From: Gavin Sinclair <gsinclair@...>
Date: 2002-10-01T11:46:37+09:00
Subject: Re: thoughts on typelessness

From: "Bryan Murphy" <bryan@terralab.com>


> Gavin Sinclair wrote:
>
> >Ruby's mixins have similar features to Java interfaces, but they are more
> >flexible, and don't play any specification role.  Also, Ruby classes don't
need
> >any interface description to be able to plug-n-play; they just need to
support
> >the rigth methods at the right time.
> >
> >In short, it's not a technical reason.  Ruby doesn't want to be as strict as
> >Java.  Result: more flexibility, but no guarantees.
> >
> >If you find yourself in a situation where you want these things, but also
want
> >to use Ruby, either:
> > - use Ruby as a prototype and then convert to Java if necessary
> > - enforce type-compliance to key methods by raising exceptions
> >
> >
> While I understand your point, I do have to disagree and say that Mixins
> aren't anything like interfaces coming from my perspective.
>
>  From my perspective, interfaces and/or contracts are a way to
> unerringly guarantee than an object *ALWAYS* provides specific
> functionality.  Mixins are a way to share common functionality amongst
> multiple objects via code-reuse, but they don't guarantee that said
> functionality will be implemented (methods can be changed after the
> fact).  A contract says I must do something.  If I no longer do
> something, I no longer follow that contract and it is therefore no
> longer valid.  I've never read anything about Ruby Mixins being
> immutible like this
>
> Also, another important reason why I think the two concepts don't serve
> the same purpose is that interfaces *only* guarantee functionality.
>  They say nothing about how that functionality is implemented (are you
> reading from a database? or from an xml file?).  A mixin is a mixin and
> it behaves a certain way.  You can have two mixins with the same
> behavior, they may look and act the same but they are in fact two
> distinct things (unless I'm misunderstanding Mixins).
>
> You can use Mixins to implement interface like funtionality, so your
> point is certainly valid.  But you can't use Mixins to GUARANTEE that
> functionality across the board for all time.  That's the missing piece
> for a lot of us.
>
> Bryan

In Ruby, the attitude always has to be "you can ...", with the disclaimer that
"you can't guarantee ...".  If you're writing code for yourself, you trust your
own "interfaces" and use them to organise code and communicate intention
better.  If you're coding with other people, you obviously need to trust that
they will not drive on the wrong side of the road just because they can.

But your points certainly do drive a wedge further between mixins and
interfaces, the latter no longer being considered a Java specialty.

Regards,
Gavin