[Grace-core] Extending types (Was Re: Type syntax)

[Andrew P. Black]

On 7 Feb 2014, at 14:00, Kim Bruce <kim at cs.pomona.edu> wrote:

> What is the definition of Collection in your example?  Your definition in collections.grace in GUnit looks different:
> 

Let’s assume something like

	type Collection<E>  = { add(element: E) -> Collection<E>; remove(element: E) -> Collection<E>; … }

I was seeing two issues.  One is insisting that the type parameters are uniform:

	type IndexableCollection = Collection<E> & type { at(ix:Number) -> E; at(ix:Number) put(e:E) }

clearly doesn’t work, because the E is unbound, but on reflection, I think that 

	type IndexableCollection<E> = Collection<E> & type { at(ix:Number) -> E; at(ix:Number) put(e:E) }

would work OK.  

The other is that the operations in IndexableCollection need to return an IndexableCollection, not a Collection.  That’s what I was meaning by SelfType.  We have to “tie the fixpoint knot”  after we have done the &, not before, to get the right type.  

	Andrew

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