NormNormalization of STLC

Set Warnings "-notation-overridden,-parsing,-deprecated-hint-without-locality".
From Coq Require Import Lists.List.
From Coq Require Import Strings.String.
From PLF Require Import Maps.
From PLF Require Import Smallstep.

Hint Constructors multi : core.

(* Chapter written and maintained by Andrew Tolmach *)
This optional chapter is based on chapter 12 of Types and Programming Languages (Pierce). It may be useful to look at the two together, as that chapter includes explanations and informal proofs that are not repeated here.
In this chapter, we consider another fundamental theoretical property of the simply typed lambda-calculus: the fact that the evaluation of a well-typed program is guaranteed to halt in a finite number of steps---i.e., every well-typed term is normalizable.
Unlike the type-safety properties we have considered so far, the normalization property does not extend to full-blown programming languages, because these languages nearly always extend the simply typed lambda-calculus with constructs, such as general recursion (see the MoreStlc chapter) or recursive types, that can be used to write nonterminating programs. However, the issue of normalization reappears at the level of types when we consider the metatheory of polymorphic versions of the lambda calculus such as System F-omega: in this system, the language of types effectively contains a copy of the simply typed lambda-calculus, and the termination of the typechecking algorithm will hinge on the fact that a "normalization" operation on type expressions is guaranteed to terminate.
Another reason for studying normalization proofs is that they are some of the most beautiful---and mind-blowing---mathematics to be found in the type theory literature, often (as here) involving the fundamental proof technique of logical relations.
The calculus we shall consider here is the simply typed lambda-calculus over a single base type bool and with pairs. We'll give most details of the development for the basic lambda-calculus terms treating bool as an uninterpreted base type, and leave the extension to the boolean operators and pairs to the reader. Even for the base calculus, normalization is not entirely trivial to prove, since each reduction of a term can duplicate redexes in subterms.

Exercise: 2 stars, standard (norm_fail)

Where do we fail if we attempt to prove normalization by a straightforward induction on the size of a well-typed term?

(* Do not modify the following line: *)
Definition manual_grade_for_norm_fail : option (nat×string) := None.

Exercise: 5 stars, standard, especially useful (norm)

The best ways to understand an intricate proof like this is are (1) to help fill it in and (2) to extend it. We've left out some parts of the following development, including some proofs of lemmas and the all the cases involving products and conditionals. Fill them in.
(* Do not modify the following line: *)
Definition manual_grade_for_norm : option (nat×string) := None.


We begin by repeating the relevant language definition, which is similar to those in the MoreStlc chapter, plus supporting results including type preservation and step determinism. (We won't need progress.) You may just wish to skip down to the Normalization section...

Syntax and Operational Semantics

Inductive ty : Type :=
  | Ty_Bool : ty
  | Ty_Arrow : ty ty ty
  | Ty_Prod : ty ty ty

Inductive tm : Type :=
    (* pure STLC *)
  | tm_var : string tm
  | tm_app : tm tm tm
  | tm_abs : string ty tm tm
    (* booleans *)
  | tm_true : tm
  | tm_false : tm
  | tm_if : tm tm tm tm
    (* pairs *)
  | tm_pair : tm tm tm
  | tm_fst : tm tm
  | tm_snd : tm tm.

Declare Custom Entry stlc.

Notation "<{ e }>" := e (e custom stlc at level 99).
Notation "( x )" := x (in custom stlc, x at level 99).
Notation "x" := x (in custom stlc at level 0, x constr at level 0).
Notation "S -> T" := (Ty_Arrow S T) (in custom stlc at level 50, right associativity).
Notation "x y" := (tm_app x y) (in custom stlc at level 1, left associativity).
Notation "\ x : t , y" :=
  (tm_abs x t y) (in custom stlc at level 90, x at level 99,
                     t custom stlc at level 99,
                     y custom stlc at level 99,
                     left associativity).
Coercion tm_var : string >-> tm.

Notation "{ x }" := x (in custom stlc at level 1, x constr).

Notation "'Bool'" := Ty_Bool (in custom stlc at level 0).
Notation "'if' x 'then' y 'else' z" :=
  (tm_if x y z) (in custom stlc at level 89,
                    x custom stlc at level 99,
                    y custom stlc at level 99,
                    z custom stlc at level 99,
                    left associativity).
Notation "'true'" := true (at level 1).
Notation "'true'" := tm_true (in custom stlc at level 0).
Notation "'false'" := false (at level 1).
Notation "'false'" := tm_false (in custom stlc at level 0).

