(** Formal Reasoning About Programs <http://adam.chlipala.net/frap/>
  * Chapter 8: Lambda Calculus and Simple Type Soundness
  * Author: Adam Chlipala
  * License: https://creativecommons.org/licenses/by-nc-nd/4.0/ *)

Require Import Frap.

(* Expression syntax *)
Inductive exp : Set :=
| Var (x : var)
| Const (n : nat)
| Plus (e1 e2 : exp)
| Abs (x : var) (e1 : exp)
| App (e1 e2 : exp).

(* Values (final results of evaluation) *)
Inductive value : exp -> Prop :=
| VConst : forall n, value (Const n)
| VAbs : forall x e1, value (Abs x e1).

(* Substitution (not applicable when [e1] isn't closed, to avoid some complexity
 * that we don't need) *)
Fixpoint subst (e1 : exp) (x : string) (e2 : exp) : exp :=
  match e2 with
    | Var y => if y ==v x then e1 else Var y
    | Const n => Const n
    | Plus e2' e2'' => Plus (subst e1 x e2') (subst e1 x e2'')
    | Abs y e2' => Abs y (if y ==v x then e2' else subst e1 x e2')
    | App e2' e2'' => App (subst e1 x e2') (subst e1 x e2'')
  end.

(* Evaluation contexts *)
Inductive context : Set :=
| Hole : context
| Plus1 : context -> exp -> context
| Plus2 : exp -> context -> context
| App1 : context -> exp -> context
| App2 : exp -> context -> context.

(* Plugging an expression into a context *)
Inductive plug : context -> exp -> exp -> Prop :=
| PlugHole : forall e, plug Hole e e
| PlugPlus1 : forall e e' C e2,
  plug C e e'
  -> plug (Plus1 C e2) e (Plus e' e2)
| PlugPlus2 : forall e e' v1 C,
  value v1
  -> plug C e e'
  -> plug (Plus2 v1 C) e (Plus v1 e')
| PlugApp1 : forall e e' C e2,
  plug C e e'
  -> plug (App1 C e2) e (App e' e2)
| PlugApp2 : forall e e' v1 C,
  value v1
  -> plug C e e'
  -> plug (App2 v1 C) e (App v1 e').

(* Small-step, call-by-value evaluation, using our evaluation contexts *)

(* First: the primitive reductions *)
Inductive step0 : exp -> exp -> Prop :=
| Beta : forall x e v,
  value v
  -> step0 (App (Abs x e) v) (subst v x e)
| Add : forall n1 n2,
  step0 (Plus (Const n1) (Const n2)) (Const (n1 + n2)).

(* Then: running them in context *)
Inductive step : exp -> exp -> Prop :=
| StepRule : forall C e1 e2 e1' e2',
  plug C e1 e1'
  -> plug C e2 e2'
  -> step0 e1 e2
  -> step e1' e2'.

(* It's easy to wrap everything as a transition system. *)
Definition trsys_of (e : exp) := {|
  Initial := {e};
  Step := step
|}.


(* Syntax of types *)
Inductive type : Set :=
| Nat
| Fun (dom ran : type).

(* Our typing judgment uses *typing contexts* (not to be confused with
 * evaluation contexts) to map free variables to their types. *)
Inductive hasty : fmap var type -> exp -> type -> Prop :=
| HtVar : forall G x t,
  G $? x = Some t
  -> hasty G (Var x) t
| HtConst : forall G n,
  hasty G (Const n) Nat
| HtPlus : forall G e1 e2,
  hasty G e1 Nat
  -> hasty G e2 Nat
  -> hasty G (Plus e1 e2) Nat
| HtAbs : forall G x e1 t1 t2,
  hasty (G $+ (x, t1)) e1 t2
  -> hasty G (Abs x e1) (Fun t1 t2)
| HtApp : forall G e1 e2 t1 t2,
  hasty G e1 (Fun t1 t2)
  -> hasty G e2 t1
  -> hasty G (App e1 e2) t2.

Hint Constructors value plug step0 step hasty.

