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  • Committer: Stephen Compall
  • Date: 2018-05-13 17:45:37 UTC
  • Revision ID: scompall@nocandysw.com-20180513174537-kdgcrr0wcxinlb08
Derive substCo instead

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opaque-types-to-infinity.org
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** Subtyping is incompatible with disagreeing instances
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Or, makes those instances *incoherent*.
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#+BEGIN_SRC scala
 
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import scalaz.syntax.semigroup._, scalaz.std.string._
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val h = Instance("hello")
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val w = Instance("world")
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scala> import scalaz.syntax.semigroup._, scalaz.std.string._
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scala> h |+| w: String
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res2: String = worldhello
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sealed abstract class LabelModule {
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  type Label
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  def substCo[F[_]: Functor](fl: F[Label]): F[String]
 
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  def substCo[F[_]: Functor](fl: F[Label]): F[String] = {
 
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    type K[A] = F[A] => F[String]
 
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    implicit val K =
 
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      Contravariant[? => F[String]].icompose[F]
 
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    substContra[K](identity).apply(fl)
 
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  }
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  def substContra[F[_]: Contravariant](fs: F[String])
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      : F[Label] = {
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    type K[A] = F[A] => F[Label]
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    implicit val K = Contravariant[? => F[Label]].compose[F]
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    substCo[K](identity).apply(fs)
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  }
 
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                                         : F[Label]
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}
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#+END_SRC
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* Infinite types