Notation "X * Y" :=
  (Ty_Prod X Y) (in custom stlc at level 2, X custom stlc, Y custom stlc at level 0).
Notation "( x ',' y )" := (tm_pair x y) (in custom stlc at level 0,
                                                x custom stlc at level 99,
                                                y custom stlc at level 99).
Notation "t '.fst'" := (tm_fst t) (in custom stlc at level 0).
Notation "t '.snd'" := (tm_snd t) (in custom stlc at level 0).


Reserved Notation "'[' x ':=' s ']' t" (in custom stlc at level 20, x constr).

Fixpoint subst (x : string) (s : tm) (t : tm) : tm :=
  match t with
  | tm_var y
      if String.eqb x y then s else t
  | <{ \ y : T, t1 }>
      if String.eqb x y then t else <{ \y:T, [x:=s] t1 }>
  | <{t1 t2}>
      <{ ([x:=s]t1) ([x:=s]t2)}>
  | <{true}>
  | <{false}>
  | <{if t1 then t2 else t3}>
      <{if ([x:=s] t1) then ([x:=s] t2) else ([x:=s] t3)}>
  | <{(t1, t2)}>
      <{( ([x:=s] t1), ([x:=s] t2) )}>
  | <{t0.fst}>
      <{ ([x:=s] t0).fst}>
  | <{t0.snd}>
      <{ ([x:=s] t0).snd}>

  where "'[' x ':=' s ']' t" := (subst x s t) (in custom stlc).


Inductive value : tm Prop :=
  | v_abs : x T2 t1,
      value <{\x:T2, t1}>
  | v_true :
      value <{true}>
  | v_false :
      value <{false}>
  | v_pair : v1 v2,
      value v1
      value v2
      value <{(v1, v2)}>.

Hint Constructors value : core.

Reserved Notation "t '-->' t'" (at level 40).

Inductive step : tm tm Prop :=
  | ST_AppAbs : x T2 t1 v2,
         value v2
         <{(\x:T2, t1) v2}> --> <{ [x:=v2]t1 }>
  | ST_App1 : t1 t1' t2,
         t1 --> t1'
         <{t1 t2}> --> <{t1' t2}>
  | ST_App2 : v1 t2 t2',
         value v1
         t2 --> t2'
         <{v1 t2}> --> <{v1 t2'}>
  | ST_IfTrue : t1 t2,
      <{if true then t1 else t2}> --> t1
  | ST_IfFalse : t1 t2,
      <{if false then t1 else t2}> --> t2
  | ST_If : t1 t1' t2 t3,
      t1 --> t1'
      <{if t1 then t2 else t3}> --> <{if t1' then t2 else t3}>
  | ST_Pair1 : t1 t1' t2,
        t1 --> t1'
        <{ (t1,t2) }> --> <{ (t1' , t2) }>
  | ST_Pair2 : v1 t2 t2',
        value v1
        t2 --> t2'
        <{ (v1, t2) }> --> <{ (v1, t2') }>
  | ST_Fst1 : t0 t0',
        t0 --> t0'
        <{ t0.fst }> --> <{ t0'.fst }>
  | ST_FstPair : v1 v2,
        value v1
        value v2
        <{ (v1,v2).fst }> --> v1
  | ST_Snd1 : t0 t0',
        t0 --> t0'
        <{ t0.snd }> --> <{ t0'.snd }>
  | ST_SndPair : v1 v2,
        value v1
        value v2
        <{ (v1,v2).snd }> --> v2

where "t '-->' t'" := (step t t').

Hint Constructors step : core.

Notation multistep := (multi step).
Notation "t1 '-->*' t2" := (multistep t1 t2) (at level 40).

Notation step_normal_form := (normal_form step).

Lemma value__normal : t, value t step_normal_form t.
Proof with eauto.
  intros t H; induction H; intros [t' ST]; inversion ST...


Definition context := partial_map ty.

Reserved Notation "Gamma '⊢' t '∈' T" (at level 40,
                                          t custom stlc, T custom stlc at level 0).

Inductive has_type : context tm ty Prop :=
  (* Same as before: *)
  (* pure STLC *)
  | T_Var : Gamma x T1,
      Gamma x = Some T1
      Gamma x \in T1
  | T_Abs : Gamma x T1 T2 t1,
    (x > T2 ; Gamma) t1 \in T1
      Gamma \x:T2, t1 \in (T2 T1)
  | T_App : T1 T2 Gamma t1 t2,
      Gamma t1 \in (T2 T1)
      Gamma t2 \in T2
      Gamma t1 t2 \in T1
  | T_True : Gamma,
       Gamma true \in Bool
  | T_False : Gamma,
       Gamma false \in Bool
  | T_If : t1 t2 t3 T1 Gamma,
       Gamma t1 \in Bool
       Gamma t2 \in T1
       Gamma t3 \in T1
       Gamma if t1 then t2 else t3 \in T1
  | T_Pair : Gamma t1 t2 T1 T2,
      Gamma t1 \in T1
      Gamma t2 \in T2
      Gamma (t1, t2) \in (T1 × T2)
  | T_Fst : Gamma t0 T1 T2,
      Gamma t0 \in (T1 × T2)
      Gamma t0.fst \in T1
  | T_Snd : Gamma t0 T1 T2,
      Gamma t0 \in (T1 × T2)
      Gamma t0.snd \in T2

where "Gamma '⊢' t '∈' T" := (has_type Gamma t T).

