Whereas Monads allow for the composition of effectful functions, Comonads allow for composition of functions that extract the value from their context.

Example:

const F = comonadOf(Eval)

F.extract(Eval.of(() => 2)) // 2

Note that having an Comonad instance implies Functor and CoflatMap implementations are also available, which is why Comonad is a subtype of Functor and CoflatMap.

Implementation notes

Even though in TypeScript the Funfix library is using abstract class to express type classes, when implementing this type class it is recommended that you implement it as a mixin using "implements", instead of extending it directly with "extends". See TypeScript: Mixins for details and note that we already have applyMixins defined.

Implementation example:

import {
  HK, Comonad,
  registerTypeClassInstance,
  applyMixins
} from "funfix"

// Type alias defined for readability.
// HK is our encoding for higher-kinded types.
type BoxK<T> = HK<Box<any>, T>

class Box<T> implements HK<Box<any>, T> {
  constructor(public value: T) {}

  // Implements HK<Box<any>, A>, not really needed, but useful in order
  // to avoid type casts. Note they can and should be undefined:
  readonly _funKindF: Box<any>
  readonly _funKindA: T
}

class BoxComonad implements Comonad<Box<any>> {
  map<A, B>(fa: BoxK<A>, f: (a: A) => B): BoxK<B> {
    const a = (fa as Box<A>).value
    return new Box(f(a))
  }

  coflatMap<A, B>(fa: BoxK<A>, ff: (a: BoxK<A>) => B): BoxK<B> {
    return new Box(ff(fa))
  }

  coflatten<A>(fa: BoxK<A>): BoxK<BoxK<A>> {
    return new Box(fa)
  }

  extract<A>(fa: BoxK<A>): A {
    return (fa as Box<A>).value
  }
}

// At the moment of writing, this call is not needed, but it is
// recommended anyway to future-proof the code ;-)
applyMixins(BoxComonad, [Comonad])

// Registering global Comonad instance for Box, needed in order
// for the `functorOf(Box)`, `coflatMapOf(Box)` and `comonadOf(Box)`
// calls to work
registerTypeClassInstance(Comonad)(Box, new BoxComonad())

We are using implements in order to support multiple inheritance and to avoid inheriting any static members. In the Flow definitions (e.g. .js.flow files) for Funfix these type classes are defined with "interface", as they are meant to be interfaces that sometimes have default implementations and not classes.

Credits

This type class is inspired by the equivalent in Haskell's standard library and the implementation is inspired by the Typelevel Cats project.