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digitalmars.D - Unclear on mixin restrictions, and an example

reply Mike Swieton <mike swieton.net> writes:
I've been following the discussion on aggregation and mixins and I've been
running into a few questions here.

Firstly, it is not clear to me what the difference between aggregation and
mixins.

Also, I am not really able to see what the meaning of mixing in a class
is. Public inheritance, of course, means "is an X", and implementing an
interface is often described as "can do X". Would a mixin mean something
like "does X like this"? This topic is only tangentially related, of
course, but I think it's important to consider. Knowing what you mean when
you program something is an important part of 'programming on purpose' (to
steal the name from an excellent book).

Also, Matthew's proposal has some the odd restriction that mixins define
no fields: Firstly, is this public or all fields? And what is the
reasoning for this? I really don't understand why.

I'd like to throw out there an example where I think mixins would be
tremendously useful. This use would not be allowed under Matthew's
proposal, and I think that's a shame if there's not very strong reasons.

Two particular needs come up for me that I'd use a mixin for: logging, and
the gang of four's Observer pattern. I'll look at the latter here. Below
is an idea for the use of mixins for it:

Please forgive any syntax errors; I work on C++ and Java by day and make
the world safe for D users by night...

class Observable(EventType)
// note this is a regular class, which is important to allow mixins to be
// subclassed. In this instance, I might subclass with a class that used
// mutexes to allow multithreaded operation. The ability to be templated
like // a regular class is extremely powerful, especially in mixins. {
	public:
		void addListener(Observer!(EventType) listener) { ... }
		void rmListener(Observer!(EventType) listener) { ... }
		void notify(EventType e) { ... }

		// would this use of Observer be forbidden under Matthew's proposal?
		void stealListeners(Observable!(EventType) speaker) { ... }
	private:
		// Matthew's proposal forbids this, doesn't it?
		list!(Observer!(EventType)) observers;
}

interface Observer(EventType)
{
	public:
		void onEvent(EventType);
}

class MyDialog :
	MyGUIComponentSuperclass,
	mixin Observable!(UserEvent),
	// multiple mixinizing is ok, different signatures. I think we forbid
	// multiple mixing of identical mixins. How do fields affect this, though?
	// This example is OK, because they are of different type (templated), but
	// what if it were, say, an int?
	mixin Observable!(ButtonEvent),
	mixin Observable!(WindowEvent)
{
	public:
		...
		void close() { notify(new WindowEvent(...)); }
}

It appears to me that the above simple example would be impossible under
the existing proposal. I'd like to argue that, from the using programmer's
perspective, these few abilities could be extremely useful.

Any thoughts on this?

Mike Swieton
__
The world is my domain.
	- Carlos the Jackal
May 01 2004
next sibling parent J Anderson <REMOVEanderson badmama.com.au> writes:
Mike Swieton wrote:

Also, Matthew's proposal has some the odd restriction that mixins define
no fields: Firstly, is this public or all fields? And what is the
reasoning for this? I really don't understand why.
  

-- -Anderson: http://badmama.com.au/~anderson/
May 01 2004
prev sibling parent Ben Hinkle <bhinkle4 juno.com> writes:
Mike Swieton wrote:

 I've been following the discussion on aggregation and mixins and I've been
 running into a few questions here.
 
 Firstly, it is not clear to me what the difference between aggregation and
 mixins.

I've heard Ruby does mixins well: http://www.rubycentral.com/faq/rubyfaq-8.html
 Also, I am not really able to see what the meaning of mixing in a class
 is. Public inheritance, of course, means "is an X", and implementing an
 interface is often described as "can do X". Would a mixin mean something
 like "does X like this"? 

Sounds accurate to me. Also something like "forwards implementation to X". see for example http://www.cs.utexas.edu/ftp/pub/predator/gcse2000.pdf
May 01 2004