(** * Representable profunctors *)
Require Import Category.Core Functor.Core Functor.Prod.Core Profunctor.Core Functor.Dual Profunctor.Identity Functor.Composition.Core Functor.Identity.

Set Implicit Arguments.
Generalizable All Variables.

Local Open Scope functor_scope.
Local Open Scope profunctor_scope.

Section representable.
  (** Quoting nLab on profunctors:

      Every functor [f : C → D] induces two profunctors [D(1, f) : C ⇸ D] and [D(f, 1) : D ⇸ C], defined by [D(1, f)(d, c) = D(d, f(c))] and [D(f, 1)(c, d) = D(f(c), d)]. These profunctors are called representable (or sometimes one of them is corepresentable). *)

  Context `{Funext}.

  Definition representable C D (F : Functor C D) : C -|-> D
    := 1%profunctor o (1, F).

  (** TODO: Is there a define this so that we get proofs by duality about representable functors?  If we had judgemental eta expansion, maybe we could do it as [swap o (representable F^op)^op]? *)
  Definition corepresentable C D (F : Functor C D) : D -|-> C
    := 1%profunctor o (F^op, 1).
End representable.