it __is__ equalivalent to Symbol, but that would mean '==' and not
'==='.

case statements can also do this

   case obj
     when String, Fixnum, Array
     when File
     when %r/foo/, %r/bar/
     else
   end

where each element in the list applies '===' to the argument 'obj' to see when
line triggers - it's really powerful.

hth.


-a
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