Suspicious Shadowing by a Type Parameter while defining Scala 3 Type Class

Question

I'm trying to implement some small quality-of-life improvements around an XML parser, basically trying to ape the way Circe looks like for Json parsing in Scala. The idea then would be to come up with a type class definition along these lines

type ParseResult[T] = ???

trait XMLCodec[T]:
  extension(t: T)
    def toXML: Elem

  extension(xml: Elem)
    def as[T]: ParseResult[T]  // <- this fails to compile

Except that, as stated in the title, I get a Suspicious Shadowing by a Type Parameter.

I could of course remove the [T] from that line as in

trait XMLCodec[T]:
  //...

  extension(xml: Elem)
    def asObject: ParseResult[T]  // <- works fine

The above is fine, but I have concerns about name collision when more than one implementation of the same type class is present in any given context. Is there an easy way out of this that I'm missing?

Answer 1

If as[T] has its own type parameter independent of the XMLCodec[T] type parameter then what is the reason to put the extension method as[T] inside the type class XMLCodec[T]?

Maybe you can have a type class

trait XMLCodec[T]:
  extension (t: T)
    def toXML: Elem

  def toParseResult(xml: Elem): ParseResult[T]

and a standalone extension method

extension (xml: Elem)
  def as[T](using codec: XMLCodec[T]): ParseResult[T] = codec.toParseResult(xml)