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Scala example source code file (Tag.scala)

This example Scala source code file (Tag.scala) is included in the "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" TM.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

some, tagof, unapply

The Tag.scala Scala example source code

package scalaz

import Id._

object Tag {
  /** `subst` specialized to `Id`.
    * @todo According to Miles, @specialized doesn't help here. Maybe manually specialize.
  @inline def apply[@specialized A, T](a: A): A @@ T = a.asInstanceOf[A @@ T]

  /** `unsubst` specialized to `Id`. */
  @inline def unwrap[@specialized A, T](a: A @@ T): A = unsubst[A, Id, T](a)

  /** Add a tag `T` to `A`.
    * NB: It is unsafe to `subst` or `unsubst` a tag in an `F` that is
    * sensitive to the `A` type within.  For example, if `F` is a
    * GADT, rather than a normal ADT, it is probably unsafe.  For
    * "normal" types like `List` and function types, it is safe.  More
    * broadly, if it is possible to write a ''legal''
    * [[scalaz.InvariantFunctor]] over the parameter, `subst` of that
    * parameter is safe.
    * We do not have a
    * <a href="">type role
    * system in Scala with which to declare the exact situations under
    * which `subst` is safe.  If we did, we would declare that `subst`
    * is safe if and only if the parameter has "representational" or
    * "phantom" role.
  def subst[A, F[_], T](fa: F[A]): F[A @@ T] = fa.asInstanceOf[F[A @@ T]]

  /** Add a tag `T` to `G[_]` */
  def subst1[G[_], F[_[_]], T](fa: F[G]): F[λ[α => G[α] @@ T]] = fa.asInstanceOf[F[λ[α => G[α] @@ T]]]

  /** Remove the tag `T`, leaving `A`. */
  def unsubst[A, F[_], T](fa: F[A @@ T]): F[A] = fa.asInstanceOf[F[A]]

  /** @see `Tag.of` */
  final class TagOf[T] private[Tag]()
      extends (Id.Id ~> (? @@ T)) {
    /** Like `Tag.apply`, but specify only the `T`. */
    def apply[A](a: A): A @@ T = Tag.apply(a)

    /** Like `Tag.unwrap`, but specify only the `T`. */
    def unwrap[A](a: A @@ T): A = Tag.unwrap(a)

    /** Like `Tag.subst`, but specify only the `T`. */
    def subst[F[_], A](fa: F[A]): F[A @@ T] = Tag.subst(fa)

    /** Like `Tag.subst1`, but specify only the `T`. */
    def subst1[F[_[_]], G[_]](fa: F[G]): F[λ[α => G[α] @@ T]] = Tag.subst1[G, F, T](fa)

    /** Tag `fa`'s return type.  Allows inference of `A` to "flow through" from
      * the enclosing context.
    def onF[A, B](fa: A => B): A => (B @@ T) =
      subst[A => ?, B](fa)

    /** One variant of `subst` with different inference. */
    def onCov[FA](fa: FA)(implicit U: Unapply[Functor, FA]): U.M[U.A @@ T] =

    /** One variant of `subst` with different inference. */
    def onContra[FA](fa: FA)(implicit U: Unapply[Contravariant, FA]): U.M[U.A @@ T] =

    /** Like `Tag.unsubst`, but specify only the `T`. */
    def unsubst[F[_], A](fa: F[A @@ T]): F[A] = Tag.unsubst(fa)

    /** Pattern match on a tagged value */
    def unapply[A](a: A @@ T): Option[A] = Some(unwrap(a))

  /** Variants of `apply`, `subst`, and `unsubst` that require
    * specifying the tag type but are more likely to infer the other
    * type parameters.
  def of[T]: TagOf[T] = new TagOf[T]

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