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pelotom/unapply trickiness

Created April 27, 2013 08:27
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unapply trickiness
// Here's the problem I was having:
scala> val unapply = implicitly[Unapply[Applicative, List[Int]]]
unapply: scalaz.Unapply[scalaz.Applicative,List[Int]] = scalaz.Unapply$$anon$14@5a977cac
scala> implicitly[Unapply[Applicative, unapply.M[unapply.A]]]
<console>:15: error: Unable to unapply type `unapply.M[unapply.A]` into a type constructor of kind `M[_]` that is classified by the type class `scalaz.Applicative`
1) Check that the type class is defined by compiling `implicitly[scalaz.Applicative[<type constructor>]]`.
2) Review the implicits in object Unapply, which only cover common type 'shapes'
(implicit not found: scalaz.Unapply[scalaz.Applicative, unapply.M[unapply.A]])
implicitly[Unapply[Applicative, unapply.M[unapply.A]]]
^
// Here's how I solved it:
scala> implicit val evidence: Applicative[unapply.M] = unapply.TC
evidence: scalaz.Applicative[unapply.M] = scalaz.std.ListInstances$$anon$1@23dc9fb9
scala> implicitly[Unapply[Applicative, unapply.M[unapply.A]]]
res1: scalaz.Unapply[scalaz.Applicative,unapply.M[unapply.A]] = scalaz.Unapply$$anon$14@d6e32d7
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