(** The exponential in cat is jsut the functor category. *)
Local Open Scope functor_scope.
(** Evaluation functor. *)
Program Definition Exp_Cat_Eval (C C' : Category) :
((Func_Cat C C') × C) –≻ C' :=
{|
FO := fun x => ((fst x) _o (snd x))%object;
FA := fun A B f => (((fst B) _a (snd f)) ∘ (@Trans _ _ _ _ (fst f) _))%morphism
|}.
Next Obligation. (* F_compose *)
(* Exp_Cat_Eval defined *)
(** The arrow map of curry functor. *)
Program Definition Exp_Cat_morph_ex_A
{C C' C'' : Category} (F : (C'' × C) –≻  C')
(a b : C'') (h : (a –≻ b)%morphism)
:
((Fix_Bi_Func_1 a F) –≻ (Fix_Bi_Func_1 b F))%nattrans :=
{|
Trans := fun c => (F _a (h, id _ c))%morphism
|}.
(* Exp_Cat_morph_ex_A defined *)
Local Hint Extern 1 => apply NatTrans_eq_simplify; cbn.
(** The curry functor. *)
Program Definition Exp_Cat_morph_ex
{C C' C'' : Category}
(F : (C'' × C) –≻ C')
:
C'' –≻ (Func_Cat C C') :=
{|
FO := fun a => Fix_Bi_Func_1 a F;
FA := Exp_Cat_morph_ex_A F
|}.

(** Proof that currying after uncurrying gives back the same functor. *)
Lemma Exp_cat_morph_ex_eval_id
{C C' C'' : Category}
(u : C'' –≻ (Func_Cat C C'))
:
(u =
Exp_Cat_morph_ex
(
(Exp_Cat_Eval C C')
∘ ((×ᶠⁿᶜ _ Cat_Has_Products) @_a (_, _) (_, _) (u, id Cat C))
)
)%morphism.