Add documentation for Bimonad

docs/src/main/mdoc/typeclasses/bimonad.md

@@ -0,0 +1,85 @@
+---
+layout: docs
+title:  "Bimonad"
+section: "typeclasses"
+source: "core/src/main/scala/cats/Bimonad.scala"
+scaladoc: "#cats.Bimonad"
+---
+# Bimonad
+
+The `Bimonad` trait directly extends `Monad` and `Comonad` without introducing new methods.  `Bimonad` is
+different from other `Bi` typeclasses like `Bifunctor`, `Bifoldable` or `Bitraverse` where the prefix describes
+a `F[_, _]`. The `Bimonad` is a `F[_]` and the `Bi` prefix has a different meaning here: it's both a `Monad` and a `Comonad`.  
+Keep in mind `Bimonad` has its own added laws so something that is both monadic
+and comonadic may not necessarily be a lawful `Bimonad`.
+
+If you use `Bimonad` as a convenience type such that:
+```scala
+def f[T[_]: Monad: Comonad, S](fa: T[S]): S
+```
+is re-written to:
+```scala
+def f[T[_]: Bimonad, S](fa: T[S]): S
+```
+then `T[_]` also needs to respect an extra set of laws.
+
+### NonEmptyList as a Bimonad
+`NonEmptyList[_]` is a lawful `Bimonad` so you can chain computations (like a `Monad`) and `extract` the result at the end (like a `Comonad`).
+
+Here is a possible implementation:
+```scala mdoc
+import cats._
+import cats.data._
+import cats.implicits._
+
+implicit val nelBimonad =
+  new Bimonad[NonEmptyList] {
+
+    // in order to have a lawful bimonad `pure` and `extract` need to respect: `nelBimonad.extract(nelBimonad.pure(a)) <-> a`
+    override def pure[A](a: A): NonEmptyList[A] =
+      NonEmptyList.one(a)
+
+    override def extract[A](fa: NonEmptyList[A]): A =
+      fa.head
+
+    // use coflatMap from NonEmptyList
+    override def coflatMap[A, B](fa: NonEmptyList[A])(f: NonEmptyList[A] => B): NonEmptyList[B] =
+      fa.coflatMap(f)
+
+    // use flatMap from NonEmptyList
+    override def flatMap[A, B](fa: NonEmptyList[A])(f: A => NonEmptyList[B]): NonEmptyList[B] =
+      fa.flatMap(f)
+
+    // the tailRecM implementation is not the subject of this material
+    // as an exercise try to implement it yourself
+    override def tailRecM[A, B](a: A)(fn: A => NonEmptyList[Either[A, B]]): NonEmptyList[B] =
+      ???
+  }
+```
+
+Note the equivalence:
+```scala mdoc
+nelBimonad.pure(true).extract === NonEmptyList.one(true).head
+```
+
+Using generic bimonad syntax we could define a function that appends and extracts a configuration:
+```scala mdoc
+def make[T[_]: Bimonad](config: T[String]): String = 
+  config
+    .flatMap(c => Bimonad[T].pure(c + " with option A"))
+    .flatMap(c => Bimonad[T].pure(c + " with option B"))
+    .flatMap(c => Bimonad[T].pure(c + " with option C"))
+    .extract
+```
+
+This works with one element non-empty lists:
+```scala mdoc
+make(NonEmptyList.one("config"))
+```
+
+`Function0[_]` and `Eval[_]` are also lawful bimonads so the following calls are also valid:
+```scala mdoc
+make(() => "config")
+
+make(Eval.later("config"))
+```

docs/src/main/mdoc/typeclasses/typeclasses.md

@@ -238,20 +238,20 @@ From [cats-infographic by @tpolecat](https://github.com/tpolecat/cats-infographi
 Originally from [@alexknvl](https://gist.github.com/alexknvl/d63508ddb6a728015ace53cb70a1fd5d)
 
 
-| Type            | Functor | Apply             | Applicative | Monad | MonoidK | ApplicativeError  | MonadError | CoflatMap | Comonad |
-| --------------- |:-------:|:-----------------:|:-----------:|:-----:|:-------:|:-----------------:|:----------:|:---------:|:-------:|
-| Id[A]           | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✔       |
-| Eval[A]         | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✔       |
-| Option[A]       | ✔       | ✔                 | ✔           | ✔     | ✔       | ✔                 | ✔          | ✔         | ✗       |
-| Const[K, A]     | ✔       | ✔ (`K:Monoid`)    | ✔           | ✗     | ✗       | ✗                 | ✗          | ✗         | ✗       |
-| Either[E, A]    | ✔       | ✔                 | ✔           | ✔     | ✔       | ✔                 | ✔          | ✗         | ✗       |
-| List[A]         | ✔       | ✔                 | ✔           | ✔     | ✔       | ✗                 | ✗          | ✔         | ✗       |
-| NonEmptyList[A] | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✔       |
-| Stream[A]       | ✔       | ✔                 | ✔           | ✔     | ✔       | ✗                 | ✗          | ✔         | ✗       |
-| Map[K, A]       | ✔       | ✔                 | ✗           | ✗     | ✔       | ✗                 | ✗          | ✗         | ✗       |
-| Validated[E, A] | ✔       | ✔ (`E: Semigroup`)| ✔           | ✗     | ✗       | ✔ (`E: Semigroup`)| ✗          | ✗         | ✗       |
-| Reader[E, A]    | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✗         | ✗       |
-| Writer[E, A]    | ✔       | ✔ (`E:Monoid`)    | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✗       |
+| Type            | Functor | Apply             | Applicative | Monad | MonoidK | ApplicativeError  | MonadError | CoflatMap | Comonad | Bimonad |
+| --------------- |:-------:|:-----------------:|:-----------:|:-----:|:-------:|:-----------------:|:----------:|:---------:|:-------:|:-------:|
+| Id[A]           | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✔       |✔       |
+| Eval[A]         | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✔       |✔       |
+| Option[A]       | ✔       | ✔                 | ✔           | ✔     | ✔       | ✔                 | ✔          | ✔         | ✗       |✗       |
+| Const[K, A]     | ✔       | ✔ (`K:Monoid`)    | ✔           | ✗     | ✗       | ✗                 | ✗          | ✗         | ✗       |✗       |
+| Either[E, A]    | ✔       | ✔                 | ✔           | ✔     | ✔       | ✔                 | ✔          | ✗         | ✗       |✗       |
+| List[A]         | ✔       | ✔                 | ✔           | ✔     | ✔       | ✗                 | ✗          | ✔         | ✗       |✗       |
+| NonEmptyList[A] | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✔       |✔       |
+| Stream[A]       | ✔       | ✔                 | ✔           | ✔     | ✔       | ✗                 | ✗          | ✔         | ✗       |✗       |
+| Map[K, A]       | ✔       | ✔                 | ✗           | ✗     | ✔       | ✗                 | ✗          | ✗         | ✗       |✗       |
+| Validated[E, A] | ✔       | ✔ (`E: Semigroup`)| ✔           | ✗     | ✗       | ✔ (`E: Semigroup`)| ✗          | ✗         | ✗       |✗       |
+| Reader[E, A]    | ✔       | ✔                 | ✔           | ✔     | ✗       | ✗                 | ✗          | ✗         | ✗       |✗       |
+| Writer[E, A]    | ✔       | ✔ (`E:Monoid`)    | ✔           | ✔     | ✗       | ✗                 | ✗          | ✔         | ✗       |✗       |