Hint Constructors has_type : core.

Hint Extern 2 (has_type _ (app _ _) _) ⇒ eapply T_App; auto : core.
Hint Extern 2 (_ = _) ⇒ compute; reflexivity : core.


The weakening lemma is proved as in pure STLC.
Lemma weakening : Gamma Gamma' t T,
     includedin Gamma Gamma'
     Gamma t \in T
     Gamma' t \in T.
  intros Gamma Gamma' t T H Ht.
  generalize dependent Gamma'.
  induction Ht; eauto using includedin_update.

Lemma weakening_empty : Gamma t T,
     empty t \in T
     Gamma t \in T.
  intros Gamma t T.
  eapply weakening.


Lemma substitution_preserves_typing : Gamma x U t v T,
  (x > U ; Gamma) t \in T
  empty v \in U
  Gamma [x:=v]t \in T.
Proof with eauto.
  intros Gamma x U t v T Ht Hv.
  generalize dependent Gamma. generalize dependent T.
  induction t; intros T Gamma H;
  (* in each case, we'll want to get at the derivation of H *)
    inversion H; clear H; subst; simpl; eauto.
  - (* var *)
    rename s into y. destruct (eqb_spec x y); subst.
    + (* x=y *)
      rewrite update_eq in H2.
      injection H2 as H2; subst.
      apply weakening_empty. assumption.
    + (* x<>y *)
      apply T_Var. rewrite update_neq in H2; auto.
  - (* abs *)
    rename s into y, t into S.
    destruct (eqb_spec x y); subst; apply T_Abs.
    + (* x=y *)
      rewrite update_shadow in H5. assumption.
    + (* x<>y *)
      apply IHt.
      rewrite update_permute; auto.


 Theorem preservation : t t' T,
   empty t \in T
   t --> t'
   empty t' \in T.
Proof with eauto.
intros t t' T HT. generalize dependent t'.
remember empty as Gamma.
induction HT;
  intros t' HE; subst; inversion HE; subst...
- (* T_App *)
  inversion HE; subst...
  + (* ST_AppAbs *)
    apply substitution_preserves_typing with T2...
    inversion HT1...
- (* T_Fst *)
  inversion HT...
- (* T_Snd *)
  inversion HT...

Context Invariance

Inductive appears_free_in : string tm Prop :=
  | afi_var : (x : string),
      appears_free_in x <{ x }>
  | afi_app1 : x t1 t2,
      appears_free_in x t1 appears_free_in x <{ t1 t2 }>
  | afi_app2 : x t1 t2,
      appears_free_in x t2 appears_free_in x <{ t1 t2 }>
  | afi_abs : x y T11 t12,
        y x
        appears_free_in x t12
        appears_free_in x <{ \y : T11, t12 }>
  (* booleans *)
  | afi_test0 : x t0 t1 t2,
      appears_free_in x t0
      appears_free_in x <{ if t0 then t1 else t2 }>
  | afi_test1 : x t0 t1 t2,
      appears_free_in x t1
      appears_free_in x <{ if t0 then t1 else t2 }>
  | afi_test2 : x t0 t1 t2,
      appears_free_in x t2
      appears_free_in x <{ if t0 then t1 else t2 }>
  (* pairs *)
  | afi_pair1 : x t1 t2,
      appears_free_in x t1
      appears_free_in x <{ (t1, t2) }>
  | afi_pair2 : x t1 t2,
      appears_free_in x t2
      appears_free_in x <{ (t1 , t2) }>
  | afi_fst : x t,
      appears_free_in x t
      appears_free_in x <{ t.fst }>
  | afi_snd : x t,
      appears_free_in x t
      appears_free_in x <{ t.snd }>

Hint Constructors appears_free_in : core.

Definition closed (t:tm) :=
   x, ¬ appears_free_in x t.

