From: Dave Thomas <dave@...>
Date: 2003-03-25T01:45:28+09:00
Subject: Re: Types (was Re: Ruby lecture slides (was Strong advantages over Python))


On Monday, Mar 24, 2003, at 10:32 US/Central, Mark Wilson wrote:
> I don't think it is necessary to name the types of 'f' and 'o' but 
> they can be described by their behavior, which is identical. To the 
> extent two different classes have different methods, of any number, 
> they are different types, which types may, but need not be, named if 
> necessary or convenient.
>

Except... you're really only interesting in the set of methods that you 
code needs to send to an object at a particular point: it's the 
protocol that you require at that point that determines type 
equivalency, rather that the complete set of methods implemented by an 
object.

For example, say I want to unit test a method that writes to a file. I 
know it writes using the '<<' operator:

    def meth(file)
       file << "hello world"
    end

I could write that test using a String as the receiver:

     str = ""
    meth(str)
    assert_equals("hello world", str)

At the point of invocation, String and File don't have identical sets 
of methods, but they do have sufficient overlap (#<<) for my purposes. 
They are therefore equivalent types.


Cheers



Dave