From: Joel VanderWerf <vjoel@...>
Date: 2008-08-29T04:44:58+09:00
Subject: Re: Subtle bug with catch/throw. Am I missing something?

Patrick Li wrote:
> Hi,
> I'm having trouble locating this bug that I'm having. I think it's
> because I don't completely understand Ruby's throw/catch mechanism.
> 
> I'm trying to write a simple example that demonstrates the same problem,
> but I can't track down the bug.
> 
> Here's my code: Is there any subtleties that may cause throw/catch to
> not behave as expected?
> 
>   def idle(engine,key)
>     catch(:exit) do                       <-- Catch :exit
>       while engine.nextFrame(key)
>         engine.drawFrame do
>           drawInContext(@img, 0, 0, 1)
>         end
> 
>         @enterEvent.listen do
>           @state = method(:active)
>           puts "player entered"
>           throw :exit                     <-- Throw :exit
>         end
>       end
>     end
>   end
> 
> Gives me: "in `throw': uncaught throw `exit' (NameError)"

I'm guessing that the problem is that the listen block has been saved 
away and is no longer above the #catch on the stack. Here's a simpler 
example:

def listen
   catch(:exit) do
     @listeners << proc {throw :exit}
   end
end

@listeners = []
listen
@listeners[0].call

This produces the same error message.

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407