Lemma context_invariance : Gamma Gamma' t S,
     Gamma t \in S
     ( x, appears_free_in x t Gamma x = Gamma' x)
     Gamma' t \in S.
  generalize dependent Gamma'.
  induction H; intros; eauto 12.
  - (* T_Var *)
    apply T_Var. rewrite <- H0; auto.
  - (* T_Abs *)
    apply T_Abs.
    apply IHhas_type. intros x1 Hafi.
    (* the only tricky step... *)
    destruct (eqb_spec x x1); subst.
    + rewrite update_eq.
      rewrite update_eq.
    + rewrite update_neq; [| assumption].
      rewrite update_neq; [| assumption].

(* A handy consequence of eqb_neq *)
Theorem false_eqb_string : x y : string,
   x y String.eqb x y = false.
  intros x y. rewrite String.eqb_neq.
  intros H. apply H. Qed.

Lemma free_in_context : x t T Gamma,
   appears_free_in x t
   Gamma t \in T
    T', Gamma x = Some T'.
Proof with eauto.
  intros x t T Gamma Hafi Htyp.
  induction Htyp; inversion Hafi; subst...
  - (* T_Abs *)
    destruct IHHtyp as [T' Hctx]... T'.
    unfold update, t_update in Hctx.
    rewrite false_eqb_string in Hctx...

Corollary typable_empty__closed : t T,
    empty t \in T
    closed t.
  intros. unfold closed. intros x H1.
  destruct (free_in_context _ _ _ _ H1 H) as [T' C].
  discriminate C. Qed.


Lemma step_deterministic :
   deterministic step.
Proof with eauto.
   unfold deterministic.
   intros t t' t'' E1 E2.
   generalize dependent t''.
   induction E1; intros t'' E2; inversion E2; subst; clear E2...
  (* ST_AppAbs *)
   - inversion H3.
   - exfalso; apply value__normal in H...
   (* ST_App1 *)
   - inversion E1.
   - f_equal...
   - exfalso; apply value__normal in H1...
   (* ST_App2 *)
   - exfalso; apply value__normal in H3...
   - exfalso; apply value__normal in H...
   - f_equal...
   - (* ST_IfTrue *)
       inversion H3.
   - (* ST_IfFalse *)
       inversion H3.
   (* ST_If *)
   - inversion E1.
   - inversion E1.
   - f_equal...
   (* ST_Pair1 *)
   - f_equal...
   - exfalso; apply value__normal in H1...
   (* ST_Pair2 *)
   - exfalso; apply value__normal in H...
   - f_equal...
   (* ST_Fst1 *)
   - f_equal...
   - exfalso.
     inversion E1; subst.
     + apply value__normal in H0...
     + apply value__normal in H1...
   (* ST_FstPair *)
   - exfalso.
     inversion H2; subst.
     + apply value__normal in H...
     + apply value__normal in H0...
   (* ST_Snd1 *)
   - f_equal...
   - exfalso.
     inversion E1; subst.
     + apply value__normal in H0...
     + apply value__normal in H1...
   (* ST_SndPair *)
   - exfalso.
     inversion H2; subst.
     + apply value__normal in H...
     + apply value__normal in H0...


Now for the actual normalization proof.
Our goal is to prove that every well-typed term reduces to a normal form. In fact, it turns out to be convenient to prove something slightly stronger, namely that every well-typed term reduces to a value. This follows from the weaker property anyway via Progress (why?) but otherwise we don't need Progress, and we didn't bother re-proving it above.
Here's the key definition:
Definition halts (t:tm) : Prop := t', t -->* t' value t'.
A trivial fact:
Lemma value_halts : v, value v halts v.
  intros v H. unfold halts.
   v. split.
  - apply multi_refl.
  - assumption.

