Type function DSL scala

typefun.scala

// example of a "type level function" resolved via implicit search using inline match - to get around named tuple types not working with match types
sealed trait Foo

type TypeFun[Cases <: Tuple] = [T] =>> TypeFun.Resolved[T, Cases]
type ->>[Case, R] = (Case, R)

object TypeFun:
  sealed trait Wild[F[_]]
  final class Resolved[T, Cases <: Tuple]:
    type Out

  object Resolved:
    type Aux[T, Cases <: Tuple, R] = Resolved[T, Cases] { type Out = R }
    transparent inline given resolved[T, Cases <: Tuple]: Resolved[T, Cases] =
      resolveCases[T, Cases, Cases]

    transparent inline def resolveCases[T, Cases <: Tuple, C <: Tuple]: Resolved[T, Cases] =
      inline compiletime.erasedValue[C] match
        case _: ((T, r) *: _)  => Resolved[T, Cases].asInstanceOf[Resolved.Aux[T, Cases, r]]
        case _: (Wild[f] *: _) => Resolved[T, Cases].asInstanceOf[Resolved.Aux[T, Cases, f[T]]]
        case _: (_ *: rest)    => resolveCases[T, Cases, rest]

type Bar = TypeFun[(Foo ->> (a: Int, b: Int), TypeFun.Wild[[T] =>> NamedTuple.From[T]])]

val x0 = summon[Bar[Foo]]
val x0Check: x0.Out = (a = 1, b = 2)

val y0 = summon[Bar[(1, 2)]]
val y0Check: y0.Out = (1, 2)

val z0 = summon[Bar[(a: Int, b: Int)]]
val z0Check: z0.Out = (a = 1, b = 2)