Derive methods with extra context parameters

[nicolasstucki]

Compiler version

3.0.0.RC-1

Minimized example

There is no way to add extra contextual parameters derived class.

For example we if we have derivation for the following

class MyToExpr[T]
object MyToExpr:
  inline given derived[T](using inline m: Mirror.Of[T], inline t: Type[T]): MyToExpr[T] =
    ${ q ?=> derivedExpr('m, 't)(using q, Type.of[T]) }

we need to explicitly write each given instance to be able to add the Type and Quotes contextual parameters

  sealed trait Opt[+T]
  object Opt:
    given [T: Type: MyToExpr](using Quotes): MyToExpr[Opt[T]] = MyToExpr.derived

  case class Sm[T](t: T) extends Opt[T]
  object Sm:
    given [T: Type: ToExpr](using Quotes): MyToExpr[Sm[T]] = MyToExpr.derived

  case object Nn extends Opt[Nothing]:
    given (using Quotes): MyToExpr[Nn.type] = MyToExpr.derived

In this example the Quotes in required to generate the Type within the given definitions of Opt, Sm and Nn.

Full example: see #11647

Expectation

We should have a way to easily derive such definitions

[nicolasstucki]

One solution to this limitation is to support #11650. Then we could summon the missing parameters after inlining the givens in Opt, Sm and Nn.

This solution is not great as it will generate duplicated code at each site the derived instance is used. But could be a compromise between writing the givens explicitly and code size.

[smarter]

This sounds like you want to generalize derives, see also lampepfl/dotty-feature-requests#184 for another way in which people would like to generalize it.

[nicolasstucki]

Possible solution: allow context functions in derivation clause

sealed trait Opt[+T] derives (Type[T], Quotes) ?=> MyToExpr[T]

This would generate the given definition

given derived$MyExpr[T](using MyExpr[T])(using Type[T], Quotes): MyToExpr[Opt[T]] = MyToExpr.derived

or

given derived$MyExpr[T](using MyExpr[T]): (Type[T], Quotes) ?=> MyToExpr[Opt[T]] = MyToExpr.derived