获取 "singleton type" 的类型

Question

我们可以通过 shapeless 创建文字类型：

import shapeless.syntax.singleton._
var x = 42.narrow
// x: Int(42) = 42

但是，如果甚至无法创建类型别名，我如何使用 Int(42) 作为类型进行操作

type Answ = Int(42) // won't compile
// or
def doSmth(value: Int(42)) = ... // won't compile

Answer 1

1) 在Typelevel Scala中你可以只写

val x: 42 = 42

type Answ = 42

def doSmth(value: 42) = ???

2) 在 Dotty Scala 中你可以写同样的东西。

3) 在 Lightbend Scala（即标准 Scala）+ Shapeless 中你可以写

import shapeless.Witness
import shapeless.syntax.singleton._

val x: Witness.`42`.T = 42.narrow

type Answ = Witness.`42`.T

def doSmth(value: Witness.`42`.T) = ???

情况 1) build.sbt 应该是

scalaOrganization := "org.typelevel"
scalaVersion := "2.12.3-bin-typelevel-4"
scalacOptions += "-Yliteral-types"

情况 2) build.sbt 应该是

scalaOrganization := "ch.epfl.lamp"
scalaVersion := "0.3.0-RC2"

和plugins.sbt

addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.1.5")

情况 3) build.sbt 应该是

scalaOrganization := "org.scala-lang"
scalaVersion := "2.12.3"
libraryDependencies += "com.chuusai" %% "shapeless" % "2.3.2"

4) 或者你可以同时使用 Typelevel Scala 和 Shapeless。

Answer 2

Int(42) 不是有效的 Scala 类型语法。

IIRC 单例类型在 scalac 中实现了一段时间，但程序员没有定义此类的语法。 Shapeless 通过宏以及一些额外的机制提供了这一点。

特别地，shapeless.Witness是一个既包含类型信息又包含关联值的对象，也可以从任何一个中调用。

import shapeless.Witness
import shapeless.syntax.singleton._
import shapeless.test.illTyped // test string for causing type-errors when compiled

// --- Type aliases ---
val w = 42.witness
type Answ1 = w.T // that is your Int(42) singleton type
type Answ2 = Witness.`42`.T // same, but without extra variable
implicitly[Answ1 =:= Answ2] // compiles, types are the same

// --- Value definitions ---
val a: Answ1 = 42 // compiles, value OK, and no need to `narrow`
illTyped { "val b: Answ1 = 43" } // would not compile
val c: Witness.`43`.T = 43 // that syntax is OK here too

// --- Summoning values ---
val answ = Witness[Answ1].value // will not compile for non-singleton
def genericSingletonMethod[A](implicit W: Witness.Aux[A]) = s"Summoning ${W.value}"
assert { genericSingletonMethod[Answ1] == "Summoning 42" }
assert { genericSingletonMethod[Witness.`"string"`.T] == "Summoning string" }

获取 "singleton type" 的类型

Get type of a "singleton type"

scala

generic-programming

shapeless