[Grace-core] Minutes of Teleconference 2013-07-24

[Andrew P Black]

On 29 Jul 2013, at 14:22, Kim Bruce wrote:

> If we tried to do some type checking, we would have to invent partial object types of the form {m:T,...,n:U,?) where ? means any compatible method types might be added (compatible means we don't break our rules on overloading).  That would allow us to pick up some type errors, but at the cost of adding a great deal of complexity to the type system.  People have looked at such systems (Mitchell among others), but they have not generally been adopted because of the complexity of often having to specify negative information as well as positive.

One of the nice things about allowing overloading by arty is that you don't need negative information.  It's _always_ possible to add a method m to an existing object (through inheritance).   If there is an existing m of a different arty, then now there are two methods m, but they are statically different.   If  the new and old ms have the same arty, then th new one replaces the old one, and the type changes appropriately.  

This isn't to say that they may not be software engineering arguments in favor of requiring explicit overriding annotations, or against overloading by arity.    I'm just pouting out that the formal system gets simpler without these features.

	Andrew

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