Question

我在Scala中有这个版本的try-with-resources。我想知道是否可以使用Shapeless和HList制作一个通用版本？

import scala.util.{Failure, Success, Try}

class Loan1[A <: AutoCloseable](resource: A) {
  def to[B](block: A => B): B = {
    Try(block(resource)) match {
      case Success(result) =>
        resource.close()
        result
      case Failure(e) =>
        resource.close()
        throw e
    }
  }
}

class Loan2[A <: AutoCloseable, B <: AutoCloseable](r1: A, r2: B){
  def to[R](block: (A,B) => R): R = {
    Try(block(r1,r2)) match {
      case Success(result) =>
        r1.close(); r2.close()
        result
      case Failure(e) =>
        r1.close(); r2.close()
        throw e
    }
  }
}

object Loan {

  def apply[A <: AutoCloseable](resource: A): Loan1[A] = new Loan1(resource)

  def apply[A <: AutoCloseable, B <: AutoCloseable] (r1: A, r2: B)= new Loan2(r1, r2)

}

有类似签名的东西，我想

  def apply[L <: HList](list: L)(implicit con: LUBConstraint[L, AutoCloseable]) = ???

另一个问题是如何在block: (A,B) => R部分以元组的形式提供元素？

这可以实现吗？

Answer 1

实际上并不那么难。您需要一种从元组（HList）获取Generic.Aux[Tup, L]的方法，以及从List[AutoClosable]（Hlist）获取ToList[L, AutoCloseable]的方法。

除了ToList部分之外，还有其他方法可以做到这一点，但它很容易融合LUBConstraint[L, AutoCloseable]以及能够在每个资源上调用close()的要求。

scala> :paste
// Entering paste mode (ctrl-D to finish)

import shapeless._, ops.hlist._
import scala.util.{Failure, Success, Try}

class Loan[Tup, L <: HList](resources: Tup)(
  implicit 
  gen: Generic.Aux[Tup, L],
  con: ToList[L, AutoCloseable]
) {
  def to[B](block: Tup => B): B = {
    Try(block(resources)) match {
      case Success(result) =>
        gen.to(resources).toList.foreach { _.close() }
        result
      case Failure(e) =>
        gen.to(resources).toList.foreach { _.close() }
        throw e
    }
  }
}

object Loan {
    def apply[Tup, L <: HList](resources: Tup)(
      implicit 
      gen: Generic.Aux[Tup, L],
      con: ToList[L, AutoCloseable]
    ) = new Loan(resources)
}

// Exiting paste mode, now interpreting.


scala> class Bar() extends AutoCloseable { def close = println("close Bar"); def IAmBar = println("doing bar stuff") }
defined class Bar

scala> class Foo() extends AutoCloseable { def close = println("close Foo"); def IAmFoo = println("doing foo stuff") }
defined class Foo

scala> Loan(new Foo, new Bar).to{ case (f, b) => f.IAmFoo; b.IAmBar }
doing foo stuff
doing bar stuff
close Foo
close Bar

唯一的问题是，对于恰好1个资源的情况，您需要编写Tuple1(new Foo)和模式匹配，如case Tuple1(f)。最简单的解决方案是保留Loan1部分并将Loan2部分替换为使用无形的LoanN部分，并且适用于每个arity＆gt; 1。这几乎等于将我的解决方案粘贴到您的解决方案并将我的Loan类重命名为LoanN：

import shapeless._, ops.hlist._, ops.nat._
import scala.util.{Failure, Success, Try}

class LoanN[Tup, L <: HList](resources: Tup)(
  implicit 
  gen: Generic.Aux[Tup, L],
  con: ToList[L, AutoCloseable]
) {
  def to[B](block: Tup => B): B = {
    Try(block(resources)) match {
      case Success(result) =>
        gen.to(resources).toList.foreach { _.close() }
        result
      case Failure(e) =>
        gen.to(resources).toList.foreach { _.close() }
        throw e
    }
  }
}

class Loan1[A <: AutoCloseable](resource: A) {
  def to[B](block: A => B): B = {
    Try(block(resource)) match {
      case Success(result) =>
        resource.close()
        result
      case Failure(e) =>
        resource.close()
        throw e
    }
  }
}


object Loan {
    def apply[A <: AutoCloseable](resource: A): Loan1[A] = new Loan1(resource)
    def apply[Tup, L <: HList, Len <: Nat](resources: Tup)(
      implicit 
      gen: Generic.Aux[Tup, L],
      con: ToList[L, AutoCloseable],
      length: Length.Aux[L, Len],
      gt: GT[Len, nat._1]
    ) = new LoanN(resources)
}

我还添加了输入长度必须大于1的约束。否则你传入case class Baz()的漏洞可以转换为List[Nothing]，这是一个List[AutoClosable] Loan1的子类型。

毫无疑问，通过自己编写一个更复杂的类型类可以消除带有HList内容的额外样板，它可以区分单个参数和参数元组。

您建议接受shapeless.ops.hlist.Tupler作为参数并将其转换为元组。这也是可能的，HList。当然，该API的用户必须自己构造Tuple1，并且您仍然遇到scala没有用于解开Tuple1[A]的漂亮语法的问题。第二个问题可以通过一个非常简单的自定义类型类来解决，该类型类将A展开到sealed trait Unwrap[In] { type Out def apply(in: In): Out } object Unwrap extends DefaultUnwrap { type Aux[In, Out0] = Unwrap[In] { type Out = Out0 } def apply[T](implicit unwrap: Unwrap[T]): Unwrap.Aux[T, unwrap.Out] = unwrap implicit def unwrapTuple1[A]: Unwrap.Aux[Tuple1[A], A] = new Unwrap[Tuple1[A]] { type Out = A def apply(in: Tuple1[A]) = in._1 } } trait DefaultUnwrap { implicit def dontUnwrapOthers[A]: Unwrap.Aux[A, A] = new Unwrap[A] { type Out = A def apply(in: A) = in } }并保持其他所有内容不受影响：

Tupler

将其与scala> :paste // Entering paste mode (ctrl-D to finish) import shapeless._, ops.hlist._ import scala.util.{Failure, Success, Try} class LoanN[Tup, L <: HList, Res](resources: L)( implicit tupler: Tupler.Aux[L, Tup], con: ToList[L, AutoCloseable], unwrap: Unwrap.Aux[Tup, Res] ) { def to[B](block: Res => B): B = { Try(block(unwrap(tupler(resources)))) match { case Success(result) => resources.toList.foreach { _.close() } result case Failure(e) => resources.toList.foreach { _.close() } throw e } } } object Loan { def apply[Tup, L <: HList, Res](resources: L)( implicit tupler: Tupler.Aux[L, Tup], con: ToList[L, AutoCloseable], unwrap: Unwrap.Aux[Tup, Res] ) = new LoanN(resources) } // Exiting paste mode, now interpreting. scala> Loan(new Foo :: new Bar :: HNil).to{ case (f,b) => f.IAmFoo; b.IAmBar } doing foo stuff doing bar stuff close Foo close Bar scala> Loan(new Foo :: HNil).to{ case (f) => f.IAmFoo } doing foo stuff close Foo相结合，你有一个相对简单的解决方案：

<form method='post' action='j_security_check'>
	<input type='text' name='j_username'>
	<input type='password' name='j_password'>
</form>

将无形HList转换为元组

1 个答案: