Question

我们可以通过无形创建文字类型：

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

但是，如果创建类型别名甚至无法创建类型别名，我如何使用Int(42)作为一种类型

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

Answer 1

1）在Typelevel Scala中你可以写

val x: 42 = 42

type Answ = 42

def doSmth(value: 42) = ???

2）在Dotty Scala中你可以写相同的内容。

3）在Lightbend Scala（即标准Scala）+无形你可以写

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) = ???

如果1）build.sbt应为

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

如果是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")

如果是3）build.sbt应该是

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

4）或者您可以同时使用Typelevel Scala和Shapeless。

Answer 2

Int(42)不是类型的有效Scala语法。

IIRC单例类型在scalac中实现了一段时间，但程序员没有定义这种类型的语法。 Shapeless提供宏，以及一些额外的机器。

特别是，shapeless.Witness是一个包含类型信息和相关值的对象，也可以从其中传唤。

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＆＃34;单身类型的类型＆＃34;

2 个答案: