From: Austin Ziegler <halostatue@...>
Date: 2005-01-10T03:30:28+09:00
Subject: Re: Type inference in ruby

On Sun, 9 Jan 2005 03:54:13 +0900, Trevor Andrade
<trevor.andrade@utoronto.ca> wrote:
> Hello all,
> 
> I was wondering whether it would be possible to have a type
> inference system in ruby. My question was asked before
> <http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/95821>
> by gabriele renzi but it never received much of a response. This
> system might try to figure out whether you are writing code with
> some obvious errors. There has been a
> <http://www.cc.gatech.edu/~lex/ti/ti.html> project on this for
> smalltalk. For instance if you have a function like the following
> 
>   def foo(object)
>     return object.call(9, 7, 3)
>   end
> 
> couldn't the type system figure out that object must respond to
> the call command and thus eliminate an error like:
> 
>   foo(5)

I'm not sure what you're trying to say here; are you wanting the
parser to detect errors like this? I'm not sure that's the right
place; sure Fixnum doesn't have a virtual class, but you can do
this:

  class Fixnum
    def call(*args)
      puts args.join("|")
    end
  end

This then totally screws up your type inference. There's no way to
be certain -- at parsing time -- that a particular call would be
invalid.


> I guess I am also wondering about a lot of other things like why
> do you need multiple assignment in Ruby? There doesn't seem to be
> any need for it since you can just modify an object by sending it
> a message eg x = x + 1 is the same as x.+(1).

Multiple assignment? Do you mean:
  x, y = y, x

Or do you mean specifically:
  x = x + 1

As far as your statement is concerned,
  x = x + 1

is NOT the same as:
  x.+(1)

It's the same as:
  x = x.+(1)

There is a distinct and important difference here.

In Ruby -- the language proper -- type inference won't really help,
at least IMO. In an interpreter or compiler for Ruby, it could make
a difference.

-austin
-- 
Austin Ziegler * halostatue@gmail.com
               * Alternate: austin@halostatue.ca