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 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.onF(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[State[Int, *], 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.
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