The key issue in the normalization proof (as in many proofs by induction) is finding a strong enough induction hypothesis. To this end, we begin by defining, for each type T, a set R_T of closed terms of type T. We will specify these sets using a relation R and write R T t when t is in R_T. (The sets R_T are sometimes called saturated sets or reducibility candidates.)
Here is the definition of R for the base language:
  • R bool t iff t is a closed term of type bool and t halts in a value
  • R (T1 T2) t iff t is a closed term of type T1 T2 and t halts in a value and for any term s such that R T1 s, we have R T2 (t s).
This definition gives us the strengthened induction hypothesis that we need. Our primary goal is to show that all programs ---i.e., all closed terms of base type---halt. But closed terms of base type can contain subterms of functional type, so we need to know something about these as well. Moreover, it is not enough to know that these subterms halt, because the application of a normalized function to a normalized argument involves a substitution, which may enable more reduction steps. So we need a stronger condition for terms of functional type: not only should they halt themselves, but, when applied to halting arguments, they should yield halting results.
The form of R is characteristic of the logical relations proof technique. (Since we are just dealing with unary relations here, we could perhaps more properly say logical properties.) If we want to prove some property P of all closed terms of type A, we proceed by proving, by induction on types, that all terms of type A possess property P, all terms of type AA preserve property P, all terms of type (AA)->(AA) preserve the property of preserving property P, and so on. We do this by defining a family of properties, indexed by types. For the base type A, the property is just P. For functional types, it says that the function should map values satisfying the property at the input type to values satisfying the property at the output type.
When we come to formalize the definition of R in Coq, we hit a problem. The most obvious formulation would be as a parameterized Inductive proposition like this:
      Inductive R : tytmProp :=
      | R_bool : b t, emptyt \in Bool
                      halts t
                      R Bool t
      | R_arrow : T1 T2 t, emptyt \in (Arrow T1 T2) →
                      halts t
                      ( s, R T1 sR T2 (app t s)) →
                      R (Arrow T1 T2) t.
Unfortunately, Coq rejects this definition because it violates the strict positivity requirement for inductive definitions, which says that the type being defined must not occur to the left of an arrow in the type of a constructor argument. Here, it is the third argument to R_arrow, namely ( s, R T1 s R TS (app t s)), and specifically the R T1 s part, that violates this rule. (The outermost arrows separating the constructor arguments don't count when applying this rule; otherwise we could never have genuinely inductive properties at all!) The reason for the rule is that types defined with non-positive recursion can be used to build non-terminating functions, which as we know would be a disaster for Coq's logical soundness. Even though the relation we want in this case might be perfectly innocent, Coq still rejects it because it fails the positivity test.
Fortunately, it turns out that we can define R using a Fixpoint:
Fixpoint R (T:ty) (t:tm) : Prop :=
  empty t \in T halts t
  (match T with
   | <{ Bool }>True
   | <{ T1 T2 }> ⇒ ( s, R T1 s R T2 <{t s}> )

   (* ... edit the next line when dealing with products *)
   | <{ T1 × T2 }>False (* FILL IN HERE *)
As immediate consequences of this definition, we have that every element of every set R_T halts in a value and is closed with type T :
Lemma R_halts : {T} {t}, R T t halts t.
  destruct T; unfold R in H; destruct H as [_ [H _]]; assumption.

Lemma R_typable_empty : {T} {t}, R T t empty t \in T.
  destruct T; unfold R in H; destruct H as [H _]; assumption.

Now we proceed to show the main result, which is that every well-typed term of type T is an element of R_T. Together with R_halts, that will show that every well-typed term halts in a value.

Membership in R_T Is Invariant Under Reduction

We start with a preliminary lemma that shows a kind of strong preservation property, namely that membership in R_T is invariant under reduction. We will need this property in both directions, i.e., both to show that a term in R_T stays in R_T when it takes a forward step, and to show that any term that ends up in R_T after a step must have been in R_T to begin with.
First of all, an easy preliminary lemma. Note that in the forward direction the proof depends on the fact that our language is determinstic. This lemma might still be true for nondeterministic languages, but the proof would be harder!
Lemma step_preserves_halting :
   t t', (t --> t') (halts t halts t').
 intros t t' ST. unfold halts.
 - (* -> *)
  intros [t'' [STM V]].
  destruct STM.
   + exfalso; apply value__normal in V; eauto.
   + rewrite (step_deterministic _ _ _ ST H). z. split; assumption.
 - (* <- *)
  intros [t'0 [STM V]].
   t'0. split; eauto.

Now the main lemma, which comes in two parts, one for each direction. Each proceeds by induction on the structure of the type T. In fact, this is where we make fundamental use of the structure of types.
One requirement for staying in R_T is to stay in type T. In the forward direction, we get this from ordinary type Preservation.
Lemma step_preserves_R : T t t', (t --> t') R T t R T t'.
 induction T; intros t t' E Rt; unfold R; fold R; unfold R in Rt; fold R in Rt;
               destruct Rt as [typable_empty_t [halts_t RRt]].
  (* Bool *)
  split. eapply preservation; eauto.
  split. apply (step_preserves_halting _ _ E); eauto.
  (* Arrow *)
  split. eapply preservation; eauto.
  split. apply (step_preserves_halting _ _ E); eauto.
  eapply IHT2.
  apply ST_App1. apply E.
  apply RRt; auto.
  (* FILL IN HERE *) Admitted.

The generalization to multiple steps is trivial:
Lemma multistep_preserves_R : T t t',
  (t -->* t') R T t R T t'.
  intros T t t' STM; induction STM; intros.
  apply IHSTM. eapply step_preserves_R. apply H. assumption.

In the reverse direction, we must add the fact that t has type T before stepping as an additional hypothesis.
Lemma step_preserves_R' : T t t',
  empty t \in T (t --> t') R T t' R T t.
  (* FILL IN HERE *) Admitted.

Lemma multistep_preserves_R' : T t t',
  empty t \in T (t -->* t') R T t' R T t.
  intros T t t' HT STM.
  induction STM; intros.
    eapply step_preserves_R'. assumption. apply H. apply IHSTM.
    eapply preservation; eauto. auto.

Closed Instances of Terms of Type t Belong to R_T

Now we proceed to show that every term of type T belongs to R_T. Here, the induction will be on typing derivations (it would be surprising to see a proof about well-typed terms that did not somewhere involve induction on typing derivations!). The only technical difficulty here is in dealing with the abstraction case. Since we are arguing by induction, the demonstration that a term abs x T1 t2 belongs to R_(T1T2) should involve applying the induction hypothesis to show that t2 belongs to R_(T2). But R_(T2) is defined to be a set of closed terms, while t2 may contain x free, so this does not make sense.
This problem is resolved by using a standard trick to suitably generalize the induction hypothesis: instead of proving a statement involving a closed term, we generalize it to cover all closed instances of an open term t. Informally, the statement of the lemma will look like this:
If x1:T1,..xn:Tn t : T and v1,...,vn are values such that R T1 v1, R T2 v2, ..., R Tn vn, then R T ([x1:=v1][x2:=v2]...[xn:=vn]t).
The proof will proceed by induction on the typing derivation x1:T1,..xn:Tn t : T; the most interesting case will be the one for abstraction.

Multisubstitutions, Multi-Extensions, and Instantiations

However, before we can proceed to formalize the statement and proof of the lemma, we'll need to build some (rather tedious) machinery to deal with the fact that we are performing multiple substitutions on term t and multiple extensions of the typing context. In particular, we must be precise about the order in which the substitutions occur and how they act on each other. Often these details are simply elided in informal paper proofs, but of course Coq won't let us do that. Since here we are substituting closed terms, we don't need to worry about how one substitution might affect the term put in place by another. But we still do need to worry about the order of substitutions, because it is quite possible for the same identifier to appear multiple times among the x1,...xn with different associated vi and Ti.
To make everything precise, we will assume that environments are extended from left to right, and multiple substitutions are performed from right to left. To see that this is consistent, suppose we have an environment written as ...,y:bool,...,y:nat,... and a corresponding term substitution written as ...[y:=(tbool true)]...[y:=(const 3)]...t. Since environments are extended from left to right, the binding y:nat hides the binding y:bool; since substitutions are performed right to left, we do the substitution y:=(const 3) first, so that the substitution y:=(tbool true) has no effect. Substitution thus correctly preserves the type of the term.
With these points in mind, the following definitions should make sense.
A multisubstitution is the result of applying a list of substitutions, which we call an environment.
Definition env := list (string × tm).

Fixpoint msubst (ss:env) (t:tm) : tm :=
match ss with
| nilt
| ((x,s)::ss') ⇒ msubst ss' <{ [x:=s]t }>
We need similar machinery to talk about repeated extension of a typing context using a list of (identifier, type) pairs, which we call a type assignment.
Definition tass := list (string × ty).

Fixpoint mupdate (Gamma : context) (xts : tass) :=
  match xts with
  | nilGamma
  | ((x,v)::xts') ⇒ update (mupdate Gamma xts') x v
We will need some simple operations that work uniformly on environments and type assigments
Fixpoint lookup {X:Set} (k : string) (l : list (string × X))
              : option X :=
  match l with
    | nilNone
    | (j,x) :: l'
      if String.eqb j k then Some x else lookup k l'

Fixpoint drop {X:Set} (n:string) (nxs:list (string × X))
            : list (string × X) :=
  match nxs with
    | nilnil
    | ((n',x)::nxs') ⇒
        if String.eqb n' n then drop n nxs'
        else (n',x)::(drop n nxs')
An instantiation combines a type assignment and a value environment with the same domains, where corresponding elements are in R.
Inductive instantiation : tass env Prop :=
| V_nil :
    instantiation nil nil
| V_cons : x T v c e,
    value v R T v
    instantiation c e
    instantiation ((x,T)::c) ((x,v)::e).
We now proceed to prove various properties of these definitions.

More Substitution Facts

First we need some additional lemmas on (ordinary) substitution.
Lemma vacuous_substitution : t x,
     ¬ appears_free_in x t
      t', <{ [x:=t']t }> = t.
Proof with eauto.
  (* FILL IN HERE *) Admitted.

