Singleton type as a type member

Question

I'm trying to define a type member to be a singleton type. Here is my attempt:

import shapeless.syntax.singleton._

trait Test{
    type Blocked <: Boolean
}

def f(t: Test{ type Blocked = false.narrow }) = ??? // does not compile

def f(t: Test{ type Blocked = false }) = ??? // argument is parsed as t: Test{ type Blocked = Boolean }

Is there a way to set a type member to a singleton type (Boolean(false) in my case)?

score 4 · Accepted Answer · answered Mar 23 '21 at 10:40

def f(t: Test{ type Blocked = false }) = ??? // argument is parsed as t: Test{ type Blocked = Boolean }

Blocked becomes literal singleton type false; note how f(true) is rejected bellow

scala> trait Test{
     |     type Blocked <: Boolean
     | }
trait Test

scala> val v = new Test { type Blocked = false }
val v: Test{type Blocked = false} = $anon$1@34aa8b61

scala> def f(x: v.Blocked) = x
def f(x: v.Blocked): v.Blocked

scala> f(false)
val res0: v.Blocked = false

scala> f(true)
         ^
       error: type mismatch;
        found   : Boolean(true)
        required: v.Blocked
           (which expands to)  false

Singleton type as a type member

1 Answers1