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From iris.program_logic Require Export ectx_language ectxi_language.
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From iris.prelude Require Export strings.
From iris.prelude Require Import gmap.
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Module heap_lang.
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Open Scope Z_scope.

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(** Expressions and vals. *)
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Definition loc := positive. (* Really, any countable type. *)
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Inductive base_lit : Set :=
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  | LitInt (n : Z) | LitBool (b : bool) | LitUnit.
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Inductive un_op : Set :=
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  | NegOp | MinusUnOp.
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Inductive bin_op : Set :=
  | PlusOp | MinusOp | LeOp | LtOp | EqOp.

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Inductive binder := BAnon | BNamed : string  binder.
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Delimit Scope binder_scope with bind.
Bind Scope binder_scope with binder.
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Definition cons_binder (mx : binder) (X : list string) : list string :=
  match mx with BAnon => X | BNamed x => x :: X end.
Infix ":b:" := cons_binder (at level 60, right associativity).
Instance binder_dec_eq (x1 x2 : binder) : Decision (x1 = x2).
Proof. solve_decision. Defined.

Instance set_unfold_cons_binder x mx X P :
  SetUnfold (x  X) P  SetUnfold (x  mx :b: X) (BNamed x = mx  P).
Proof.
  constructor. rewrite -(set_unfold (x  X) P).
  destruct mx; rewrite /= ?elem_of_cons; naive_solver.
Qed.

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(** A typeclass for whether a variable is bound in a given
   context. Making this a typeclass means we can use tpeclass search
   to program solving these constraints, so this becomes extensible.
   Also, since typeclass search runs *after* unification, Coq has already
   inferred the X for us; if we were to go for embedded proof terms ot
   tactics, Coq would do things in the wrong order. *)
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Class VarBound (x : string) (X : list string) :=
  var_bound : bool_decide (x  X).
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(* There is no need to restrict this hint to terms without evars, [vm_compute]
will fail in case evars are arround. *)
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Hint Extern 0 (VarBound _ _) => vm_compute; exact I : typeclass_instances. 

