问题描述
在使用镜像在 Scala 3 中执行类型类派生时,是否有一种干净的方法来访问案例类字段的默认值?例如:
case class Foo(s: String = "bar",i: Int,d: Double = Math.PI)
Mirror.Product.MirroredElemLabels
将设置为 ("s","i","d")
。有没有类似的东西:(Some["bar"],None,Some[3.141592653589793])
?
如果不能,可以使用宏来实现吗?我可以同时使用镜像和宏来派生类型类实例吗?
解决方法
您必须编写一个宏,使用名为 concurrency:
# Documentation suggests ${{ github.head_ref }},but that's only available on pull_request/pull_request_target triggers,so using ${{ github.ref }}.
# On master,we want all builds to complete even if merging happens faster to make it easier to discover at which point something broke.
group: ${{ github.ref == 'refs/heads/master' && format('ci-master-{0}',github.sha) || format('ci-{0}',github.ref) }}
,<init>$default$1
,... 的方法在伴生对象中工作
<init>$default$2
镜像和宏可以协同工作:
import scala.quoted.*
inline def printDefaults[T]: Unit = ${printDefaultsImpl[T]}
def printDefaultsImpl[T](using Quotes,Type[T]): Expr[Unit] = {
import quotes.reflect.*
(1 to 3).map(i =>
TypeRepr.of[T].typeSymbol
.companionClass
.declaredMethod(s"$$lessinit$$greater$$default$$$i")
.headOption
.flatMap(_.tree.asInstanceOf[DefDef].rhs)
).foreach(println)
'{()}
}
printDefaults[Foo]
//Some(Literal(Constant(bar)))
//None
//Some(Select(Ident(Math),PI))
用法:
import scala.quoted.*
import scala.deriving.*
trait Default[T] {
type Out <: Tuple
def defaults: Out
}
object Default {
transparent inline given mkDefault[T](using
m: Mirror.ProductOf[T],s: ValueOf[Tuple.Size[m.MirroredElemTypes]]
): Default[T] =
new Default[T] {
type Out = Tuple.Map[m.MirroredElemTypes,Option]
def defaults = getDefaults[T](s.value).asInstanceOf[Out]
}
inline def getDefaults[T](inline s: Int): Tuple = ${getDefaultsImpl[T]('s)}
def getDefaultsImpl[T](s: Expr[Int])(using Quotes,Type[T]): Expr[Tuple] = {
import quotes.reflect.*
val n = s.asTerm.underlying.asInstanceOf[Literal].constant.value.asInstanceOf[Int]
val terms: List[Option[Term]] =
(1 to n).toList.map(i =>
TypeRepr.of[T].typeSymbol
.companionClass
.declaredMethod(s"$$lessinit$$greater$$default$$$i")
.headOption
.flatMap(_.tree.asInstanceOf[DefDef].rhs)
)
def exprOfOption[T](oet: Option[Expr[T]])(using Type[T],Quotes): Expr[Option[T]] = oet match {
case None => Expr(None)
case Some(et) => '{Some($et)}
}
val exprs: List[Option[Expr[Any]]] = terms.map(_.map(_.asExprOf[Any]))
val exprs1: List[Expr[Option[Any]]] = exprs.map(exprOfOption)
Expr.ofTupleFromSeq(exprs1)
}
}