HoareListHoare Logic with Lists

Require Export SfLib.

Imp Programs with Lists

There are only so many numeric functions with interesting properties that have simple proofs. (Of course, there are lots of interesting functions on numbers and they have many interesting properties — this is the whole field of number theory! — but proving these properties often requires developing a lot of supporting lemmas.) In order to able to write a few more programs to reason about, we introduce here an extended version of Imp where variables can range over both numbers and lists of numbers. The basic operations are extended to also include taking the head and tail of lists, and testing lists for nonemptyness.

To do this, we only need to change the definitions of state, aexp, aeval, bexp, and beval. The definitions of com and ceval can be reused verbatim, although we need to copy-and-paste them in the context of the new definitions.

We start by repeating some material from chapter Imp.

Repeated Definitions

Inductive id : Type :=
  Id : nat → id.

Definition beq_id id1 id2 :=
  match (id1, id2) with
    (Id n1, Id n2) => beq_nat n1 n2
  end.

Theorem beq_id_refl : ∀i,
  true = beq_id i i.
Proof.
  intros. destruct i.
  apply beq_nat_refl. Qed.

Theorem beq_id_eq : ∀i1 i2,
  true = beq_id i1 i2 → i1 = i2.
Proof.
  intros i1 i2 H.
  destruct i1. destruct i2.
  apply beq_nat_eq in H. subst.
  reflexivity. Qed.

Theorem beq_id_false_not_eq : ∀i1 i2,
  beq_id i1 i2 = false → i1 <> i2.
Proof.
  intros i1 i2 H.
  destruct i1. destruct i2.
  apply beq_nat_false in H.
  intros C. apply H. inversion C. reflexivity. Qed.

Theorem not_eq_beq_id_false : ∀i1 i2,
  i1 <> i2 → beq_id i1 i2 = false.
Proof.
  intros i1 i2 H.
  destruct i1. destruct i2.
  assert (n <> n0).
    intros C. subst. apply H. reflexivity.
  apply not_eq_beq_false. assumption. Qed.

Definition X : id := Id 0.
Definition Y : id := Id 1.
Definition Z : id := Id 2.

Extensions

Now we come to the key changes.

Rather than evaluating to a nat, an aexp in our new language will evaluate to a value — an element of type val — which can be either a nat or a list of nats.

Similarly, states will now map identifiers to vals rather than nats, so that we can store lists in mutable variables.

Inductive val : Type :=
| VNat : nat → val
| VList : list nat → val.

Definition state := id → val.

Definition empty_state : state := fun _ => VNat 0.
Definition update (st : state) (X:id) (v : val) : state :=
fun X' => if beq_id X X' then v else st X'.

Imp does not have a static type system, so nothing prevents the programmer from e.g. adding two lists or taking the head of a number. We have to decide what to do in such nonsensical situations.

We adopt a simple solution: if an arithmetic function is given a list as an argument we treat the list as if it was the number 0. Similarly, if a list function is given a number as an argument we treat the number as if it was nil. (Cf. Javascript, where adding 3 to the empty list evaluates to 3...)

The two functions asnat and aslist interpret vals in a numeric or a list context; aeval calls these whenever it evaluates an arithmetic or a list operation.

Definition asnat (v : val) : nat :=
  match v with
  | VNat n => n
  | VList _ => 0
  end.

Definition aslist (v : val) : list nat :=
  match v with
  | VNat n => []
  | VList xs => xs
  end.

Now we fill in the definitions of abstract syntax and evaluation functions for arithmetic and boolean expressions.

We extend bexps with an operation to test if a list is nonempty and adapt beval acordingly.

Repeated Definitions

Now we need to repeat a little bit of low-level work from Imp.v, plus the definitions of com and ceval. There are no interesting changes — it's just a matter of repeating the same definitions, lemmas, and proofs in the context of the new definitions of arithmetic and boolean expressions.

(Is all this cutting and pasting really necessary? No: Coq includes a powerful module system that we could use to abstract the repeated definitions with respect to the varying parts. But explaining how it works would distract us from the topic at hand.)