Instance var_bound_proof_irrel x X : ProofIrrel (VarBound x X).
Proof. rewrite /VarBound. apply _. Qed.
Instance set_unfold_var_bound x X P :
  SetUnfold (x  X) P  SetUnfold (VarBound x X) P.
Proof.
  constructor. by rewrite /VarBound bool_decide_spec (set_unfold (x  X) P).
Qed.
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Inductive expr (X : list string) :=
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  (* Base lambda calculus *)
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      (* Var is the only place where the terms contain a proof. The fact that they
       contain a proof at all is suboptimal, since this means two seeminlgy
       convertible terms could differ in their proofs. However, this also has
       some advantages:
       * We can make the [X] an index, so we can do non-dependent match.
       * In expr_weaken, we can push the proof all the way into Var, making
         sure that proofs never block computation. *)
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  | Var (x : string) `{VarBound x X}
  | Rec (f x : binder) (e : expr (f :b: x :b: X))
  | App (e1 e2 : expr X)
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  (* Base types and their operations *)
  | Lit (l : base_lit)
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  | UnOp (op : un_op) (e : expr X)
  | BinOp (op : bin_op) (e1 e2 : expr X)
  | If (e0 e1 e2 : expr X)
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  (* Products *)
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  | Pair (e1 e2 : expr X)
  | Fst (e : expr X)
  | Snd (e : expr X)
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  (* Sums *)
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  | InjL (e : expr X)
  | InjR (e : expr X)
  | Case (e0 : expr X) (e1 : expr X) (e2 : expr X)
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  (* Concurrency *)
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  | Fork (e : expr X)
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  (* Heap *)
  | Loc (l : loc)
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  | Alloc (e : expr X)
  | Load (e : expr X)
  | Store (e1 : expr X) (e2 : expr X)
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  | CAS (e0 : expr X) (e1 : expr X) (e2 : expr X).
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Bind Scope expr_scope with expr.
Delimit Scope expr_scope with E.
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Arguments Var {_} _ {_}.
Arguments Rec {_} _ _ _%E.
Arguments App {_} _%E _%E.
Arguments Lit {_} _.
Arguments UnOp {_} _ _%E.
Arguments BinOp {_} _ _%E _%E.
Arguments If {_} _%E _%E _%E.
Arguments Pair {_} _%E _%E.
Arguments Fst {_} _%E.
Arguments Snd {_} _%E.
Arguments InjL {_} _%E.
Arguments InjR {_} _%E.
Arguments Case {_} _%E _%E _%E.
Arguments Fork {_} _%E.
Arguments Loc {_} _.
Arguments Alloc {_} _%E.
Arguments Load {_} _%E.
Arguments Store {_} _%E _%E.
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Arguments CAS {_} _%E _%E _%E.
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Inductive val :=
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  | RecV (f x : binder) (e : expr (f :b: x :b: []))
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  | LitV (l : base_lit)
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  | PairV (v1 v2 : val)
  | InjLV (v : val)
  | InjRV (v : val)
  | LocV (l : loc).
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Bind Scope val_scope with val.
Delimit Scope val_scope with V.
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Arguments PairV _%V _%V.
Arguments InjLV _%V.
Arguments InjRV _%V.
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Definition signal : val := RecV BAnon (BNamed "x") (Store (Var "x") (Lit (LitInt 1))).
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Fixpoint of_val (v : val) : expr [] :=
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  match v with
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  | RecV f x e => Rec f x e
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  | LitV l => Lit l
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  | PairV v1 v2 => Pair (of_val v1) (of_val v2)
  | InjLV v => InjL (of_val v)
  | InjRV v => InjR (of_val v)
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  | LocV l => Loc l
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  end.
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Fixpoint to_val (e : expr []) : option val :=
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  match e with
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  | Rec f x e => Some (RecV f x e)
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  | Lit l => Some (LitV l)
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  | Pair e1 e2 => v1  to_val e1; v2  to_val e2; Some (PairV v1 v2)
  | InjL e => InjLV <$> to_val e
  | InjR e => InjRV <$> to_val e
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  | Loc l => Some (LocV l)
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  | _ => None
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  end.

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(** The state: heaps of vals. *)
Definition state := gmap loc val.
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(** Evaluation contexts *)
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Inductive ectx_item :=
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  | AppLCtx (e2 : expr [])
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  | AppRCtx (v1 : val)
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  | UnOpCtx (op : un_op)
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  | BinOpLCtx (op : bin_op) (e2 : expr [])
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  | BinOpRCtx (op : bin_op) (v1 : val)
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  | IfCtx (e1 e2 : expr [])
  | PairLCtx (e2 : expr [])
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  | PairRCtx (v1 : val)
  | FstCtx
  | SndCtx
  | InjLCtx
  | InjRCtx
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  | CaseCtx (e1 : expr []) (e2 : expr [])
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  | AllocCtx
  | LoadCtx
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  | StoreLCtx (e2 : expr [])
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  | StoreRCtx (v1 : val)
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  | CasLCtx (e1 : expr [])  (e2 : expr [])
  | CasMCtx (v0 : val) (e2 : expr [])
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  | CasRCtx (v0 : val) (v1 : val).
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Definition fill_item (Ki : ectx_item) (e : expr []) : expr [] :=
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  match Ki with
  | AppLCtx e2 => App e e2
  | AppRCtx v1 => App (of_val v1) e
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  | UnOpCtx op => UnOp op e
  | BinOpLCtx op e2 => BinOp op e e2
  | BinOpRCtx op v1 => BinOp op (of_val v1) e
  | IfCtx e1 e2 => If e e1 e2
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  | PairLCtx e2 => Pair e e2
  | PairRCtx v1 => Pair (of_val v1) e
  | FstCtx => Fst e
  | SndCtx => Snd e
  | InjLCtx => InjL e
  | InjRCtx => InjR e
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  | CaseCtx e1 e2 => Case e e1 e2
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  | AllocCtx => Alloc e
  | LoadCtx => Load e
  | StoreLCtx e2 => Store e e2
  | StoreRCtx v1 => Store (of_val v1) e
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  | CasLCtx e1 e2 => CAS e e1 e2
  | CasMCtx v0 e2 => CAS (of_val v0) e e2
  | CasRCtx v0 v1 => CAS (of_val v0) (of_val v1) e
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  end.