Lemma subst_closed: t,
     closed t
      x t', <{ [x:=t']t }> = t.
  intros. apply vacuous_substitution. apply H. Qed.

Lemma subst_not_afi : t x v,
    closed v ¬ appears_free_in x <{ [x:=v]t }>.
Proof with eauto. (* rather slow this way *)
  unfold closed, not.
  induction t; intros x v P A; simpl in A.
    - (* var *)
     destruct (eqb_spec x s)...
     inversion A; subst. auto.
    - (* app *)
     inversion A; subst...
    - (* abs *)
     destruct (eqb_spec x s)...
     + inversion A; subst...
     + inversion A; subst...
    - (* tru *)
     inversion A.
    - (* fls *)
     inversion A.
    - (* test *)
     inversion A; subst...
    - (* pair *)
     inversion A; subst...
    - (* fst *)
     inversion A; subst...
    - (* snd *)
     inversion A; subst...

Lemma duplicate_subst : t' x t v,
  closed v <{ [x:=t]([x:=v]t') }> = <{ [x:=v]t' }>.
  intros. eapply vacuous_substitution. apply subst_not_afi. assumption.

Lemma swap_subst : t x x1 v v1,
    x x1
    closed v closed v1
    <{ [x1:=v1]([x:=v]t) }> = <{ [x:=v]([x1:=v1]t) }>.
Proof with eauto.
 induction t; intros; simpl.
  - (* var *)
   destruct (eqb_spec x s); destruct (eqb_spec x1 s).
   + subst. exfalso...
   + subst. simpl. rewrite String.eqb_refl. apply subst_closed...
   + subst. simpl. rewrite String.eqb_refl. rewrite subst_closed...
   + simpl. rewrite false_eqb_string... rewrite false_eqb_string...
  (* FILL IN HERE *) Admitted.

Properties of Multi-Substitutions

Lemma msubst_closed: t, closed t ss, msubst ss t = t.
  induction ss.
    destruct a. simpl. rewrite subst_closed; assumption.

Closed environments are those that contain only closed terms.
Fixpoint closed_env (env:env) :=
  match env with
  | nilTrue
  | (x,t)::env'closed t closed_env env'
Next come a series of lemmas charcterizing how msubst of closed terms distributes over subst and over each term form
Lemma subst_msubst: env x v t, closed v closed_env env
    msubst env <{ [x:=v]t }> = <{ [x:=v] { msubst (drop x env) t } }> .
  induction env0; intros; auto.
  destruct a. simpl.
  inversion H0.
  destruct (eqb_spec s x).
  - subst. rewrite duplicate_subst; auto.
  - simpl. rewrite swap_subst; eauto.

Lemma msubst_var: ss x, closed_env ss
   msubst ss (tm_var x) =
   match lookup x ss with
   | Some tt
   | Nonetm_var x
  induction ss; intros.
    destruct a.
     simpl. destruct (String.eqb s x).
      apply msubst_closed. inversion H; auto.
      apply IHss. inversion H; auto.

Lemma msubst_abs: ss x T t,
  msubst ss <{ \ x : T, t }> = <{ \x : T, {msubst (drop x ss) t} }>.
  induction ss; intros.
    destruct a.
      simpl. destruct (String.eqb s x); simpl; auto.

Lemma msubst_app : ss t1 t2,
    msubst ss <{ t1 t2 }> = <{ {msubst ss t1} ({msubst ss t2}) }>.
 induction ss; intros.
   destruct a.
    simpl. rewrite <- IHss. auto.

You'll need similar functions for the other term constructors.

Properties of Multi-Extensions

We need to connect the behavior of type assignments with that of their corresponding contexts.
Lemma mupdate_lookup : (c : tass) (x:string),
    lookup x c = (mupdate empty c) x.
  induction c; intros.
    destruct a. unfold lookup, mupdate, update, t_update. destruct (String.eqb s x); auto.

Lemma mupdate_drop : (c: tass) Gamma x x',
      mupdate Gamma (drop x c) x'
    = if String.eqb x x' then Gamma x' else mupdate Gamma c x'.
  induction c; intros.
  - destruct (eqb_spec x x'); auto.
  - destruct a. simpl.
    destruct (eqb_spec s x).
    + subst. rewrite IHc.
      unfold update, t_update. destruct (eqb_spec x x'); auto.
    + simpl. unfold update, t_update. destruct (eqb_spec s x'); auto.
      subst. rewrite false_eqb_string; congruence.

Properties of Instantiations

These are strightforward.
Lemma instantiation_domains_match: {c} {e},
    instantiation c e
     {x} {T},
      lookup x c = Some T t, lookup x e = Some t.
  intros c e V. induction V; intros x0 T0 C.
    simpl in ×.
    destruct (String.eqb x x0); eauto.

