Rename `promap`

Hi there. Would you please consider renaming the promap method to lens (or something like this)?

This implementation of promap does not appear to unify with the type signature for the promap operation specified in Fantasy-Land (or more generally with how profunctors are specified in other languages). The expected signature is p a b ~> (a' -> a, b -> b') -> p a' b', which does not line up in terms of arity with:

  const celsiusProf = appProf.promap(
    state => fToC(state.fahrenheit),
    (celsius, state) => ({ ...state, fahrenheit: cToF(celsius) })
  );

AFAICT this is a concrete getter setter lens being passed into a method that applies it to state and setState.

While there is a way to utilize a profunctor lens for the same purpose, to do this you have to lawfully implement promap, and then use a profunctor lens to transform a value of the lawful profunctor type.

[DylanRJohnston]

Yep, the expanded definition is super easy to write

type Lens<A, B, S, T> = {
  get: (s: S) => A;
  set: (b: B, s: S) => T;
};

const promap = <A, B, S, T, U, V>(
  lens: Lens<A, B, S, T>,
  f: (u: U) => S,
  g: (t: T) => V,
): Lens<A, B, U, V> => ({
  get: u => lens.get(f(u)),
  set: (b, u) => g(lens.set(b, f(u))),
});

[masaeedu]

The type Lens_<-, -> does not support an instance of Profunctor (although it’s probably a profunctor between a different set of categories), if that’s what you’re trying to figure out.

The type Lens is contravariant in S and B, and covariant in A and T. Once you set S and A to be the same type you can’t really contravariantly map A and likewise you can’t covariantly map B.