[Grace-core] On Gradual Typing

[Andrew P Black]

I was going to propose something a little different.  

Michael pointed out that a type-checker might prohibit


> 1    object {
> 2        method x -> Done {
> 3            self.y
>                self.z
> 4        }
> 5    }

on the grounds that self doesn't have a y method.    To make it clear that this is intentional, we need to use an annotation to keep the type checker happy.  I propose

> 1    object is requiring(y, z) {
> 2        method x -> Done {
> 3            self.y
>                self.z
> 4        }
> 5    }

The point is that we annotate the object constructor to say that we know that this object needs more methods before it is useful as a standalone object, and list their names.   Of corse, I expect that an IDE will tell me that I need this annotation, and offer to put it there, as does Eclipse/Java with import statements.

I admit that "is requiring" sounds a bit like Bombay English.   If we follow James' suggestion that we use colon for all annotation, including type annotation, then we could write

	object: requires(y,z) {

which is a lot better.

I prefer this to declaring abstract methods because it lists, in one place, in a compact form, all the methods an inheritor needs to provide.

	Andrew