shapeless.compat

Ignoring

trait Ignoring[M, T] extends AnyRef

Allows to ignore some implicits in a LowPriority[T].

Use like LowPriority[Ignoring[Witness."ignoredMethod".T, T]].

Typical usage is when a fallback for type class TC is defined in its companion, like

object TC {
implicit def anyTC[T]: TC[T] = ...
}

With the example of LowPriority[T] above,

trait TC[T] {
  def prop: Option[Boolean]
}

trait LowPriTC {
  // default low priority TC[T] for any T, with field `prop` equal to `None`
  implicit def anyTC[T]: TC[T] = new TC[T] { def prop = None }
}

object TC extends LowPriTC {
  // TC[Int] available by default, with field `prop` equal to `Some(true)`
  implicit val intTC: TC[Int] = new TC[Int] { def prop = Some(true) }
}

// extra `TC[T]`, with field `prop` equal to `Some(false)`
implicit def extraTC[T](implicit ev: LowPriority[Ignoring[Witness.`"anyTC"`.T, TC[T]]]): TC[T] =
  new TC[T] { def prop = Some(false) }

// Already available instance `intTC` is still found, because `extraTC[Int]` requires a
// `LowPriority[TC[Int]]`, that will refuse to materialize (because `LowPriority` is able to
// know about the already available `intTC`.)
assert(implicitly[TC[Int]].prop == true)

// `extraTC[String]` is found, as the default `anyTC[String]` is ignored,
assert(implicitly[TC[String]].prop == false)
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