Boost.Hana: Comonad

| Boost.Hana 1.7.1 Your standard library for metaprogramming |

Comonad
Concepts

The `Comonad` concept represents context-sensitive computations and data.

Formally, the Comonad concept is dual to the Monad concept. But unless you're a mathematician, you don't care about that and it's fine. So intuitively, a Comonad represents context sensitive values and computations. First, Comonads make it possible to extract context-sensitive values from their context with `extract`. In contrast, Monads make it possible to wrap raw values into a given context with `lift` (from Applicative).

Secondly, Comonads make it possible to apply context-sensitive values to functions accepting those, and to return the result as a context-sensitive value using `extend`. In contrast, Monads make it possible to apply a monadic value to a function accepting a normal value and returning a monadic value, and to return the result as a monadic value (with `chain`).

Finally, Comonads make it possible to wrap a context-sensitive value into an extra layer of context using `duplicate`, while Monads make it possible to take a value with an extra layer of context and to strip it with `flatten`.

Whereas `lift`, `chain` and `flatten` from Applicative and Monad have signatures

\begin{align*} \mathtt{lift}_M &: T \to M(T) \\ \mathtt{chain} &: M(T) \times (T \to M(U)) \to M(U) \\ \mathtt{flatten} &: M(M(T)) \to M(T) \end{align*}

`extract`, `extend` and `duplicate` from Comonad have signatures

\begin{align*} \mathtt{extract} &: W(T) \to T \\ \mathtt{extend} &: W(T) \times (W(T) \to U) \to W(U) \\ \mathtt{duplicate} &: W(T) \to W(W(T)) \end{align*}

Notice how the "arrows" are reversed. This symmetry is essentially what we mean by Comonad being the dual of Monad.

Note
  The Typeclassopedia is a nice Haskell-oriented resource for further reading about Comonads.

## Minimal complete definition

`extract` and (`extend` or `duplicate`) satisfying the laws below. A `Comonad` must also be a `Functor`.

## Laws

For all Comonads `w`, the following laws must be satisfied:

extract(duplicate(w)) == w
transform(duplicate(w), extract) == w
duplicate(duplicate(w)) == transform(duplicate(w), duplicate)

Note
  There are several equivalent ways of defining Comonads, and this one is just one that was picked arbitrarily for simplicity.

## Refined concept

1. Functor
   Every Comonad is also required to be a Functor. At first, one might think that it should instead be some imaginary concept CoFunctor. However, it turns out that a CoFunctor is the same as a `Functor`, hence the requirement that a `Comonad` also is a `Functor`.

## Concrete models

`hana::lazy`