Theorem update_eq : ∀n V st,
  (update st V n) V = n.
Proof.
  intros n V st.
  unfold update.
  rewrite ← beq_id_refl.
  reflexivity.
Qed.

Theorem update_neq : ∀V2 V1 n st,
  beq_id V2 V1 = false →
  (update st V2 n) V1 = (st V1).
Proof.
  intros V2 V1 n st Hneq.
  unfold update.
  rewrite → Hneq.
  reflexivity. Qed.

Theorem update_shadow : ∀x1 x2 k1 k2 (f : state),
   (update (update f k2 x1) k2 x2) k1 = (update f k2 x2) k1.
Proof.
  intros x1 x2 k1 k2 f.
  unfold update.
  destruct (beq_id k2 k1); reflexivity. Qed.

Theorem update_same : ∀x1 k1 k2 (f : state),
  f k1 = x1 →
  (update f k1 x1) k2 = f k2.
Proof.
  intros x1 k1 k2 f Heq.
  unfold update. subst.
  remember (beq_id k1 k2) as b.
  destruct b.
  Case "true".
    apply beq_id_eq in Heqb. subst. reflexivity.
  Case "false".
    reflexivity. Qed.

Theorem update_permute : ∀x1 x2 k1 k2 k3 f,
  beq_id k2 k1 = false →
  (update (update f k2 x1) k1 x2) k3 = (update (update f k1 x2) k2 x1) k3.
Proof.
  intros x1 x2 k1 k2 k3 f H.
  unfold update.
  remember (beq_id k1 k3) as b13.
  remember (beq_id k2 k3) as b23.
  apply beq_id_false_not_eq in H.
  destruct b13; try reflexivity.
  Case "true".
    destruct b23; try reflexivity.
    SCase "true".
      apply beq_id_eq in Heqb13.
      apply beq_id_eq in Heqb23.
      subst. apply ex_falso_quodlibet. apply H. reflexivity. Qed.

We can keep exactly the same old definitions of com and ceval.

Inductive com : Type :=
  | CSkip : com
  | CAss : id → aexp → com
  | CSeq : com → com → com
  | CIf : bexp → com → com → com
  | CWhile : bexp → com → com.

Tactic Notation "com_cases" tactic(first) ident(c) :=
  first;
  [ Case_aux c "SKIP" | Case_aux c "::=" | Case_aux c ";"
  | Case_aux c "IFB" | Case_aux c "WHILE" ].

Notation "'SKIP'" :=
  CSkip.
Notation "X '::=' a" :=
  (CAss X a) (at level 60).
Notation "c1 ; c2" :=
  (CSeq c1 c2) (at level 80, right associativity).
Notation "'WHILE' b 'DO' c 'END'" :=
  (CWhile b c) (at level 80, right associativity).
Notation "'IFB' e1 'THEN' e2 'ELSE' e3 'FI'" :=
  (CIf e1 e2 e3) (at level 80, right associativity).

Reserved Notation "c1 '/' st '⇓' st'" (at level 40, st at level 39).

Inductive ceval : state → com → state → Prop :=
  | E_Skip : ∀st,
      SKIP / st ⇓ st
  | E_Asgn : ∀st a1 n X,
      aeval st a1 = n →
      (X ::= a1) / st ⇓ (update st X n)
  | E_Seq : ∀c1 c2 st st' st'',
      c1 / st ⇓ st' →
      c2 / st' ⇓ st'' →
      (c1 ; c2) / st ⇓ st''
  | E_IfTrue : ∀st st' b1 c1 c2,
      beval st b1 = true →
      c1 / st ⇓ st' →
      (IFB b1 THEN c1 ELSE c2 FI) / st ⇓ st'
  | E_IfFalse : ∀st st' b1 c1 c2,
      beval st b1 = false →
      c2 / st ⇓ st' →
      (IFB b1 THEN c1 ELSE c2 FI) / st ⇓ st'
  | E_WhileEnd : ∀b1 st c1,
      beval st b1 = false →
      (WHILE b1 DO c1 END) / st ⇓ st
  | E_WhileLoop : ∀st st' st'' b1 c1,
      beval st b1 = true →
      c1 / st ⇓ st' →
      (WHILE b1 DO c1 END) / st' ⇓ st'' →
      (WHILE b1 DO c1 END) / st ⇓ st''

  where "c1 '/' st '⇓' st'" := (ceval st c1 st').

