Relaxed assignment conversions for sealed types

Brian Goetz brian.goetz at
Sun Nov 22 18:32:38 UTC 2020

I guess I deserved that, because I wasn't being specific, but I was 
looking for slightly different feedback :)

What I was trying to ask for feedback on was, that we have two problems 
in search of solutions, and I was proposing that they are both, at some 
level, the same problem -- and might benefit from a common approach.

On 11/19/2020 4:13 PM, Remi Forax wrote:
> We have avoided in the past to mix the class restriction and the type 
> system.
> By example, even if a class A is declared final, List<? extends A> is 
> not equivalent to List<A>.
> For me, you want to steer the type system to work more like a close 
> world type system,
> so it's not very consistent with the existing semantics and the fact 
> that we have chosen to add the keyword 'sealed' instead of introducing 
> a special construct to define a sum type.
> regards,
> Rémi
> ------------------------------------------------------------------------
>     *De: *"Brian Goetz" <brian.goetz at>
>     *À: *"Tagir Valeev" <amaembo at>
>     *Cc: *"amber-spec-experts" <amber-spec-experts at>
>     *Envoyé: *Jeudi 19 Novembre 2020 21:47:59
>     *Objet: *Re: Relaxed assignment conversions for sealed types
>     Bump -- hoping for feedback on this thought:
>         BarImpl bi = (__static BarImpl) b;
>         Pulling on this string some more — I think there’s a
>         connection between this feature and total statement switches.
>          We’ve been looking for a way to make statement switches
>         total, and here, what we’re looking for is a way to make
>         _casts_ total.  Which suggests this is one feature, not two.
>          So perhaps:
>             switch-total (x) { … }  // a switch, but with added bonus
>         totality checking
>             BarImpl b = (total BarImpl) bar  // a cast, but with added
>         bonus totality checking
>         Obviously we can use another word besides `total`, but it’s a
>         pretty good straw man.

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