(* --- an environment-based interpreter for named terms ------- *)
(* The result of this interpreter is a "closure" --- a lambda expression paired with a substitution for all of the free variables in the term. We call such substitutions "environments".  *)
Inductive nom_val : Set :=
| nom_closure : atom -> nom_env -> n_exp -> nom_val.
Fixpoint rho_lookup x rho : option (n_exp * nom_env) :=
match rho with
| nom_nil => None
| nom_cons y e r rho' =>  if (x == y) then Some (e,r) else rho_lookup x rho'
end.
Inductive res (a : Set) : Set :=
| val     : a -> res a
| timeout : res a
| stuck   : res a.
Fixpoint nom_interp (n:nat) (e:n_exp) (rho: nom_env) : res nom_val :=
match n with
| 0 => timeout _
| S m =>
match e with
| n_var x =>  match (rho_lookup x rho) with
| Some (e',r') => nom_interp m e' r'
| None   => stuck _
end
| n_app e1 e2 =>
match nom_interp m e1 rho with
| val _ (nom_closure x rho' e1') =>
nom_interp m e1' (nom_cons x e2 rho rho')
| r       => r
end
| n_abs x e   => val _ (nom_closure x rho e)
end
end.
(* --- translation from nominal terms and values to LN terms --- *)
(* Our goal is to prove the correctness of this interpreter. In otherwords, we want to show that if it produces a value, then we can translate to an evaluation of LN terms.  The property that we ultimately want to prove looks something like this:  << Lemma nom_soundness0 : forall n e v, val _ v = nom_interp n e nom_nil  -> bigstep (nom_to_exp e) (nom_val_to_exp v). >>  where we straightforwardly translate nominal terms to LN terms. (Note that we cannot write the function in the other direction --- there are many equally valid names that we can choose for the bound variable.) *)
Fixpoint nom_to_exp (ne : n_exp) : exp :=
match ne with
| n_var x => var_f x
| n_app e1 e2 => app (nom_to_exp e1) (nom_to_exp e2)
| n_abs x e1 => abs (close_exp_wrt_exp x (nom_to_exp e1))
end.
(* The following function for environments substitutes all of the mappings in the environment in a LN expression, using the LN substitution function. This is good because we don't need to define capture-avoiding substitution for nominal terms. *)
Fixpoint nom_envsubst (ne:nom_env) : exp -> exp :=
match ne with
| nom_nil => fun e => e
| nom_cons x e' r' rho =>
fun e => nom_envsubst rho ([ x ~> nom_envsubst r' (nom_to_exp e')] e)
end.
Fixpoint nom_val_to_exp (cv:nom_val) : exp :=
match cv with
| nom_closure x rho e =>
nom_envsubst rho (nom_to_exp (n_abs x e))
end.
(* ----------------- closed values, environments and terms ------------------- *)
(* To prove the soundness lemma, we will need to strengthen it so that it says something about non-empty environments.   << Lemma nom_soundness1 : forall n rho e v, val _ v = nom_interp n e rho  -> .... -> bigstep (nom_envsubst rho (nom_to_exp e)) (nom_val_to_exp v). >>  However, this lemma will not be true for all environments and nominal terms, we need to make sure that (1) the domain of the environment includes definitions for all of the free variables in e, and (2) the range of the environment includes only *closed* nominal values.  *)
Fixpoint dom_rho (rho :nom_env) :=
match rho with
| nom_nil => {}
| nom_cons x _ _ rho => {{x}} \u dom_rho rho
end.
Inductive closed_env : nom_env -> Prop :=
| closed_nil : closed_env nom_nil
| closed_cons : forall x e r rho,
closed_env r ->
fv_exp (nom_to_exp e) [<=] dom_rho r ->
closed_env rho -> closed_env (nom_cons x e r rho).
Inductive closed_val : nom_val -> Prop :=
| closed_closure : forall x rho e,
closed_env rho ->
fv_exp (nom_to_exp e) [<=] add x (dom_rho rho) ->
closed_val (nom_closure x rho e).
Hint Constructors closed_env closed_val.
(* Our definition of closed_env means that any value that we lookup from this environment will be closed. *)

Lemma rho_lookup_closed_val : forall x rho e r,
closed_env rho ->
Some (e,r) = rho_lookup x rho ->
fv_exp (nom_to_exp e) [<=] dom_rho r.