From: Florian Gross <flgr@...>
Date: 2004-06-09T02:28:38+09:00
Subject: Re: Interfaces and semantics (or, how to hashpipe a duck)

Hal Fulton wrote:

> I think when Rich says "semantics" he means something that can't
> really be checked by a machine -- the "purpose or function," as it
> were.

> I do sometimes wish there were a way to detect semantics -- but the
> best we could do is simply remove the problem a step further.

> For example, imagine that we are concerned with a conceptual "append"
> operation. We want to know whether an object is capable of being
> appended onto.
> 
> Well, a string is "appendable." So is an array. So is a file.

> I have at times (almost) wished for a thing rather like Java interfaces:
> 
>   obj.implements?(Append)   # where Append is some sort of interface
>                             # whose conceptual semantics are agreed on

I think I'm going to share my ideas about all this. Oh, and sorry for 
the heavy quoting, but it's hard to snip well-written text. :)

However this also applies to me right now:

>> Warning: I'm tired, so I'm not expressing myself as well as I might
>> at a different time of day.

I would really like to do it like this:

EachContract = Contract.new do
   def test(obj)
     assert(obj.respond_to?(:each))
   end
end

AppendContract = Contract.new do
   implicates EachContract

   def test_append(obj)
     assert(obj.respond_to?(:<<))
     initial = []; obj.each { |item| initial << item }
     new_obj = obj.clone; new_obj << "x"
     appended = []; obj.each { |item| appended << item }
     assert(appended.size > initial.size)
     assert(initial.all? { |item| appended.include?(item) })
   end
end

obj = []
case obj
   when AppendContract then obj << 5
end

This still needs to be thought to the end, however.

Regards,
Florian Gross