[External] : Re: User model stacking

[Maurizio Cimadamore]

On 28/04/2022 14:51, Kevin Bourrillion wrote:
>
>     If we ride with that horse, it means that in universal generics,
>     we should not use T but T.val apart when we want T.val | null that
>     can be spelled T.
>
>
> I'm not following, but again I think I'm naively assuming a type 
> variable might need to be projected in either direction.

In my opinion, this does represent a change that is "less syntactic" 
than it looks.

Saying "T.ref" means "give me the reference projection of T". Reference 
projections are defined for both reference classes (String.ref = String) 
and for value classes (of course!).

By analogy, saying "T.val" means "give me the value projection of T". 
But here we have an issue: while value classes do have value projections 
(again, of course!), reference classes do not.

This seems to be at odds with the "for all" semantics of type-variables.

What we need to make "for all" works is a function that gives us the 
value projection _if it exists_, or the type unchanged otherwise.

T.valOrT

Naming asides (this is a bikeshed that will be painted later), one thing 
stands: this is no longer a "projection", this feels like a more ad-hoc 
type mapping (and one which might be a bit hard to explain).

Maurizio