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使用freshName作为参数,但不显式指定类型

scala-quasiquotes scala-macros metaprogramming scala

0

Some Name · 技术社区 · 5 年前

我试图在以下宏中使用freshName作为参数名:

我

def test: Unit = macro implTst

def implTst(c: blackbox.Context): c.Expr[Unit] = {
  import c.universe._

  def withImplicitsM(exprs: List[c.Tree], expr: c.Tree): c.Tree =
    exprs match {
      case Nil =>
        expr
      case head :: tail =>
        //error here
        q"""$head.flatMap(implicit ${c.freshName()} => ${withImplicitsM(tail, expr)})"""

    }

  val exprsIo    = List(q"cats.effect.IO.apply(1)", q"cats.effect.IO.apply(2)")
  val resultTree = q"""println(${withImplicitsM(exprsIo, q"cats.effect.IO.apply(3)")}.unsafeRunSync())"""

  c.Expr[Unit](resultTree)
}

它抛出编译错误:

[error] Main.scala:25:9: exception during macro expansion: 
[error] java.lang.IllegalArgumentException: "fresh$macro$2" is not valid representation of a parameter, consider reformatting it into q"val $name: $T = $default" shape

二、

用硬编码标识符替换freshname可以使其工作:

def test: Unit = macro implTst

def implTst(c: blackbox.Context): c.Expr[Unit] = {
  import c.universe._

  def withImplicitsM(exprs: List[c.Tree], expr: c.Tree): c.Tree =
    exprs match {
      case Nil =>
        expr
      case head :: tail =>
        q"""$head.flatMap(implicit i => ${withImplicitsM(tail, expr)})"""

    }

  val exprsIo    = List(q"cats.effect.IO.apply(1)", q"cats.effect.IO.apply(2)")
  val resultTree = q"""println(${withImplicitsM(exprsIo, q"cats.effect.IO.apply(3)")}.unsafeRunSync())"""

  c.Expr[Unit](resultTree)
}

有没有办法使用 implicit ${c.freshName()} 不显式指定参数类型?

0 回复 | 直到 4 年前

1

Some Name 5 年前

解决方案:

在参数定义中显式使用空类型。

def withImplicitsM(exprs: List[c.Tree], expr: c.Tree): c.Tree =
  exprs match {
    case Nil =>
      expr
    case head :: tail =>
      val emptyType = tq""
      val v         = q"implicit val ${TermName(c.freshName())}: $emptyType"
      q"""$head.flatMap($v => ${withImplicitsM(tail, expr)})"""

  }

我怎么知道的 :

我破坏了一个类似的 flatMap 调用并查看参数定义的样子:

  val flatmapExpression               = q"cats.effect.IO.apply(1).flatMap(implicit i => cats.effect.IO.apply(2))"
  val q"$foo($args)"                  = flatmapExpression
  val q"(..$params) => $body"         = args
  val q"$mods val $name: $tpt = $rhs" = params(0)
  println(mods)
  println(name)
  println(tpt)
  println(rhs)

以下是打印出来的内容:

Modifiers(implicit <param>, , Map())
i
<type ?>
<empty>

注意 <type ?> 这是一个空类型。