[Grace-core] Fwd: [important] Traits

[Andrew P Black]

One thing that I don't like at all in our current proposal is that a client can make an object by saying

	object trait-expresssion

and if I want that constructor to do something else, then I have to do two things:

(1) Write the factory method that does the two things, e.g.

	method interval.from(base)to(limit)by(increment) {
		if (increment=0) then { return error "interval with 0 increment" }
		if (increment>0) then { object intervalUp(base,limit,increment) }
					else { object intervalDown(base,limit,increment) }

(2) change every occurrence of object interval(base,limit,increment) in the client code to interval.from(base)to(limit)by(increment).

Number (1) is what you would expect, but number (2) is very bad.  We can set a good example in our libraries by ALWAYS providing factory objects.  But we have to make the traits public too,  so that clients can inherit from them.   So I don't see a way of preventing clients from client using them directly.   Maybe allow object trait-expression ONLY in method that are annotated as factory methods?

On 11 Mar 2013, at 22:14 , James Noble wrote:

> Off the top of my head, I don't know why a "first class" trait couldn't
> simply implement its constructor method itself - making it a bit
> more like our original class design, closer to a Smalltalk class,
> so you wouldn't need to make factories that did nothing but called
> the trait "new" method.  Presumably we can stop there and avoid
> the temptation towards the whole static mess...

We did something like this in Emerald, where an object could be both a class and a type.   That is, a class was an object with a factory method, and a type was an object with s "signature" method.    The same object could have both.

IF traits are first class, then we could have an object with methods

	template // answers a trait that can be used as a template

	new // answers an object that looks like it was made from self.template

	signature // answers a type such that self.new:self.signature

But, at least for the beginning versions of the language, I think that we want to continue assuming that traits, type and objects are three different "sorts" of things.

So everything is not an object :-(

	Andrew