Lemma instantiation_env_closed : c e,
  instantiation c e closed_env e.
  intros c e V; induction V; intros.
    unfold closed_env. fold closed_env.
    split; [|assumption].
    eapply typable_empty__closed. eapply R_typable_empty. eauto.

Lemma instantiation_R : c e,
    instantiation c e
     x t T,
      lookup x c = Some T
      lookup x e = Some t R T t.
  intros c e V. induction V; intros x' t' T' G E.
    unfold lookup in ×. destruct (String.eqb x x').
      inversion G; inversion E; subst. auto.

Lemma instantiation_drop : c env,
    instantiation c env
     x, instantiation (drop x c) (drop x env).
  intros c e V. induction V.
    intros. simpl. constructor.
    intros. unfold drop.
    destruct (String.eqb x x0); auto. constructor; eauto.

Congruence Lemmas on Multistep

We'll need just a few of these; add them as the demand arises.
Lemma multistep_App2 : v t t',
  value v (t -->* t') <{ v t }> -->* <{ v t' }>.
  intros v t t' V STM. induction STM.
   apply multi_refl.
   eapply multi_step.
     apply ST_App2; eauto. auto.


The R Lemma

We can finally put everything together.
The key lemma about preservation of typing under substitution can be lifted to multi-substitutions:
Lemma msubst_preserves_typing : c e,
     instantiation c e
      Gamma t S, (mupdate Gamma c) t \in S
     Gamma { (msubst e t) } \in S.
    intros c e H. induction H; intros.
    simpl in H. simpl. auto.
    simpl in H2. simpl.
    apply IHinstantiation.
    eapply substitution_preserves_typing; eauto.
    apply (R_typable_empty H0).

And at long last, the main lemma.
Lemma msubst_R : c env t T,
    (mupdate empty c) t \in T
    instantiation c env
    R T (msubst env t).
  intros c env0 t T HT V.
  generalize dependent env0.
  (* We need to generalize the hypothesis a bit before setting up the induction. *)
  remember (mupdate empty c) as Gamma.
  assert ( x, Gamma x = lookup x c).
    intros. rewrite HeqGamma. rewrite mupdate_lookup. auto.
  clear HeqGamma.
  generalize dependent c.
  induction HT; intros.

  - (* T_Var *)
   rewrite H0 in H. destruct (instantiation_domains_match V H) as [t P].
   eapply instantiation_R; eauto.
   rewrite msubst_var. rewrite P. auto. eapply instantiation_env_closed; eauto.

  - (* T_Abs *)
    rewrite msubst_abs.
    (* We'll need variants of the following fact several times, so its simplest to
       establish it just once. *)

    assert (WT : empty \x : T2, {msubst (drop x env0) t1 } \in (T2 T1) ).
    { eapply T_Abs. eapply msubst_preserves_typing.
      { eapply instantiation_drop; eauto. }
      eapply context_invariance.
      { apply HT. }
      unfold update, t_update. rewrite mupdate_drop. destruct (eqb_spec x x0).
      + auto.
      + rewrite H.
        clear - c n. induction c.
        simpl. rewrite false_eqb_string; auto.
        simpl. destruct a. unfold update, t_update.
        destruct (String.eqb s x0); auto. }
    unfold R. fold R. split.
     split. apply value_halts. apply v_abs.
     destruct (R_halts H0) as [v [P Q]].
     pose proof (multistep_preserves_R _ _ _ P H0).
     apply multistep_preserves_R' with (msubst ((x,v)::env0) t1).
       eapply T_App. eauto.
       apply R_typable_empty; auto.
       eapply multi_trans. eapply multistep_App2; eauto.
       eapply multi_R.
       simpl. rewrite subst_msubst.
       eapply ST_AppAbs; eauto.
       eapply typable_empty__closed.
       apply (R_typable_empty H1).
       eapply instantiation_env_closed; eauto.
       eapply (IHHT ((x,T2)::c)).
          intros. unfold update, t_update, lookup. destruct (String.eqb x x0); auto.
       constructor; auto.

  - (* T_App *)
    rewrite msubst_app.
    destruct (IHHT1 c H env0 V) as [_ [_ P1]].
    pose proof (IHHT2 c H env0 V) as P2. fold R in P1. auto.

  (* FILL IN HERE *) Admitted.

Normalization Theorem

And the final theorem:
Theorem normalization : t T, empty t \in T halts t.
  replace t with (msubst nil t) by reflexivity.
  apply (@R_halts T).
  apply (msubst_R nil); eauto.
  eapply V_nil.

(* 2022-09-20 16:43 *)