### view src/Intensional.v @ 258:4c9031b62cd0

Wf_wf
author Adam Chlipala Wed, 16 Dec 2009 16:21:29 -0500 108ec446fbaf 0c5fb678bfe2
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```(* Copyright (c) 2008-2009, Adam Chlipala
*
* Creative Commons Attribution-Noncommercial-No Derivative Works 3.0
* The license text is available at:
*)

(* begin hide *)
Require Import Arith List.

Require Import Axioms DepList Tactics.

Set Implicit Arguments.
(* end hide *)

(** %\chapter{Intensional Transformations}% *)

(* begin hide *)

Inductive type : Type :=
| Nat : type
| Arrow : type -> type -> type.

Infix "-->" := Arrow (right associativity, at level 60).

Fixpoint typeDenote (t : type) : Set :=
match t with
| Nat => nat
| t1 --> t2 => typeDenote t1 -> typeDenote t2
end.

Module Phoas.
Section vars.
Variable var : type -> Type.

Inductive exp : type -> Type :=
| Var : forall t,
var t
-> exp t

| Const : nat -> exp Nat
| Plus : exp Nat -> exp Nat -> exp Nat

| App : forall t1 t2,
exp (t1 --> t2)
-> exp t1
-> exp t2
| Abs : forall t1 t2,
(var t1 -> exp t2)
-> exp (t1 --> t2).
End vars.

Definition Exp t := forall var, exp var t.

Implicit Arguments Var [var t].
Implicit Arguments Const [var].
Implicit Arguments Plus [var].
Implicit Arguments App [var t1 t2].
Implicit Arguments Abs [var t1 t2].

Notation "# v" := (Var v) (at level 70).

Notation "^ n" := (Const n) (at level 70).
Infix "+^" := Plus (left associativity, at level 79).

Infix "@" := App (left associativity, at level 77).
Notation "\ x , e" := (Abs (fun x => e)) (at level 78).
Notation "\ ! , e" := (Abs (fun _ => e)) (at level 78).

Fixpoint expDenote t (e : exp typeDenote t) : typeDenote t :=
match e with
| Var _ v => v

| Const n => n
| Plus e1 e2 => expDenote e1 + expDenote e2

| App _ _ e1 e2 => (expDenote e1) (expDenote e2)
| Abs _ _ e' => fun x => expDenote (e' x)
end.

Definition ExpDenote t (e : Exp t) := expDenote (e _).

Section exp_equiv.
Variables var1 var2 : type -> Type.

Inductive exp_equiv : list { t : type & var1 t * var2 t }%type
-> forall t, exp var1 t -> exp var2 t -> Prop :=
| EqVar : forall G t (v1 : var1 t) v2,
In (existT _ t (v1, v2)) G
-> exp_equiv G (#v1) (#v2)

| EqConst : forall G n,
exp_equiv G (^n) (^n)
| EqPlus : forall G x1 y1 x2 y2,
exp_equiv G x1 x2
-> exp_equiv G y1 y2
-> exp_equiv G (x1 +^ y1) (x2 +^ y2)

| EqApp : forall G t1 t2 (f1 : exp _ (t1 --> t2)) (x1 : exp _ t1) f2 x2,
exp_equiv G f1 f2
-> exp_equiv G x1 x2
-> exp_equiv G (f1 @ x1) (f2 @ x2)
| EqAbs : forall G t1 t2 (f1 : var1 t1 -> exp var1 t2) f2,
(forall v1 v2, exp_equiv (existT _ t1 (v1, v2) :: G) (f1 v1) (f2 v2))
-> exp_equiv G (Abs f1) (Abs f2).
End exp_equiv.

Definition Wf t (E : Exp t) := forall var1 var2, exp_equiv nil (E var1) (E var2).
End Phoas.
(* end hide *)

Module DeBruijn.
Inductive exp : list type -> type -> Type :=
| Var : forall G t,
member t G
-> exp G t

| Const : forall G, nat -> exp G Nat
| Plus : forall G, exp G Nat -> exp G Nat -> exp G Nat

| App : forall G t1 t2,
exp G (t1 --> t2)
-> exp G t1
-> exp G t2
| Abs : forall G t1 t2,
exp (t1 :: G) t2
-> exp G (t1 --> t2).

Implicit Arguments Const [G].

Fixpoint expDenote G t (e : exp G t) : hlist typeDenote G -> typeDenote t :=
match e with
| Var _ _ v => fun s => hget s v

| Const _ n => fun _ => n
| Plus _ e1 e2 => fun s => expDenote e1 s + expDenote e2 s

| App _ _ _ e1 e2 => fun s => (expDenote e1 s) (expDenote e2 s)
| Abs _ _ _ e' => fun s x => expDenote e' (x ::: s)
end.
End DeBruijn.

Import Phoas DeBruijn.

(** * From De Bruijn to PHOAS *)

Section phoasify.
Variable var : type -> Type.