Tactic Notation "ceval_cases" tactic(first) ident(c) :=
  first;
  [ Case_aux c "E_Skip" | Case_aux c "E_Asgn" | Case_aux c "E_Seq"
  | Case_aux c "E_IfTrue" | Case_aux c "E_IfFalse"
  | Case_aux c "E_WhileEnd" | Case_aux c "E_WhileLoop" ].

Hoare Rules

We copy verbatim the Hoare rules from Hoare.v.

	(hoare_asgn)

{{assn_sub X a Q}} X::=a {{Q}}

	(hoare_skip)

{{ P }} SKIP {{ P }}

{{ P }} c1 {{ Q }}
{{ Q }} c2 {{ R }}	(hoare_seq)

{{ P }} c1;c2 {{ R }}

{{P ∧ b}} c1 {{Q}}
{{P ∧ ~b}} c2 {{Q}}	(hoare_if)

{{P}} IFB b THEN c1 ELSE c2 FI {{Q}}

{{P ∧ b}} c {{P}}	(hoare_while)

{{P}} WHILE b DO c END {{P ∧ ~b}}

{{P'}} c {{Q'}}
P ⇾ P'
Q' ⇾ Q	(hoare_consequence)

{{P}} c {{Q}}

Definition Assertion := state → Prop.

Definition hoare_triple (P:Assertion) (c:com) (Q:Assertion) : Prop :=
  ∀st st',
       c / st ⇓ st' →
       P st →
       Q st'.

Notation "{{ P }} c {{ Q }}" := (hoare_triple P c Q)
                                  (at level 90, c at next level)
                                  : hoare_spec_scope.
Open Scope hoare_spec_scope.

Definition assn_sub X a Q : Assertion :=
  fun (st : state) =>
    Q (update st X (aeval st a)).

Theorem hoare_asgn : ∀Q X a,
  {{assn_sub X a Q}} (X ::= a) {{Q}}.
Proof.
  unfold hoare_triple.
  intros Q X a st st' HE HQ.
  inversion HE. subst.
  unfold assn_sub in HQ. assumption. Qed.

Theorem hoare_skip : ∀P,
     {{P}} SKIP {{P}}.
Proof.
  intros P st st' H HP. inversion H. subst.
  assumption. Qed.

Theorem hoare_seq : ∀P Q R c1 c2,
     {{Q}} c2 {{R}} →
     {{P}} c1 {{Q}} →
     {{P}} c1;c2 {{R}}.
Proof.
  intros P Q R c1 c2 H1 H2 st st' H12 Pre.
  inversion H12; subst.
  apply (H1 st'0 st'); try assumption.
  apply (H2 st st'0); assumption. Qed.

Definition bassn b : Assertion :=
  fun st => (beval st b = true).

Lemma bexp_eval_true : ∀b st,
  beval st b = true → (bassn b) st.
Proof.
  intros b st Hbe.
  unfold bassn. assumption. Qed.

Lemma bexp_eval_false : ∀b st,
  beval st b = false → ~ ((bassn b) st).
Proof.
  intros b st Hbe contra.
  unfold bassn in contra.
  rewrite → contra in Hbe. inversion Hbe. Qed.

Theorem hoare_if : ∀P Q b c1 c2,
  {{fun st => P st ∧ bassn b st}} c1 {{Q}} →
  {{fun st => P st ∧ ~(bassn b st)}} c2 {{Q}} →
  {{P}} (IFB b THEN c1 ELSE c2 FI) {{Q}}.
Proof.
  intros P Q b c1 c2 HTrue HFalse st st' HE HP.
  inversion HE; subst.
  Case "b is true".
    apply (HTrue st st').
      assumption.
      split. assumption.
             apply bexp_eval_true. assumption.
  Case "b is false".
    apply (HFalse st st').
      assumption.
      split. assumption.
             apply bexp_eval_false. assumption. Qed.