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(** Substitution *)
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(** We have [subst' e BAnon v = e] to deal with anonymous binders *)
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Lemma wexpr_rec_prf {X Y} (H : X `included` Y) {f x} :
  f :b: x :b: X `included` f :b: x :b: Y.
Proof. set_solver. Qed.

Program Fixpoint wexpr {X Y} (H : X `included` Y) (e : expr X) : expr Y :=
  match e return expr Y with
  | Var x _ => @Var _ x _
  | Rec f x e => Rec f x (wexpr (wexpr_rec_prf H) e)
  | App e1 e2 => App (wexpr H e1) (wexpr H e2)
  | Lit l => Lit l
  | UnOp op e => UnOp op (wexpr H e)
  | BinOp op e1 e2 => BinOp op (wexpr H e1) (wexpr H e2)
  | If e0 e1 e2 => If (wexpr H e0) (wexpr H e1) (wexpr H e2)
  | Pair e1 e2 => Pair (wexpr H e1) (wexpr H e2)
  | Fst e => Fst (wexpr H e)
  | Snd e => Snd (wexpr H e)
  | InjL e => InjL (wexpr H e)
  | InjR e => InjR (wexpr H e)
  | Case e0 e1 e2 => Case (wexpr H e0) (wexpr H e1) (wexpr H e2)
  | Fork e => Fork (wexpr H e)
  | Loc l => Loc l
  | Alloc e => Alloc (wexpr H e)
  | Load  e => Load (wexpr H e)
  | Store e1 e2 => Store (wexpr H e1) (wexpr H e2)
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  | CAS e0 e1 e2 => CAS (wexpr H e0) (wexpr H e1) (wexpr H e2)
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  end.
Solve Obligations with set_solver.

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Definition wexpr' {X} (e : expr []) : expr X := wexpr (included_nil _) e.
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Definition of_val' {X} (v : val) : expr X := wexpr (included_nil _) (of_val v).

Lemma wsubst_rec_true_prf {X Y x} (H : X `included` x :: Y) {f y}
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    (Hfy : BNamed x  f  BNamed x  y) :
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  f :b: y :b: X `included` x :: f :b: y :b: Y.
Proof. set_solver. Qed.
Lemma wsubst_rec_false_prf {X Y x} (H : X `included` x :: Y) {f y}
    (Hfy : ¬(BNamed x  f  BNamed x  y)) :
  f :b: y :b: X `included` f :b: y :b: Y.
Proof. move: Hfy=>/not_and_l [/dec_stable|/dec_stable]; set_solver. Qed.

Program Fixpoint wsubst {X Y} (x : string) (es : expr [])
    (H : X `included` x :: Y) (e : expr X)  : expr Y :=
  match e return expr Y with
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  | Var y _ => if decide (x = y) then wexpr' es else @Var _ y _
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  | Rec f y e =>
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     Rec f y $ match decide (BNamed x  f  BNamed x  y) return _ with
               | left Hfy => wsubst x es (wsubst_rec_true_prf H Hfy) e
               | right Hfy => wexpr (wsubst_rec_false_prf H Hfy) e
               end
  | App e1 e2 => App (wsubst x es H e1) (wsubst x es H e2)
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  | Lit l => Lit l
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  | UnOp op e => UnOp op (wsubst x es H e)
  | BinOp op e1 e2 => BinOp op (wsubst x es H e1) (wsubst x es H e2)
  | If e0 e1 e2 => If (wsubst x es H e0) (wsubst x es H e1) (wsubst x es H e2)
  | Pair e1 e2 => Pair (wsubst x es H e1) (wsubst x es H e2)
  | Fst e => Fst (wsubst x es H e)
  | Snd e => Snd (wsubst x es H e)
  | InjL e => InjL (wsubst x es H e)
  | InjR e => InjR (wsubst x es H e)
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  | Case e0 e1 e2 =>
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     Case (wsubst x es H e0) (wsubst x es H e1) (wsubst x es H e2)
  | Fork e => Fork (wsubst x es H e)
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  | Loc l => Loc l
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  | Alloc e => Alloc (wsubst x es H e)
  | Load e => Load (wsubst x es H e)
  | Store e1 e2 => Store (wsubst x es H e1) (wsubst x es H e2)
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  | CAS e0 e1 e2 => CAS (wsubst x es H e0) (wsubst x es H e1) (wsubst x es H e2)
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  end.
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Solve Obligations with set_solver.