Fixpoint phoasify G t (e : DeBruijn.exp G t) : hlist var G -> Phoas.exp var t :=
match e with
| Var _ _ v => fun s => #(hget s v)

| Const _ n => fun _ => ^n
| Plus _ e1 e2 => fun s => phoasify e1 s +^ phoasify e2 s

| App _ _ _ e1 e2 => fun s => phoasify e1 s @ phoasify e2 s
| Abs _ _ _ e' => fun s => \x, phoasify e' (x ::: s)
end.
End phoasify.

Definition Phoasify t (e : DeBruijn.exp nil t) : Phoas.Exp t :=
fun _ => phoasify e HNil.

Theorem phoasify_sound : forall G t (e : DeBruijn.exp G t) s,
Phoas.expDenote (phoasify e s) = DeBruijn.expDenote e s.
induction e; crush; ext_eq; crush.
Qed.

Section vars.
Variables var1 var2 : type -> Type.

Fixpoint zip G (s1 : hlist var1 G) : hlist var2 G -> list {t : type & var1 t * var2 t}%type :=
match s1 with
| HNil => fun _ => nil
| HCons _ _ v1 s1' => fun s2 => existT _ _ (v1, hhd s2) :: zip s1' (htl s2)
end.

Lemma In_zip : forall t G (s1 : hlist _ G) s2 (m : member t G),
In (existT _ t (hget s1 m, hget s2 m)) (zip s1 s2).
induction s1; intro s2; dep_destruct s2; intro m; dep_destruct m; crush.
Qed.

Lemma unsimpl_zip : forall t (v1 : var1 t) (v2 : var2 t)
G (s1 : hlist _ G) s2 t' (e1 : Phoas.exp _ t') e2,
exp_equiv (zip (v1 ::: s1) (v2 ::: s2)) e1 e2
-> exp_equiv (existT _ _ (v1, v2) :: zip s1 s2) e1 e2.
trivial.
Qed.

Hint Resolve In_zip unsimpl_zip.

Theorem phoasify_wf : forall G t (e : DeBruijn.exp G t) s1 s2,
exp_equiv (zip s1 s2) (phoasify e s1) (phoasify e s2).
Hint Constructors exp_equiv.

induction e; crush.
Qed.
End vars.

Theorem Phoasify_wf : forall t (e : DeBruijn.exp nil t),
Wf (Phoasify e).
unfold Wf, Phoasify; intros;
apply (phoasify_wf e (HNil (B := var1)) (HNil (B := var2))).
Qed.

(** * From PHOAS to De Bruijn *)

Fixpoint lookup (G : list type) (n : nat) : option type :=
match G with
| nil => None
| t :: G' => if eq_nat_dec n (length G') then Some t else lookup G' n
end.

Infix "##" := lookup (left associativity, at level 1).

Fixpoint wf (ts : list type) t (e : Phoas.exp (fun _ => nat) t) : Prop :=
match e with
| Phoas.Var t n => ts ## n = Some t
| Phoas.Const _ => True
| Phoas.Plus e1 e2 => wf ts e1 /\ wf ts e2
| Phoas.App _ _ e1 e2 => wf ts e1 /\ wf ts e2
| Phoas.Abs t1 _ e1 => wf (t1 :: ts) (e1 (length ts))
end.

Fixpoint makeG (ts : list type) : list { t : type & nat * nat }%type :=
match ts with
| nil => nil
| t :: ts' => existT _ t (length ts', length ts') :: makeG ts'
end.

Opaque eq_nat_dec.
Hint Extern 1 (_ >= _) => omega.

Lemma lookup_contra' : forall t ts n,
ts ## n = Some t
-> n >= length ts
-> False.
induction ts; crush;
match goal with
| [ _ : context[if ?E then _ else _] |- _ ] => destruct E; crush
end; eauto.
Qed.

Lemma lookup_contra : forall t ts,
ts ## (length ts) = Some t
-> False.
intros; eapply lookup_contra'; eauto.
Qed.

Hint Resolve lookup_contra.

Lemma lookup_In : forall t v1 v2 ts,
In (existT (fun _ : type => (nat * nat)%type) t (v1, v2)) (makeG ts)
-> ts ## v1 = Some t.
induction ts; crush;
match goal with
| [ |- context[if ?E then _ else _] ] => destruct E; crush
end; elimtype False; eauto.
Qed.

Hint Resolve lookup_In.

Hint Extern 1 (_ :: _ = makeG (_ :: _)) => reflexivity.

Lemma Wf_wf' : forall G t e1 (e2 : Phoas.exp (fun _ => nat) t),
exp_equiv G e1 e2
-> forall ts, G = makeG ts
-> wf ts e1.
induction 1; crush; eauto.
Qed.

Lemma Wf_wf : forall t (E : Exp t),
Wf E
-> wf nil (E (fun _ => nat)).
intros; eapply Wf_wf'; eauto.
Qed.```