ValueOf typeclass to inhabit singleton types

[OlivierBlanvillain]

Currently there is no way to obtain the runtime value corresponding to a singleton type. SIP-23 - Literal-based singleton types mentions a ValueOf (or SingleInhabitant) type class that is currently missing from Dotty.

For reference, a ValueOf type class was implemented in Typelevel Scala in this commit:
typelevel/scala@d777d40

[Glavo]

It looks like this feature has been added in Scala 2.13.0-M3 (scala.ValueOf), when will we add it to Dotty?

[OlivierBlanvillain]

It should probably be implemented as another fallback case in implicit search. @Glavo I think it could be a good first issue if you want to give it a shot!

[smarter]

I have an implementation, but I haven't made a pr for it yet

[milessabin]

I'll pick this up if nobody else wants it.

[milessabin]

PR here: #5647.

[Swoorup]

Shouldn't this also work for Tuple Singletons?

scala> valueOf[("orders", "trades")];;
1 |valueOf[("orders", "trades")];;
  |^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |cannot reduce summonFrom with
  | patterns :  case ev @ _:ValueOf[(("orders" : String), ("trades" : String))]
  | This location contains code that was inlined from Predef.scala:25

Use case would be making type-safe channel subscription?

case class TradeEvent()
case class OrderEvent()

type Channel = "trades" | "orders"
type Trades = "trades"
type Orders = "orders"

type SelChannel[C] = C match {
  case Trades => TradeEvent 
  case Orders => OrderEvent 
  case x *: xs => SelChannel[x] | SelChannel[xs]
  case EmptyTuple => Nothing
}

def subscribeOne[Channel <: Singleton](): List[SelChannel[Channel]] = 
  val channels: List[String] = List(valueOf[Channel]: String)
  ???

def subscribe[Channels <: Tuple](): List[SelChannel[Channels]] = 
  val channels: List[String] = (valueOf[Channels]: Tuple).toList
  ???

val orderSubscription: List[OrderEvent] = subscribeOne[("orders")]()
val orderSubscription2: List[OrderEvent] = subscribe[Tuple1["orders"]]()
val allSubscription: List[TradeEvent | OrderEvent] = subscribe[("orders", "trades")]()

[bishabosha]

Edit: comment moved to lampepfl/dotty-feature-requests#172 (comment)