From: Jim Weirich <jweirich@...>
Date: 2003-09-13T09:23:16+09:00
Subject: Duck Typing

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 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?

-- 
-- Jim Weirich       jweirich@one.net      http://onestepback.org
-----------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct, 
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)