scalaz

Unapply

trait Unapply[TC[_[_]], MA] extends AnyRef

Represents a type MA that has been destructured into as a type constructor M[_] applied to type A, along with a corresponding type class instance TC[M].

The implicit conversions in the companion object provide a means to obtain type class instances for partially applied type constructors, in lieu of direct compiler support as described in SI-2712.

// Directly depending on Applicative[G]
def traverse[G[_], B](f: A => G[B])(implicit G: Applicative[G]): G[F[B]] =
  G.traverse(self)(f)

// Indirect lookup of the Applicative instance
// Requires -Ydep-method-types or Scala trunk circa Oct 2011
def traverseI[GB](f: A => GB)(implicit G: Unapply[Applicative, GB]): G.M[F[G.A]] /*G[F[B]*/ = {
  G.TC.traverse(self)(a => G(f(a)))
}

// Deforested version of traverseI
def traverseI2[GB](f: A => GB)(implicit G: Unapply[Applicative, GB]): G.M[F[G.A]] /*G[F[B]*/ = {
  G.TC.traverse(self)(G.leibniz.subst[({type ￎﾻ[ￎﾱ] = A => ￎﾱ})#ￎﾻ](f))
}

// Old usage
def stateTraverse1 {
  import scalaz._, Scalaz._
  import State.{State, stateMonad}
  val ls = List(1, 2, 3)
  val traverseOpt: Option[List[Int]] = ls.traverse(a => Some(a))
  val traverseState: State[Int, List[Int]] = ls.traverse[({type ￎﾻ[ￎﾱ]=State[Int, ￎﾱ]})#ￎﾻ, Int](a => State((x: Int) => (x + 1, a)))
}

// New usage
def stateTraverse2 {
  import scalaz._, Scalaz._
  val ls = List(1, 2, 3)
  val traverseOpt: Option[List[Int]] = ls.traverseI(a => some(a))
  val traverseState = ls.traverseI(a => State((x: Int) => (x + 1, a)))
}

Credits to Miles Sabin.

Annotations
@implicitNotFound( ... )
Source
Unapply.scala
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Type Members

  1. abstract type A

    The type that M was applied to

  2. abstract type M[_]

    The type constructor

Abstract Value Members

  1. abstract def TC: TC[M]

    The instance of the type class

  2. abstract def leibniz: ===[MA, M[A]]

    Evidence that MA =:= M[A]

Concrete Value Members

  1. final def !=(arg0: Any): Boolean

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  2. final def ##(): Int

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  3. final def ==(arg0: Any): Boolean

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  4. final def apply(ma: MA): M[A]

    Compatibility.

    Compatibility.

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  5. final def asInstanceOf[T0]: T0

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  6. def clone(): AnyRef

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  10. final def getClass(): Class[_]

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  13. final def ne(arg0: AnyRef): Boolean

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  14. final def notify(): Unit

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