[ruby-talk:14348] Re: case and to_s

Hello --

On Sat, 28 Apr 2001, Jim Freeze wrote:

> Hi:
>
> I am trying to add a clase to a list depending upon the
> type of the class. Since there are several types of
> classes, I am using a case statement. But, the way I
> am using the case statement causes the << to call
> #to_s, which I don't want it to do.
> Here is the error followed by the code.
> Can someone please explain this behavior to me?
> Tnanks
>
> ./dev.rb
> ./dev.rb:21:in `===': failed to convert Class into String (TypeError)
>         from ./dev.rb:21:in `add_device'
>         from ./dev.rb:32
>         from ./dev.rb:32:in `each'
>         from ./dev.rb:32
>

I don't think it's a matter of #to_s getting called -- but
read on.

[...]

>     case d.type
>       when /DeviceA/
>         @devlist << d
>     end

This is in effect performing the test:

   /DeviceA/ === d.type

but Regexp#=== (case equality method for Regexp) expects a String on
the other end of the line, not a Class.

You could force a regexp-to-string test:

  case d.type.to_s
  (etc.)

Or you could do:

  case d
  when DeviceA
    (etc.)

because then your test is:

  DeviceA === d

and Module#===, to paraphrase the pickaxe (p. 345), will return true
if d is an instance of DeviceA or one of DeviceA's descendants.  (The
descendants part may not be desireable, but you can always use the
string-based test above instead.)


David

-- 
David Alan Black
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Web:  http://pirate.shu.edu/~blackdav