Convert Shapeless HList to a Tuple

Question

I have this version of try-with-resources in Scala. I wonder if it is possible to make a generic version of this using Shapeless and HList?

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)

}

Something with the similar signature, I guess

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

One more problem is how to make elements available in form of a tuple in block: (A,B) => R section?

Is this possible to implement?

Answer 1

It's actually not that hard. You need a way to get an HList from a tuple ( Generic.Aux[Tup, L] ) and a way to get a List[AutoClosable] from the Hlist ( ToList[L, AutoCloseable] ).

There are probably other ways to do this than the ToList part, but it's an easy fusion of the LUBConstraint[L, AutoCloseable] and the requirement of being able to call close() on every resource.

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

The only problem is that for the case of exactly 1 resource you need to write Tuple1(new Foo) and pattern match like case Tuple1(f) . The easiest solution is to keep the Loan1 part and replace the Loan2 part with a LoanN that is implemented with shapeless and works for every arity >1. So that's almost equal to copy pasting my solution into yours and renaming my Loan class to 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)
}

I also added the constraint that the length of the input has to be greater than 1. Otherwise there is a loophole where you pass in a case class Baz() which can be converted to a List[Nothing] which is a subtype of List[AutoClosable] .

Undoubtedly the extra boilerplate with the Loan1 stuff could still be eliminated by writing a more complex typeclass yourself that is able to make a distinction between a single argument and a tuple of arguments.

---

You proposed to accept an HList as argument and transform it to a tuple. That is also possible, with shapeless.ops.hlist.Tupler . Then of course the users of that API will have to construct the HList themselves, and you still have the problem of scala not having the pretty syntax for unwrapping a Tuple1 . That second problem can be solved with a really simple custom typeclass that unwraps a Tuple1[A] to an A and leaves everything else untouched:

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
  }
}

Combine that with Tupler and you have a relatively simple solution:

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