High Order Function with Type Parameter

[shelby3]

@keean

I mean an example of how you avoid this with intersection types. If you are proposing intersection types solve the issue, I would like to see how. Ceylon has first class intersection types, you could try that?

What is the intersection of a Natural and a Positive ?

I already explained this:

@shelby3

Moreover the immutability isn't necessary. The problem at-hand arose from forming a shared interface for the trait NatPos because we don't have first-class intersections with tagged instances. Afair, the research paper was assuming that the types were untagged and thus we could write to an instance as a Nat and read from it as Pos. Tagging forces de-structuring that disallows doing that.

My real motivation has been first-class intersections and unions as I think they will be necessary for composability, but I will want to discuss that in a different thread.

@keean

The answer is Natural /\ Positive = Positive

@shelby3

Natural /\ Positive is not the empty set. Where we have arrived is they are not disjoint and the problem at-hand is not requiring the programmer to conceive of 4 traits to be able to write NatPos instead of Natural /\ Positive. The utility is Don't Repeat Yourself (DRY) and greater degrees-of-freedom.

Afaics, Ceylon won't work for my designs because it doesn't support typeclasses and instead offers the anti-pattern of subclassing. I anticipate this will become more clear as I will try to explain how I think the design patterns for first-class genericity must be handled with a global dictionary hashtable.