Definition subst {X} (x : string) (es : expr []) (e : expr (x :: X)) : expr X :=
  wsubst x es (λ z, id) e.
Definition subst' {X} (mx : binder) (es : expr []) : expr (mx :b: X)  expr X :=
  match mx with BNamed x => subst x es | BAnon => id end.
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(** The stepping relation *)
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Definition un_op_eval (op : un_op) (l : base_lit) : option base_lit :=
  match op, l with
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  | NegOp, LitBool b => Some (LitBool (negb b))
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  | MinusUnOp, LitInt n => Some (LitInt (- n))
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  | _, _ => None
  end.

Definition bin_op_eval (op : bin_op) (l1 l2 : base_lit) : option base_lit :=
  match op, l1, l2 with
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  | PlusOp, LitInt n1, LitInt n2 => Some $ LitInt (n1 + n2)
  | MinusOp, LitInt n1, LitInt n2 => Some $ LitInt (n1 - n2)
  | LeOp, LitInt n1, LitInt n2 => Some $ LitBool $ bool_decide (n1  n2)
  | LtOp, LitInt n1, LitInt n2 => Some $ LitBool $ bool_decide (n1 < n2)
  | EqOp, LitInt n1, LitInt n2 => Some $ LitBool $ bool_decide (n1 = n2)
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  | _, _, _ => None
  end.

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Inductive head_step : expr []  state  expr []  state  option (expr [])  Prop :=
  | BetaS f x e1 e2 v2 e' σ :
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     to_val e2 = Some v2 
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     e' = subst' x (of_val v2) (subst' f (Rec f x e1) e1) 
     head_step (App (Rec f x e1) e2) σ e' σ None
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  | UnOpS op l l' σ :
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     un_op_eval op l = Some l'  
     head_step (UnOp op (Lit l)) σ (Lit l') σ None
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  | BinOpS op l1 l2 l' σ :
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     bin_op_eval op l1 l2 = Some l'  
     head_step (BinOp op (Lit l1) (Lit l2)) σ (Lit l') σ None
  | IfTrueS e1 e2 σ :
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     head_step (If (Lit $ LitBool true) e1 e2) σ e1 σ None
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  | IfFalseS e1 e2 σ :
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     head_step (If (Lit $ LitBool false) e1 e2) σ e2 σ None
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  | FstS e1 v1 e2 v2 σ :
     to_val e1 = Some v1  to_val e2 = Some v2 
     head_step (Fst (Pair e1 e2)) σ e1 σ None
  | SndS e1 v1 e2 v2 σ :
     to_val e1 = Some v1  to_val e2 = Some v2 
     head_step (Snd (Pair e1 e2)) σ e2 σ None
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  | CaseLS e0 v0 e1 e2 σ :
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     to_val e0 = Some v0 
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     head_step (Case (InjL e0) e1 e2) σ (App e1 e0) σ None
  | CaseRS e0 v0 e1 e2 σ :
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     to_val e0 = Some v0 
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     head_step (Case (InjR e0) e1 e2) σ (App e2 e0) σ None
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  | ForkS e σ:
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     head_step (Fork e) σ (Lit LitUnit) σ (Some e)
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  | AllocS e v σ l :
     to_val e = Some v  σ !! l = None 
     head_step (Alloc e) σ (Loc l) (<[l:=v]>σ) None
  | LoadS l v σ :
     σ !! l = Some v 
     head_step (Load (Loc l)) σ (of_val v) σ None
  | StoreS l e v σ :
     to_val e = Some v  is_Some (σ !! l) 
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     head_step (Store (Loc l) e) σ (Lit LitUnit) (<[l:=v]>σ) None
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  | CasFailS l e1 v1 e2 v2 vl σ :
     to_val e1 = Some v1  to_val e2 = Some v2 
     σ !! l = Some vl  vl  v1 
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     head_step (CAS (Loc l) e1 e2) σ (Lit $ LitBool false) σ None
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  | CasSucS l e1 v1 e2 v2 σ :
     to_val e1 = Some v1  to_val e2 = Some v2 
     σ !! l = Some v1 
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     head_step (CAS (Loc l) e1 e2) σ (Lit $ LitBool true) (<[l:=v2]>σ) None.
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(** Atomic expressions *)
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Definition atomic (e: expr []) : bool :=
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  match e with
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  | Alloc e => bool_decide (is_Some (to_val e))
  | Load e => bool_decide (is_Some (to_val e))
  | Store e1 e2 => bool_decide (is_Some (to_val e1)  is_Some (to_val e2))
  | CAS e0 e1 e2 =>
    bool_decide (is_Some (to_val e0)  is_Some (to_val e1)  is_Some (to_val e2))
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  (* Make "skip" atomic *)
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  | App (Rec _ _ (Lit _)) (Lit _) => true
  | _ => false
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  end.
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(** Substitution *)
Lemma var_proof_irrel X x H1 H2 : @Var X x H1 = @Var X x H2.
Proof. f_equal. by apply (proof_irrel _). Qed.

