From: Jason Creighton <androflux@...>
Date: 2003-09-14T00:40:54+09:00
Subject: Re: Duck Typing

On Sat, 13 Sep 2003 09:23:16 +0900
Jim Weirich <jweirich@one.net> wrote:

> In the Method Redefinition thread, this explanation of Duck Typing is
> offered ...
> 
> > What Duck typing is based mostly on realising what sort of operations
> > you want to do with the object and testing for those operations,
> > rather than testing for the class. As Dave is fond of saying: type and
> > class aren't the same.
> 
> This is slightly different than my understanding of Duck Typing.  I
> would phrase it more like this:
> 
>   Duck typing is based mostly on realising what sort of operations 
>   you want to do with the object and just doing them, rather than
>   worrying if the object inherits from the proper base class or
>   interface.
> 
> I've heard others also explain duck typing in terms of explicitly
> testing for particular methods and I feel that leaves the wrong
> impression.  If we say Ruby supports duck typing, then newcomers are
> left with the impression that you need to do a lot of testing for
> particular methods (which you don't).

I agree completely.

> I would call this an example of Duck Typing ...
> 
>   class Dog
>     def talk
>       puts "Woof"
>     end
>   end
>   class Duck
>     def talk
>       puts "Quack"
>     end
>   end
>   [Dog.new, Duck.new].each { |a|  a.talk }
> 
> .. even though there is no explicit method testing going on.  After
> all, if it walks and talks like a duck ...
> 
> So, am I off base?

No. That is how I think of Duck Typing as well. Just call the methods.
Don't bother with method existence checking, Ruby does it for you. (By
raising NoMethodError)

Jason Creighton