Singleton subtyping, and can we mark scala.Singleton as experimental?


#21

Just to try to draw out something actionable before diving further into alt-designs, do we all agree that the current incarnation of Singleton as an upper bound should be marked experimental for the 2.13 release, while these alt-designs cook?


#22

That’s part of SIP23 which was approved (and already used is some projects). Personally, I would rather any changes in this concept are passed through a SIP process, including a deprecation/language flag.


#23

I’m not sure what’s special about 2.13 here, scala.Singleton has existed and has had this behavior since before 2.13.


#24

:man_facepalming: wow, I did not know this, sorry folks


#25

Or even bolder! Explicit kinds :slight_smile: (sneaking them in through a magic context bound?)

def foo[T: Kind[Singleton]](t: T) = ...

#26

I do like the idea of a magic context bounds, it’s the closest I’ve seen to Miles’ desire for a general purpose elaboration mechanism.

I recant my idea of a caret in the same position as +/- and instead give you… elaboration bounds

def foo[T ^ Singleton](t: T) = ... 

Just as bold, but slightly less magical.

Given how commonly ^ is used for higher-kinded multiplication (a.k.a exponentiation), I like how the intuition works on this.


#27

CPS did that for return types, for type parameters (e.g. @specialized) it’s a prefix.


#28

I’m not sure, if the only way to define an “elaboration bound” is by hardcoding its behavior in the compiler, then the syntax you propose doesn’t seem any better than using a keyword for each hardcoded behavior, and it raises more questions, like what’s Singleton ? Is it a type ? If so, what are the values of this type ? etc.