Lemma wexpr_id X (H : X `included` X) e : wexpr H e = e.
Proof. induction e; f_equal/=; auto. by apply (proof_irrel _). Qed.
Lemma wexpr_proof_irrel X Y (H1 H2 : X `included` Y) e : wexpr H1 e = wexpr H2 e.
Proof.
  revert Y H1 H2; induction e; simpl; auto using var_proof_irrel with f_equal.
Qed.
Lemma wexpr_wexpr X Y Z (H1 : X `included` Y) (H2 : Y `included` Z) H3 e :
  wexpr H2 (wexpr H1 e) = wexpr H3 e.
Proof.
  revert Y Z H1 H2 H3.
  induction e; simpl; auto using var_proof_irrel with f_equal.
Qed.
Lemma wexpr_wexpr' X Y Z (H1 : X `included` Y) (H2 : Y `included` Z) e :
  wexpr H2 (wexpr H1 e) = wexpr (transitivity H1 H2) e.
Proof. apply wexpr_wexpr. Qed.

Lemma wsubst_proof_irrel X Y x es (H1 H2 : X `included` x :: Y) e :
  wsubst x es H1 e = wsubst x es H2 e.
Proof.
  revert Y H1 H2; induction e; simpl; intros; repeat case_decide;
    auto using var_proof_irrel, wexpr_proof_irrel with f_equal.
Qed.
Lemma wexpr_wsubst X Y Z x es (H1: X `included` x::Y) (H2: Y `included` Z) H3 e:
  wexpr H2 (wsubst x es H1 e) = wsubst x es H3 e.
Proof.
  revert Y Z H1 H2 H3.
  induction e; intros; repeat (case_decide || simplify_eq/=);
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    unfold wexpr'; auto using var_proof_irrel, wexpr_wexpr with f_equal.
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Qed.
Lemma wsubst_wexpr X Y Z x es (H1: X `included` Y) (H2: Y `included` x::Z) H3 e:
  wsubst x es H2 (wexpr H1 e) = wsubst x es H3 e.
Proof.
  revert Y Z H1 H2 H3.
  induction e; intros; repeat (case_decide || simplify_eq/=);
    auto using var_proof_irrel, wexpr_wexpr with f_equal.
Qed.
Lemma wsubst_wexpr' X Y Z x es (H1: X `included` Y) (H2: Y `included` x::Z) e:
  wsubst x es H2 (wexpr H1 e) = wsubst x es (transitivity H1 H2) e.
Proof. apply wsubst_wexpr. Qed.

