Scala:如何强制将语句转换为文字？

Question

我正在试验它的一个库中提供的 scala 的精炼类型特性:

https://github.com/fthomas/refined

下面的代码代表一个简单的案例:

  import eu.timepit.refined.auto._
  import shapeless.{Witness => W}

    type Vec5 = List[Int] Refined Size[Equal[W.`5`.T]]

    val v1: Vec5 = List(1, 2, 3, 4, 5)

    val v2: Vec5 = List(1 to 5: _*)

尝试编译它时，我收到以下错误:

[Error] /home/peng/git/scalaspike/common/src/test/scala/com/tribbloids/spike/refined_spike/Example.scala:32: compile-time refinement only works with literals
[Error] /home/peng/git/scalaspike/common/src/test/scala/com/tribbloids/spike/refined_spike/Example.scala:34: compile-time refinement only works with literals
[Error] /home/peng/git/scalaspike/common/src/test/scala/com/tribbloids/spike/singleton_ops_spike/Example.scala:32: Cannot prove requirement Require[...]
three errors found

应该注意的是，v1 和 v2 都可以在编译时轻松评估和内联，但是 Scala 编译器似乎拒绝这样做，并且对于 List 类型似乎没有办法建议这一点。

那么这个功能怎么会有用呢？

Answer 1

问题是 eu.timepit.refined 宏适用于文字、 BigDecimal 、 BigInt
https://github.com/fthomas/refined/blob/master/modules/core/shared/src/main/scala/eu/timepit/refined/macros/RefineMacro.scala#L14-L23

def impl[F[_, _], T: c.WeakTypeTag, P: c.WeakTypeTag](t: c.Expr[T])(
    rt: c.Expr[RefType[F]],
    v: c.Expr[Validate[T, P]]
): c.Expr[F[T, P]] = {
  val tValue: T = t.tree match {
    case Literal(Constant(value)) => value.asInstanceOf[T]
    case BigDecimalMatcher(value) => value.asInstanceOf[T]
    case BigIntMatcher(value)     => value.asInstanceOf[T]
    case _                        => abort(Resources.refineNonCompileTimeConstant)
  }

List(1, 2, 3, 4, 5) 不是文字。

对于像 List(1, 2, 3, 4, 5) 这样的非文字值，有 refineV 在运行时工作

val v1 = List(1, 2, 3, 4, 5)
val v2 = List(1, 2, 3, 4, 5, 6)
refineV[Size[Equal[5]]](v1) 
// Right(List(1, 2, 3, 4, 5))
refineV[Size[Equal[5]]](v2) 
// Left(Predicate taking size(List(1, 2, 3, 4, 5, 6)) = 6 failed: Predicate failed: (6 == 5).)

幸运的是你可以在编译时运行 refineV

object myAuto {
  implicit def compileTimeRefineV[T, P](t: T): T Refined P = 
    macro compileTimeRefineVImpl[T, P]

  def compileTimeRefineVImpl[T: c.WeakTypeTag, 
                             P: c.WeakTypeTag](c: blackbox.Context)(t: c.Tree): c.Tree = {
    import c.universe._
    val pTyp = weakTypeOf[P]
    val tTyp = weakTypeOf[T]
    c.eval(c.Expr[Either[String, T Refined P]](c.untypecheck(
      q"_root_.eu.timepit.refined.`package`.refineV[$pTyp].apply[$tTyp]($t)"
    ))).fold(
      c.abort(c.enclosingPosition, _),
      _ => q"$t.asInstanceOf[_root_.eu.timepit.refined.api.Refined[$tTyp, $pTyp]]"
    )
  }
}

import myAuto._ // don't import eu.timepit.refined.auto._
type Vec5 = List[Int] Refined Size[Equal[5]]
val v1: Vec5 = List(1, 2, 3, 4, 5) // compiles
// val v2: Vec5 = List(1, 2, 3, 4, 5, 6) 
  // Error: Predicate taking size(List(1, 2, 3, 4, 5, 6)) = 6 failed: Predicate failed: (6 == 5).

如果您只需要静态大小的集合，您可以使用 shapeless.Sized
https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0#collections-with-statically-known-sizes