3
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 trait Comonad[M[_]] {
   // map
   def >>[A,B](a: M[A])(f: A => B): M[B]

   // extract | coeta 
   def counit[A](a:M[A]): A

   // coflatten | comu
   def cojoin[A](a: M[A]): M[M[A]]
}

object Comonad {

  implicit def listComonad[A]: Comonad[List]
  =
    new Comonad[List] {

      def counit[A](lsa: List[A])
      =
        lsa match { case List(a) => a }

      def cojoin[A](lsa:List[A]): List[List[A]]
      =
        List(lsa)

      def >>[A,B](lsa: List[A])(f: A => B): List[B]
      =
        lsa map f
  }

}

So yeah I'm looking at this and I don't have that correct feeling...

Anyone mind correcting this and maybe offerring one or two other simple comonads?

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2
\$\begingroup\$

I believe, what's bothering you is a non-total definition of counit, right? (for cojoin one possible variation is lsa.tails) Indeed, a List does not have a valid comonad instance specifically bacause of that. It does have a valid semicomonad instance though.

Things that have a valid comonad instance are, for example: Identity, NonEmptyList, Zipper, Tuple. Here's a reddit question with more examples of comonads.

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