Lemma hoare_while : ∀P b c,
  {{fun st => P st ∧ bassn b st}} c {{P}} →
  {{P}} WHILE b DO c END {{fun st => P st ∧ ~ (bassn b st)}}.
Proof.
  intros P b c Hhoare st st' He HP.
  (* Like we've seen before, we need to reason by induction
     on He, because, in the "keep looping" case, its hypotheses
     talk about the whole loop instead of just c *)
  remember (WHILE b DO c END) as wcom.
  ceval_cases (induction He) Case; try (inversion Heqwcom); subst.

  Case "E_WhileEnd".
    split. assumption. apply bexp_eval_false. assumption.

  Case "E_WhileLoop".
    apply IHHe2. reflexivity.
    apply (Hhoare st st'); try assumption.
      split. assumption. apply bexp_eval_true. assumption. Qed.

Definition assert_implies (P Q : Assertion) : Prop :=
  ∀st, P st → Q st.

Notation "P ⇾ Q" := (assert_implies P Q) (at level 80).
Notation "P ⇿ Q" := (P ⇾ Q ∧ Q ⇾ P) (at level 80).

Theorem hoare_consequence_pre : ∀(P P' Q : Assertion) c,
  {{P'}} c {{Q}} →
  P ⇾ P' →
  {{P}} c {{Q}}.
Proof.
  intros P P' Q c Hhoare Himp.
  intros st st' Hc HP. apply (Hhoare st st').
  assumption. apply Himp. assumption. Qed.

Theorem hoare_consequence_post : ∀(P Q Q' : Assertion) c,
  {{P}} c {{Q'}} →
  Q' ⇾ Q →
  {{P}} c {{Q}}.
Proof.
  intros P Q Q' c Hhoare Himp.
  intros st st' Hc HP.
  apply Himp.
  apply (Hhoare st st').
  assumption. assumption. Qed.

Theorem hoare_consequence : ∀(P P' Q Q' : Assertion) c,
  {{P'}} c {{Q'}} →
  P ⇾ P' →
  Q' ⇾ Q →
  {{P}} c {{Q}}.
Proof.
  intros P P' Q Q' c Hht HPP' HQ'Q.
  intros st st' Hc HP.
  apply HQ'Q. apply (Hht st st'). assumption.
  apply HPP'. assumption. Qed.

Reasoning About Programs with Lists

Now let's look at a formal Hoare Logic proof for a program that works with lists. We will verify the following program, which checks if the number Y occurs in the list X, and if so sets Z to 1.

Definition list_member :=
  WHILE BIsCons (AId X) DO
    IFB (BEq (AId Y) (AHead (AId X))) THEN
      Z ::= (ANum 1)
    ELSE
      SKIP
    FI;
    X ::= ATail (AId X)
  END.

The informal proof looks like this:

