From: "Mauricio Fernández" <batsman.geo@...>
Date: 2003-09-14T20:45:12+09:00
Subject: Re: Duck Typing

On Sun, Sep 14, 2003 at 07:19:07PM +0900, Chad Fowler wrote:
> On Sun, 14 Sep 2003, Ryan Pavlik wrote:
> class String
>   undef_method :chomp
> end
> 
> def some_method(a,b)
>   expect(a, String, b, Numeric) # I'm using your "Strong"Typing module. ;)
>   a.chomp #Whoops!  "a" is not the duck you're looking for
>           # (but it *is* a String)!
>   ...etc.
> end
> 
> Since I happen to be on the topic of your "Strong"Typing module, let's 
> have a look at the FAQ:
> 
> <quote source="http://mephle.org/StrongTyping">
> Q: Yeah, but really, why bother?  Why not just let ruby sort out the
>    errors as they occur?  
> A: This is incorrect thinking.  Allowing errors to just occur when
>    they happen is naive programming.  Consider the following:
>       
>       # Wait N seconds, then open the bridge for M seconds
>       def sendMsg(bridge, n, m)
>          sleep(n)
>          bridge.open
>          sleep(m)
>          bridge.close
>       end


StrongTyping doesn't address that problem, careful programming does:

def sendMsg(bridge, n, m)
	open = false
	begin
		sleep n
		bridge.open  # either completes correctly or raises an exception before opening
		open = true
		sleep m
		bridge.close
	ensure
		bridge.close if open
	end
end

As Chad points out, even StrongTyping assumes that the interface of
a class will remain stable. This means that as long as methods can be
removed/modified, the notion of "type" enforced by StrongTyping doesn't
match Ruby's native one, so you cannot get 100% safety.


-- 
 _           _                             
| |__   __ _| |_ ___ _ __ ___   __ _ _ __  
| '_ \ / _` | __/ __| '_ ` _ \ / _` | '_ \ 
| |_) | (_| | |_\__ \ | | | | | (_| | | | |
|_.__/ \__,_|\__|___/_| |_| |_|\__,_|_| |_|
	Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com

'Ooohh.. "FreeBSD is faster over loopback, when compared to Linux
over the wire". Film at 11.'
	-- Linus Torvalds