[Functional Programming] Function modelling -- 8. Compose Functors

Path: Compose Functors -> Monad Transformers -> Free Monad

Compose Functors:

Let's say we have a Task holding a Either. And we want simply apply a .map() function to transform the value inside TaskEither.

Like this:

const TaskEither = Compose(Task, Either);

TaskEither.of(2)
  .map((two) => two + 1)
  .extract()
  .fork(console.error, (either) => either.fold(console.log, console.log));

So how to make TaskEither composion?

const Task = require("data.task");
const Either = require("data.either");

const Compose = (F, G) => {
  const M = (fg) => ({
    extract: () => fg,
    map: (f) => M(fg.map((g) => g.map(f))),
  });
  M.of = (x) => M(F.of(G.of(x)));
  return M;
};

const TaskEither = Compose(Task, Either);

TaskEither.of(2)
  .map((two) => two + 1)
  .extract()
  .fork(console.error, (either) => either.fold(console.log, console.log));

Compose Functors can provide a way to map into nested Functors. But it cannot do chain.

So it is useful in one way, but useless in another. 

Not commonly used.

In next post, we will have a look Monads Transfomers to solve the problem that we cannot do chain in compose functors.