ListFromExpr fails when list has inline element

[deusaquilus]

Minimized code

(Note: Full code sample available here: https://github.com/deusaquilus/list_inline_bug)

Let's first create some objects to hold something with non-trivial content.

case class Entity(value: String)
case class Input(ent: Entity)
case class Container(ents: List[Entity])

Let's make a macro that pulls out Entity from Input assuming Input was created via the standard .apply function.

  inline def container(inline c: Input):Container = ${ containerImpl('c) }
  def containerImpl(c: Expr[Input])(using Quotes): Expr[Container] =
    import quotes.reflect._
    val entExpr = c match
      case '{ Input($ent) } => ent
      case _ => report.throwError("Cannot Extract Entity from Input")
    '{ Container(List($entExpr)) }

Then let's make a second function to take the result of the first, pull out the entity from the container, and then do a ListFromExpr on the list. This involves writing a FromExpr for Entity:

  given FromExpr[Entity] with
    def unapply(expr: Expr[Entity])(using Quotes) = expr match
      case '{ Entity(${Expr(value)}) } => Some(Entity(value))
      case _ => None

... and then the actual function that will do list.valueOrError.

  inline def pull(inline c: Container): Entity = ${ pullImpl('c) }
  def pullImpl(c: Expr[Container])(using Quotes): Expr[Entity] =
    import quotes.reflect._
    val inputs = c match
      case '{ Container($list) } => 
        println("List Value: " + Printer.TreeStructure.show(list.asTerm))
        list.valueOrError
      case _ => report.throwError("Cannot Extract List from Container")
    '{ Entity(${Expr(inputs.head.value)}) }

Now let's run the application:

  def makeEnt = Entity("foo")

  inline def input = Input(makeEnt)
  inline def contained = container(input)
  inline def output = pull(contained)

  def main(args: Array[String]): Unit = {
    println( output )
  }

Output

This will fail with the following error

[error] 16 |    println( output )
[error]    |             ^^^^^^
[error]    |Expected a known value. 
[error]    |
[error]    |The value of: scala.List.apply[io.getquill.Entity](io.getquill.Test.makeEnt)
[error]    |could not be extracted using scala.quoted.FromExpr$ListFromExpr@3b76a5b3
[error]    | This location contains code that was inlined from Dsl.scala:17
[error]    | This location contains code that was inlined from Test.scala:13

The source of the problem seems to be that there is an Inlined node in the list apply:

Apply(TypeApply(
  Select(Ident("List"), "apply"), List(Inferred())
), List(Typed(Repeated(
    List(/* Here */ Inlined(None, Nil, Ident("makeEnt"))), Inferred()), Inferred())))

If I change makeEnt into an inline def then the problem will go away.

Expectation

The code should compile and return the Entity object.

Repository

Repo with full code sample available here:
https://github.com/deusaquilus/list_inline_bug

[deusaquilus]

Also note that the following slight variation will also fail with the same error:

  def makeEnt = Entity("foo")

  inline def entity = makeEnt // This case breaks to
  inline def input = Input(entity)
  inline def contained = container(input)
  inline def output = pull(contained)

  def main(args: Array[String]): Unit = {
    println( output )
  }

[deusaquilus]

One thing to note is that I haven't managed to solve this issue when using .asTerm.underlyingArgument.asExpr either.

[nicolasstucki]

The code is doing exactly what it is supposed to. The issue here is trying to get the value of an Entity that is not represented with its constructor. The FromExpr[Entity] is correctly implemented as

  given FromExpr[Entity] with
    def unapply(expr: Expr[Entity])(using Quotes) = expr match
      case '{ Entity(${Expr(value)}) } => Some(Entity(value))
      case _ => None

but then we ask it to unapply '{ makeEnt } which clearly does not match case '{ Entity(${Expr(value)}) }. This failure to match is propagated out to the ListFromExpr.