    {{ X = l ∧ Y = n ∧ Z = 0 }} =>
    {{ Y = n ∧ ∃p, p ++ X = l ∧ (Z = 1 ↔ appears_in n p) }}
  WHILE (BIsCons X)
  DO
      {{ Y = n ∧ (∃p, p ++ X = l ∧ (Z = 1 ↔ appears_in n p))
               ∧ (BIsCons X) }}
    IFB (Y == head X) THEN
        {{ Y = n
           ∧ (∃p, p ++ X = l ∧ (Z = 1 ↔ appears_in n p))
           ∧ (BIsCons X)
           ∧ Y == AHead X }} =>
        {{ Y = n ∧ (∃p, p ++ tail X = l
                    ∧ (1 = 1 ↔ appears_in n p)) }}
      Z ::= 1
        {{ Y = n
           ∧ (∃p, p ++ tail X = l ∧ (Z = 1 ↔ appears_in n p)) }}
    ELSE
        {{ Y = n
           ∧ (∃p, p ++ X = l ∧ (Z = 1 ↔ appears_in n p))
           ∧ (BIsCons X)
           ∧ ~ (Y == head X) }} =>
        {{ Y = n
           ∧ (∃p, p ++ tail X = l ∧ (Z = 1 ↔ appears_in n p)) }}
      SKIP
        {{ Y = n
           ∧ (∃p, p ++ tail X = l ∧ (Z = 1 ↔ appears_in n p)) }}
    FI;
       {{ Y = n
           ∧ (∃p, p ++ tail X = l ∧ (Z = 1 ↔ appears_in n p)) }}
    X ::= ATail X
        {{ Y = n
           ∧ (∃p, p ++ X = l ∧ (Z = 1 ↔ appears_in n p)) }}
  END
     {{ Y = n
        ∧ (∃p, p ++ X = l ∧ (Z = 1 ↔ appears_in n p))
        ∧ ~ (BIsCons X) }} =>
     {{ Z = 1 ↔ appears_in n l }}

The only interesting part of the proof is the choice of loop invariant:

∃p, p ++ X = l ∧ (Z = 1 ↔ appears_in n p)

This states that at each iteration of the loop, the original list l is equal to the append of the current value of X and some other list p which is not the value of any variable in the program, but keeps track of enough information from the original state to make the proof go through. (Such a p is sometimes called a "ghost variable").

In order to show that such a list p exists, in each iteration we add the head of X to the end of p. This needs the function snoc, from Poly.v.

Fixpoint snoc {X:Type} (l:list X) (v:X) : (list X) :=
  match l with
  | nil => [ v ]
  | cons h t => h :: (snoc t v)
  end.

The main proof uses several lemmas about snoc and ++.

Lemma snoc_equation : ∀(A : Type) (h:A) (x y : list A),
  snoc x h ++ y = x ++ h :: y.
Proof.
  intros A h x y.
  induction x.
    Case "x = []". reflexivity.
    Case "x = cons". simpl. rewrite IHx. reflexivity.
Qed.

Lemma appears_in_snoc1 : ∀a l,
  appears_in a (snoc l a).
Proof.
  induction l.
    Case "l = []". apply ai_here.
    Case "l = cons". simpl. apply ai_later. apply IHl.
Qed.

Lemma appears_in_snoc2 : ∀a b l,
  appears_in a l →
  appears_in a (snoc l b).
Proof.
  induction l; intros H; inversion H; subst; simpl.
    Case "l = []". apply ai_here.
    Case "l = cons". apply ai_later. apply IHl. assumption.
Qed.

Lemma appears_in_snoc3 : ∀a b l,
   appears_in a (snoc l b) →
   (appears_in a l ∨ a = b).
Proof.
   induction l; intros H.
   Case "l = []". inversion H.
     SCase "ai_here". right. reflexivity.
     SCase "ai_later". left. assumption.
   Case "l = cons". inversion H; subst.
     SCase "ai_here". left. apply ai_here.
     SCase "ai_later". destruct (IHl H1).
       left. apply ai_later. assumption.
       right. assumption.
Qed.

Lemma append_singleton_equation : ∀(x : nat) l l',
  (l ++ [x]) ++ l' = l ++ x :: l'.
Proof.
  intros x l l'.
  induction l.
    reflexivity.
    simpl. rewrite IHl. reflexivity.
Qed.

Lemma append_nil : ∀(A : Type) (l : list A),
  l ++ [] = l.
Proof.
  induction l.
    reflexivity.
    simpl. rewrite IHl. reflexivity.
Qed.

