From: Dave Thomas <dave@...>
Date: 2003-03-24T16:33:35+09:00
Subject: Re: Types (was Re: Ruby lecture slides (was Strong advantages over Python))


On Sunday, Mar 23, 2003, at 23:56 US/Central, Mark Wilson wrote:
> Ruby has types. In Ruby, they are generally called classes. 
> Conceptually, there should not be a distinction between type and class 
> and this is certainly true of Ruby.
>
The first sentence is true, but the second is only a convention.

Take this example:

   class Fred
     def shout
       puts "wilma"
     end
   end

   f = Fred.new

   o = Object.new
   def o.shout
     puts "wilma"
   end

'f' and 'o' have different classes (Fred and Object respectively) but 
they have indistinguishable types (for all intents and purposes). In 
general, types and classes are only loosely connected in Ruby, which is 
one of the languages strengths. For example, I recently wrote a search 
engine that initially stored a vector of bits in an Array. I was able 
to change the code to store the bits in a Bignum instead by changing 
just two lines of code (and one of those was because I was being lazy 
in the original implementation). For the subset of operations I used, 
Bignum and Array had the same type.

This more commonly occurs with operators such as '<<' : we quite 
commonly write code where we don't care what class something is: we 
just need to be able to append to it.

That's the essence of duck typing: if something walks and talks like a 
duck, then it may as well be a duck.


Cheers


Dave