(* Some automation *)

Ltac t0 := match goal with
           | [ H : ex _ |- _ ] => destruct H
           | [ H : _ /\ _ |- _ ] => destruct H
           | [ |- context[?x ==v ?y] ] => destruct (x ==v y)
           | [ H : Some _ = Some _ |- _ ] => invert H

           | [ H : step _ _ |- _ ] => invert H
           | [ H : step0 _ _ |- _ ] => invert1 H
           | [ H : hasty _ ?e _, H' : value ?e |- _ ] => invert H'; invert H
           | [ H : hasty _ _ _ |- _ ] => invert1 H
           | [ H : plug _ _ _ |- _ ] => invert1 H
           end; subst.

Ltac t := simplify; propositional; repeat (t0; simplify); try congruence; eauto 6.

(* Now we're ready for the first of the two key properties, to show that "has
 * type t in the empty typing context" is an invariant. *)
Lemma progress : forall e t,
  hasty $0 e t
  -> value e
  \/ (exists e' : exp, step e e').
Proof.
  induct 1; t.
Qed.

(* Inclusion between typing contexts is preserved by adding the same new mapping
 * to both. *)
Lemma weakening_override : forall (G G' : fmap var type) x t,
  (forall x' t', G $? x' = Some t' -> G' $? x' = Some t')
  -> (forall x' t', G $+ (x, t) $? x' = Some t'
                    -> G' $+ (x, t) $? x' = Some t').
Proof.
  simplify.
  cases (x ==v x'); simplify; eauto.
Qed.

Hint Resolve weakening_override.

(** Raising a typing derivation to a larger typing context *)
Lemma weakening : forall G e t,
  hasty G e t
  -> forall G', (forall x t, G $? x = Some t -> G' $? x = Some t)
    -> hasty G' e t.
Proof.
  induct 1; t.
Qed.

Hint Resolve weakening.

(* Replacing a typing context with an equal one has no effect (useful to guide
 * proof search). *)
Lemma hasty_change : forall G e t,
  hasty G e t
  -> forall G', G' = G
    -> hasty G' e t.
Proof.
  t.
Qed.

Hint Resolve hasty_change.

(* Replacing a variable with a properly typed term preserves typing. *)
Lemma substitution : forall G x t' e t e',
  hasty (G $+ (x, t')) e t
  -> hasty $0 e' t'
  -> hasty G (subst e' x e) t.
Proof.
  induct 1; t.
Qed.

Hint Resolve substitution.

(* We're almost ready for the main preservation property.  Let's prove it first
 * for the more basic [step0] relation. *)
Lemma preservation0 : forall e1 e2,
  step0 e1 e2
  -> forall t, hasty $0 e1 t
    -> hasty $0 e2 t.
Proof.
  invert 1; t.
Qed.

Hint Resolve preservation0.

(* We also need this key property, essentially saying that, if [e1] and [e2] are
 * "type-equivalent," then they remain "type-equivalent" after wrapping the same
 * context around both. *)
Lemma generalize_plug : forall e1 C e1',
  plug C e1 e1'
  -> forall e2 e2', plug C e2 e2'
    -> (forall t, hasty $0 e1 t -> hasty $0 e2 t)
    -> (forall t, hasty $0 e1' t -> hasty $0 e2' t).
Proof.
  induct 1; t.
Qed.

Hint Resolve generalize_plug.

(* OK, now we're out of the woods. *)
Lemma preservation : forall e1 e2,
  step e1 e2
  -> forall t, hasty $0 e1 t
    -> hasty $0 e2 t.
Proof.
  invert 1; t.
Qed.

Hint Resolve progress preservation.

(* Now watch this!  Though the syntactic approach to type soundness is usually
 * presented from scratch as a proof technique, it turns out that the two key
 * properties, progress and preservation, are just instances of the same methods
 * we've been applying all along with invariants of transition systems! *)
Theorem safety : forall e t, hasty $0 e t
  -> invariantFor (trsys_of e)
                  (fun e' => value e'
                             \/ exists e'', step e' e'').
Proof.
  simplify.

  (* Step 1: strengthen the invariant.  In particular, the typing relation is
   * exactly the right stronger invariant!  Our progress theorem proves the
   * required invariant inclusion. *)
  apply invariant_weaken with (invariant1 := fun e' => hasty $0 e' t); eauto.

  (* Step 2: apply invariant induction, whose induction step turns out to match
   * our preservation theorem exactly! *)
  apply invariant_induction; simplify.
  equality.
  eauto. (* We use preservation here. *)
Qed.