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hmf opened this issue Nov 17, 2020 · 3 comments
Closed

Macro fails to match on more specific case: HList example #10358

hmf opened this issue Nov 17, 2020 · 3 comments
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@nicolasstucki
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area:metaprogramming:quotes Issues related to quotes and splices

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@hmf
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hmf commented Nov 17, 2020 •
edited
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Minimized code

  sealed trait HList
  case class HCons[+HD, TL <: HList](hd: HD, tl: TL) extends HList
  case object HNil extends HList

  private def flattenImpl[Xs <: HList](e: Expr[Xs])(using xt:Type[Xs], qctx:QuoteContext):
  Expr[_ <:HList] = {
    import qctx.reflect._

    e match {
      case '{HNil} =>
        '{HNil}

      //case '{HCons($h, $_)} =>
      case '{HCons($h, HCons($_,$_))} =>
        '{???}

      case _ =>
        report.error(s"flattenImpl ???????  ${e.show}", e); '{???}
    }
  }


  transparent inline def flattenM[Xs <: HList](inline xs: Xs): Any = {
    ${flattenImpl('xs)}
  }

  val ffl00: HCons[Int, HCons[Int, HNil.type]] = flattenM( HCons(0, HCons(1, HNil)) )

Output

[error] -- Error: HyperParameters.scala:448:66 
[error] 448 |    val ffl00: HCons[Int, HCons[Int, HNil.type]] = flattenM( HCons(0, HCons(1, HNil)) )
[error]     |                                                             ^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |flattenImpl ???????  automl.Recorder.HCons.apply[scala.Int, automl.Recorder.HCons[scala.Int, automl.Recorder.HNil]](0, automl.Recorder.HCons.apply[scala.Int, automl.Recorder.HNil.type](1, automl.Recorder.HNil))
[error]     | This location contains code that was inlined from HyperParameters.scala:448

Expectation

I expected the match to succeed. If I use the more general case that is commented out, the match occurs.
@hmf hmf added the itype:bug label Nov 17, 2020
@prolativ
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prolativ commented Nov 17, 2020 •
edited by nicolasstucki
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It looks like the problem is with the second parameter of HCons. Let's take a slightly simpler example:

import scala.quoted._

sealed trait HList
case class HCons[HD](hd: HD, tl: HList) extends HList
case object HNil extends HList

def showFirstTwoImpl(e: Expr[HList])(using Quotes): Expr[String] = {
  e match {
      case '{HCons($h1, HCons($h2, $_))} => '{$h1.toString ++ $h2.toString}
      case _ => '{""}
  }
}

transparent inline def showFirstTwo(inline xs: HList) = ${ showFirstTwoImpl('xs) }
object Main:
  @main
  def run(): Unit = 
    println(showFirstTwo(HCons(123, HCons("abc", HNil))))

Running the code above prints "123abc" but if you redefine HCons to use 2 type parameters like this

case class HCons[HD, TL](hd: HD, tl: TL) extends HList

then it prints an empty string

@nicolasstucki
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Using

case class HCons[HD, TL](hd: HD, tl: TL) extends HList

we can make it match by changing

-      case '{HCons($h1, HCons($h2, $_))} => '{$h1.toString ++ $h2.toString}
+      case '{HCons($h1: hd1, HCons($h2: hd2, $_ : tl))} => '{$h1.toString ++ $h2.toString}

or

-      case '{HCons($h1, HCons($h2, $_))} => '{$h1.toString ++ $h2.toString}
+      case '{HCons[hd, HCons[sd, tl]]($h1, HCons($h2, $_))} => '{$h1.toString ++ $h2.toString}

The issue with the original one is that we infer

case '{HCons[Any, HCons[Any, Any]]($h1, HCons[Any, Any]($h2, $_))} => ...

That will not match because the type parameters of HCons are invariant.

@nicolasstucki nicolasstucki added area:metaprogramming:quotes Issues related to quotes and splices and removed area:metaprogramming labels Jun 2, 2022
@hmf
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hmf commented Dec 13, 2022 •
edited
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@nicolasstucki

I have gotten bitten by this again. On reviewing this example, the solution:

-      case '{HCons($h1, HCons($h2, $_))} => '{$h1.toString ++ $h2.toString}
+      case '{HCons($h1: hd1, HCons($h2: hd2, $_ : tl))} => '{$h1.toString ++ $h2.toString}

now fails to compile in Scala version 3.2.1 with the following error:

Found:    tl$given1.Underlying
Required: data.Macros3.HList

The second solution seems to work. Is this a regression issue or need I do something else?

