Cannot Refer to Path Dependent Type inside of type-quote

[deusaquilus]

Minimized code

Let's say we want to create a class with a PDT for example:

trait Context {
  type RowType
}

Then inside of a macro, I want to be able to use this PDT (i.e. RowType) inside of a type-quote

val tpe = '[$context.RowType]

I would want to do this for many reasons... for example to summon an object with it:

val decodeOpt = Expr.summon(using '[Decoder[$context.RowType, T]])

Only it doesn't work...

Output

The compiler fails with the following error:

[error] 34 |    val tpe = '[$context.RowType]
[error]    |                        ^
[error]    |                        ']' expected, but '.' found

Expectation

The code should compile and run properly. Statements like '[$foo.Bar] should be possible.

Code

A full code sample of this issue and a minimized version of my use-case is available here:
https://github.com/deusaquilus/pdt_typesummon

[deusaquilus]

FYI, I was using this functionality to implement Quill's QueryMeta functionality in a type-safe way. Going to look for workarounds.

[LPTK]

How would this work? context could be an arbitrary expression which is not a stable path, which would probably result in generating a program with an illegal type projection.

Maybe the macro API could provide a Stable[T] <: Expr[T] type for such use cases.

[nicolasstucki]

@deusaquilus could you provide a small self contained example that show from where you get the context in those cases?

[deusaquilus]

@nicolasstucki The attached code sums it up. I have removed everything extraneous.
https://github.com/deusaquilus/pdt_typesummon/blob/master/src/main/scala/bug/Macro.scala

The idea is:

1. Create a object with a type member used in internal implicit defs:

trait Context {
  type RowType
  implicit def stringDecoder: Decoder[RowType, String]
  def serve[T](decoder: Decoder[RowType, T]): T
}

2. Summon one of these implicit defs inside of a macro. Then pass it back into the object.

  val tpe = '[Decoder[$context.RowType, T]]
  val decoderExpr = 
    Expr.summon(using tpe) match {
      case Some(decoder) => decoder
      case None => qctx.throwError(s"Cannot find decoder for: ${tpe.show}")
    }
  
  '{ $context.serve[T]($decoderExpr) }

That's basically it!

[nicolasstucki]

@deusaquilus it looks like this can be encoded as

import scala.quoted._
import scala.quoted.matching._

trait Decoder[RowType, T] {
  def decode(row:RowType): T
}

trait Context {
  type RowType
  implicit def stringDecoder: Decoder[RowType, String]
  def serve[T](decoder: Decoder[RowType, T]): T
}

object Macro {

  inline def serveDecoder[T](context: Context): T = ${ serveDecoderImpl('context) }
  def serveDecoderImpl[T: Type, U: Type, C <: Context { type RowType = U }: Type](context: Expr[C])(using qctx: QuoteContext): Expr[T] = {
    import qctx.tasty._

    val tpe = '[Decoder[U, T]]
    val decoderExpr =
      Expr.summon(using tpe) match {
        case Some(decoder) => decoder
        case None => qctx.throwError(s"Cannot find decoder for: ${tpe.show}")
      }

    '{ $context.serve[T]($decoderExpr) }
  }
}

I have not tried to expand the macro.

[deusaquilus]

@nicolasstucki I'll give that a try. My context (in the real Quill) itself has parameters. Should I encode that like this?

def serveDecoderImpl[T: Type, U: Type, CN <: Naming, C <: Context[CN] { type RowType = U }: Type](context: Expr[C])(using qctx: QuoteContext): Expr[T]

Or like this?

def serveDecoderImpl[T: Type, U: Type, CN <: Naming, C <: Context[_] { type RowType = U }: Type](context: Expr[C[CN]])(using qctx: QuoteContext): Expr[T]

[nicolasstucki]

The first one looks better

[deusaquilus]

Edit: Removing my previous comments as they were incorrect.

@nicolasstucki Okay, so your example above does not actually work because you need the type T in order to know type of T to be able to summon a decoder.

[error] 20 |  println( serveDecoder[String](ctx) )
[error]    |           ^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |        Cannot find decoder for: bug.Decoder[ctx.RowType, scala.Nothing]
[error]    | This location contains code that was inlined from Test.scala:20

You can have a look at that here:
https://github.com/deusaquilus/pdt_typesummon/tree/decoder_type_is_nothing

[deusaquilus]

However, if I add a parameter to specify T as a specific type (e.g. via Class[T]), it will work:
This works:

inline def serveDecoder[T](context: Context, cls: Class[T]): T = ${ serveDecoderImpl('context, 'cls) }
def serveDecoderImpl[T: Type, U: Type, C <: Context { type RowType = U }: Type](context: Expr[C], cls: Expr[Class[T]])(using qctx: QuoteContext): Expr[T] = {

You can see it in this branch:
https://github.com/deusaquilus/pdt_typesummon/tree/working_with_inferred_types

[deusaquilus]

If I want to manually type the inline serveDecoder method how do I do that?
I tried this:

inline def serveDecoder[T](context: Context): T = ${ serveDecoderImpl[T, context.RowType, Context]('context) }

... and I get:

[error] -- [E057] Type Mismatch Error: /home/alexander/git/dotty/pdt_typesummon/src/main/scala/bug/Macro.scala:18:92 
[error] 18 |  inline def serveDecoder[T](context: Context): T = ${ serveDecoderImpl[T, context.RowType, Context]('context) }
[error]    |                                                                                            ^
[error]    |Type argument bug.Context does not conform to upper bound bug.Context{RowType = context.RowType}

I tried adding Context { type RowType = context.RowType } like this:

inline def serveDecoder[T](context: Context): T = ${ serveDecoderImpl[T, context.RowType, Context { type RowType = context.RowType }]('context) }

... but then this error happens:

[error] Cyclic macro dependencies in /home/alexander/git/dotty/pdt_typesummon/src/main/scala/bug/Test.scala.
[error] Compilation stopped since no further progress can be made.

Code here:
https://github.com/deusaquilus/pdt_typesummon/tree/manual_typing_not_working

[nicolasstucki]

That seems to be another bug. I will investigate it.

The context type parameter should probably be context.type.

[deusaquilus]

Just tried that. That causes this error:

[error] 21 |  inline def serveDecoder[T](context: Context): T = ${ serveDecoderImpl[T, context.RowType, context.type]('context) }
[error]    |                                                                                            ^^^^^^^
[error]    |                       access to value context from wrong staging level:
[error]    |                        - the definition is at level 0,
[error]    |                        - but the access is at level -1.

Just checked that into the following branch:
https://github.com/deusaquilus/pdt_typesummon/tree/using_context.type

[nicolasstucki]

That should be allowed.

[nicolasstucki]

Minimized to

import scala.quoted._
inline def foo(x: Any): Any = ${ expr[x.type] }
def expr[X](using Quotes): Expr[Any] = ???

3 |inline def foo(x: Any): Any = ${ expr[x.type] }
  |                                      ^
  |                           access to parameter x from wrong staging level:
  |                            - the definition is at level 0,
  |                            - but the access is at level -1.