Lemma wsubst_closed X Y x es (H1 : X `included` x :: Y) H2 (e : expr X) :
  x  X  wsubst x es H1 e = wexpr H2 e.
Proof.
  revert Y H1 H2.
  induction e; intros; repeat (case_decide || simplify_eq/=);
    auto using var_proof_irrel, wexpr_proof_irrel with f_equal set_solver.
  exfalso; set_solver.
Qed.
Lemma wsubst_closed_nil x es H (e : expr []) : wsubst x es H e = e.
Proof.
  rewrite -{2}(wexpr_id _ (reflexivity []) e).
  apply wsubst_closed, not_elem_of_nil.
Qed.

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(** Basic properties about the language *)
Lemma to_of_val v : to_val (of_val v) = Some v.
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Proof. by induction v; simplify_option_eq. Qed.
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Lemma of_to_val e v : to_val e = Some v  of_val v = e.
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Proof.
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  revert e v. cut ( X (e : expr X) (H : X = ) v,
    to_val (eq_rect _ expr e _ H) = Some v  of_val v = eq_rect _ expr e _ H).
  { intros help e v. apply (help  e eq_refl). }
  intros X e; induction e; intros HX ??; simplify_option_eq;
    repeat match goal with
    | IH :  _ :  = , _ |- _ => specialize (IH eq_refl); simpl in IH
    end; auto with f_equal.
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Qed.
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Instance of_val_inj : Inj (=) (=) of_val.
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Proof. by intros ?? Hv; apply (inj Some); rewrite -!to_of_val Hv. Qed.
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Instance fill_item_inj Ki : Inj (=) (=) (fill_item Ki).
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Proof. destruct Ki; intros ???; simplify_eq/=; auto with f_equal. Qed.
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Lemma fill_item_val Ki e :
  is_Some (to_val (fill_item Ki e))  is_Some (to_val e).
Proof. intros [v ?]. destruct Ki; simplify_option_eq; eauto. Qed.
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Lemma val_stuck e1 σ1 e2 σ2 ef :
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  head_step e1 σ1 e2 σ2 ef  to_val e1 = None.
Proof. destruct 1; naive_solver. Qed.
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Lemma atomic_not_val e : atomic e  to_val e = None.
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Proof. by destruct e. Qed.
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Lemma atomic_fill_item Ki e : atomic (fill_item Ki e)  is_Some (to_val e).
Proof.
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  intros. destruct Ki; simplify_eq/=; destruct_and?;
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    repeat (case_match || contradiction); eauto.
Qed.