Compiler explanation 
[error] 777 |      case '{HCons($h1: hd1, HCons($h2: hd2, $_ : tl))} => 
[error]     |                                             ^^^^^^^
[error]     |                                        Found:    tl$given1.Underlying
[error]     |                                        Required: data.Macros3.HList
[error]     |---------------------------------------------------------------------------
[error]     | Explanation (enabled by `-explain`)
[error]     |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]     |
[error]     | Tree: ${_}:(tl$given1 @ _).Underlying
[error]     | I tried to show that
[error]     |   tl$given1.Underlying
[error]     | conforms to
[error]     |   TL
[error]     | but the comparison trace ended with `false`:
[error]     |
[error]     |   ==> tl$given1.Underlying  <:  TL
[error]     |     ==> tl$given1.Underlying  <:  TL
[error]     |       ==> tl  <:  TL
[error]     |         ==> tl  <:  TL in frozen constraint
[error]     |           ==> tl  <:  Nothing in frozen constraint
[error]     |             ==> Any  <:  Nothing (left is approximated) in frozen constraint
[error]     |             <== Any  <:  Nothing (left is approximated) in frozen constraint = false
[error]     |           <== tl  <:  Nothing in frozen constraint = false
[error]     |           ==> Any  <:  TL (left is approximated) in frozen constraint
[error]     |             ==> Any  <:  Nothing (left is approximated) in frozen constraint
[error]     |             <== Any  <:  Nothing (left is approximated) in frozen constraint = false
[error]     |           <== Any  <:  TL (left is approximated) in frozen constraint = false
[error]     |         <== tl  <:  TL in frozen constraint = false
[error]     |         ==> add constraint TL >: tl , constraint =  uninstantiated variables: TL, HD, TL, HD, TL, HD, TL, HD, TL  constrained types:    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]  bounds:       HD := Any      TL >: data.Macros3.HCons[Any, data.Macros3.HList] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd1      TL >: data.Macros3.HCons[HD, TL] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd2      TL <: data.Macros3.HList  ordering: 
[error]     |           ==> lub(Nothing, tl, canConstrain=false, isSoft=true)
[error]     |           <== lub(Nothing, tl, canConstrain=false, isSoft=true) = tl
[error]     |           ==> tl  <:  data.Macros3.HList
[error]     |             ==> Any  <:  data.Macros3.HList (left is approximated)
[error]     |             <== Any  <:  data.Macros3.HList (left is approximated) = false
[error]     |           <== tl  <:  data.Macros3.HList = false
[error]     |         <== add constraint TL >: tl , constraint =  uninstantiated variables: TL, HD, TL, HD, TL, HD, TL, HD, TL  constrained types:    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]  bounds:       HD := Any      TL >: data.Macros3.HCons[Any, data.Macros3.HList] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd1      TL >: data.Macros3.HCons[HD, TL] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd2      TL <: data.Macros3.HList  ordering:  = false
[error]     |       <== tl  <:  TL = false
[error]     |     <== tl$given1.Underlying  <:  TL = false
[error]     |   <== tl$given1.Underlying  <:  TL = false
[error]     |
[error]     | The tests were made under a constraint with:
[error]     |  uninstantiated variables: TL, HD, TL, HD, TL, HD, TL, HD, TL
[error]     |  constrained types: 
[error]     |   [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]
[error]     | , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL], 
[error]     |   [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]
[error]     | , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL], 
[error]     |   [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]
[error]     |  bounds: 
[error]     |      HD := Any
[error]     |      TL >: data.Macros3.HCons[Any, data.Macros3.HList] <: data.Macros3.HList
[error]     |      HD
[error]     |      TL <: data.Macros3.HList
[error]     |      HD >: hd1
[error]     |      TL >: data.Macros3.HCons[HD, TL] <: data.Macros3.HList
[error]     |      HD
[error]     |      TL <: data.Macros3.HList
[error]     |      HD >: hd2
[error]     |      TL <: data.Macros3.HList
[error]     |  ordering: 
[error]      ---------------------------------------------------------------------------

@hmf hmf mentioned this issue Dec 14, 2022
Closed
@hmf hmf closed this as completed Dec 14, 2022
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