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adds implicits to Unapply to unpack A into [α]A #153

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merged 1 commit into from Sep 21, 2012

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commented Sep 19, 2012

steps

currently Int => Int cannot be passed into traverseU. Int is indirectly available as Applicative via Monoid[Int].applicative, which is Applicative[[α]Int].

scala> Applicative[({type λ[α] = Int})#λ]
res0: scalaz.Applicative[[α]Int] = scalaz.Monoid$$anon$1@26b0207f

scala> List(1, 2, 3) traverseU {_ + 1}
<console>:14: error: Unable to unapply type `Int` into a type constructor of kind `M[_]` that is classified by the type class `scalaz.Applicative`
1) Check that the type class is defined by compiling `implicitly[scalaz.Applicative[<type constructor>]]`.
2) Review the implicits in object Unapply, which only cover common type 'shapes'
(implicit not found: scalaz.Unapply[scalaz.Applicative, Int])
              List(1, 2, 3) traverseU {_ + 1}
                            ^

what this changes

I added unapplyA as the lowest priority implicit, which unpacks A into [α]A.

trait Unapply_3 {
  // /** Unpack a value of type `A0` into type `[a]A0`, given a instance of `TC` */
  implicit def unapplyA[TC[_[_]], A0](implicit TC0: TC[({type λ[α] = A0})#λ]): Unapply[TC, A0] {
    type M[X] = A0
    type A = A0
  } = new Unapply[TC, A0] {
    type M[X] = A0
    type A = A0
    def TC = TC0
    def apply(ma: M[A0]) = ma
  }
}

With this, traverseU works as follows:

scala> List(1, 2, 3) traverseU {_ + 1}
res0: Int = 9

tonymorris added a commit that referenced this pull request Sep 21, 2012

Merge pull request #153 from eed3si9n/topic/unapplya
adds implicits to Unapply to unpack A into [α]A

@tonymorris tonymorris merged commit 8cabc17 into scalaz:scalaz-seven Sep 21, 2012

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