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Lemma atomic_step e1 σ1 e2 σ2 ef :
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  atomic e1  head_step e1 σ1 e2 σ2 ef  is_Some (to_val e2).
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Proof.
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  destruct 2; simpl; rewrite ?to_of_val; try by eauto. subst.
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  unfold subst'; repeat (case_match || contradiction || simplify_eq/=); eauto.
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Qed.
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Lemma head_ctx_step_val Ki e σ1 e2 σ2 ef :
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  head_step (fill_item Ki e) σ1 e2 σ2 ef  is_Some (to_val e).
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Proof. destruct Ki; inversion_clear 1; simplify_option_eq; eauto. Qed.
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Lemma fill_item_no_val_inj Ki1 Ki2 e1 e2 :
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  to_val e1 = None  to_val e2 = None 
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  fill_item Ki1 e1 = fill_item Ki2 e2  Ki1 = Ki2.
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Proof.
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  destruct Ki1, Ki2; intros; try discriminate; simplify_eq/=;
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    repeat match goal with
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    | H : to_val (of_val _) = None |- _ => by rewrite to_of_val in H
    end; auto.
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Qed.
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Lemma alloc_fresh e v σ :
  let l := fresh (dom _ σ) in
  to_val e = Some v  head_step (Alloc e) σ (Loc l) (<[l:=v]>σ) None.
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Proof. by intros; apply AllocS, (not_elem_of_dom (D:=gset _)), is_fresh. Qed.
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(** Equality and other typeclass stuff *)
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Instance base_lit_dec_eq (l1 l2 : base_lit) : Decision (l1 = l2).
Proof. solve_decision. Defined.
Instance un_op_dec_eq (op1 op2 : un_op) : Decision (op1 = op2).
Proof. solve_decision. Defined.
Instance bin_op_dec_eq (op1 op2 : bin_op) : Decision (op1 = op2).
Proof. solve_decision. Defined.
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Fixpoint expr_beq {X Y} (e : expr X) (e' : expr Y) : bool :=
  match e, e' with
  | Var x _, Var x' _ => bool_decide (x = x')
  | Rec f x e, Rec f' x' e' =>
     bool_decide (f = f') && bool_decide (x = x') && expr_beq e e'
  | App e1 e2, App e1' e2' | Pair e1 e2, Pair e1' e2' |
    Store e1 e2, Store e1' e2' => expr_beq e1 e1' && expr_beq e2 e2'
  | Lit l, Lit l' => bool_decide (l = l')
  | UnOp op e, UnOp op' e' => bool_decide (op = op') && expr_beq e e'
  | BinOp op e1 e2, BinOp op' e1' e2' =>
     bool_decide (op = op') && expr_beq e1 e1' && expr_beq e2 e2'
  | If e0 e1 e2, If e0' e1' e2' | Case e0 e1 e2, Case e0' e1' e2' |
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    CAS e0 e1 e2, CAS e0' e1' e2' =>
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     expr_beq e0 e0' && expr_beq e1 e1' && expr_beq e2 e2'
  | Fst e, Fst e' | Snd e, Snd e' | InjL e, InjL e' | InjR e, InjR e' |
    Fork e, Fork e' | Alloc e, Alloc e' | Load e, Load e' => expr_beq e e'
  | Loc l, Loc l' => bool_decide (l = l')
  | _, _ => false
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  end.
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Lemma expr_beq_correct {X} (e1 e2 : expr X) : expr_beq e1 e2  e1 = e2.
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Proof.
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  split.
  * revert e2; induction e1; intros [] * ?; simpl in *;
      destruct_and?; subst; repeat f_equal/=; auto; try apply proof_irrel.
  * intros ->. induction e2; naive_solver.
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Qed.
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Instance expr_dec_eq {X} (e1 e2 : expr X) : Decision (e1 = e2).
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Proof.
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 refine (cast_if (decide (expr_beq e1 e2))); by rewrite -expr_beq_correct.
Defined.
Instance val_dec_eq (v1 v2 : val) : Decision (v1 = v2).
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Proof.
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 refine (cast_if (decide (of_val v1 = of_val v2))); abstract naive_solver.
Defined.
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Instance expr_inhabited X : Inhabited (expr X) := populate (Lit LitUnit).
Instance val_inhabited : Inhabited val := populate (LitV LitUnit).
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End heap_lang.

(** Language *)
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Program Instance heap_ectxi_lang :
  EctxiLanguage
    (heap_lang.expr []) heap_lang.val heap_lang.ectx_item heap_lang.state := {|
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  of_val := heap_lang.of_val; to_val := heap_lang.to_val;
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  fill_item := heap_lang.fill_item; 
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  atomic := heap_lang.atomic; head_step := heap_lang.head_step;
|}.
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Solve Obligations with eauto using heap_lang.to_of_val, heap_lang.of_to_val,
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  heap_lang.val_stuck, heap_lang.atomic_not_val, heap_lang.atomic_step,
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  heap_lang.fill_item_val, heap_lang.atomic_fill_item,
  heap_lang.fill_item_no_val_inj, heap_lang.head_ctx_step_val.
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Canonical Structure heap_lang := ectx_lang (heap_lang.expr []).
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(* Prefer heap_lang names over ectx_language names. *)
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Export heap_lang.