Theorem list_member_correct : ∀l n,
  {{ fun st => aslist (st X) = l ∧ asnat (st Y) = n ∧ asnat (st Z) = 0 }}
  list_member
  {{ fun st => asnat (st Z) = 1 ↔ appears_in n l }}.
Proof.
  intros l n.
  eapply hoare_consequence.
  apply hoare_while with (P := fun st =>
     asnat (st Y) = n
     ∧ ∃p, p ++ aslist (st X) = l
                  ∧ (asnat (st Z) = 1 ↔ appears_in n p)).
    (* The loop body preserves the invariant: *)
    eapply hoare_seq.
    apply hoare_asgn.
    apply hoare_if.
    Case "If taken".
      eapply hoare_consequence_pre.
      apply hoare_asgn.
      intros st [[[H1 [p [H2 H3]]] H9] H10].
      unfold assn_sub. split.
      (* (st Y) is still n *)
        rewrite update_neq; try reflexivity.
        rewrite update_neq; try reflexivity.
        assumption.
      (* and the interesting part of the invariant is preserved: *)
        (* X has to be a cons *)
        remember (aslist (st X)) as x.
        destruct x as [|h x'].
          unfold bassn in H9. unfold beval in H9. unfold aeval in H9.
          rewrite ← Heqx in H9. inversion H9.

          ∃(snoc p h).
          rewrite update_eq.
          unfold aeval. rewrite update_neq; try reflexivity.
          rewrite ← Heqx.
          split.
            rewrite snoc_equation. assumption.

            rewrite update_neq; try reflexivity.
            rewrite update_eq.
            split.
              simpl.
              unfold bassn in H10. unfold beval in H10.
              unfold aeval in H10. rewrite H1 in H10.
              rewrite ← Heqx in H10. simpl in H10.
              rewrite (beq_nat_true _ _ H10).
              intros. apply appears_in_snoc1.

              intros. reflexivity.
    Case "If not taken".
      eapply hoare_consequence_pre. apply hoare_skip.
      unfold assn_sub.
      intros st [[[H1 [p [H2 H3]]] H9] H10].
      split.
      (* (st Y) is still n *)
        rewrite update_neq; try reflexivity.
        assumption.
      (* and the interesting part of the invariant is preserved: *)
        (* X has to be a cons *)
        remember (aslist (st X)) as x.
        destruct x as [|h x'].
          unfold bassn in H9. unfold beval in H9. unfold aeval in H9.
          rewrite ← Heqx in H9. inversion H9.

          ∃(snoc p h).
          split.
            rewrite update_eq.
            unfold aeval. rewrite ← Heqx.
            rewrite snoc_equation. assumption.

            rewrite update_neq; try reflexivity.
            split.
              intros. apply appears_in_snoc2. apply H3. assumption.

              intros. destruct (appears_in_snoc3 _ _ _ H).
              SCase "later".
                inversion H3 as [_ H3'].
                apply H3'. assumption.
              SCase "here (absurd)".
                subst.
                unfold bassn, beval, aeval in H10.
                rewrite not_true_iff_false in H10.
                apply beq_nat_false in H10.
                rewrite ← Heqx in H10. simpl in H10.
                apply ex_falso_quodlibet. apply H10. assumption.
  (* The invariant holds at the start of the loop: *)
  intros st [H1 [H2 H3]].
  rewrite H1. rewrite H2. rewrite H3.
  split.
    reflexivity.
    ∃[]. split.
      reflexivity.
      split; intros H; inversion H.
  (* At the end of the loop the invariant implies the right thing. *)
  simpl. intros st [[H1 [p [H2 H3]]] H5].
  (* x must be  *)
  unfold bassn in H5. unfold beval in H5. unfold aeval in H5.
  destruct (aslist (st X)) as [|h x'].
    rewrite append_nil in H2.
    rewrite ← H2.
    assumption.

    apply ex_falso_quodlibet. apply H5. reflexivity.
Qed.

Exercise: 3 stars (list_sum)

Here is a direct definition of the sum of the elements of a list, and an Imp program that computes the sum.

Definition sum l := fold_right plus 0 l.

Definition sum_program :=
  Y ::= ANum 0;
  WHILE (BIsCons (AId X)) DO
    Y ::= APlus (AId Y) (AHead (AId X)) ;
    X ::= ATail (AId X)
  END.

Provide an informal proof of the following specification of sum_program in the form of a decorated version of the program.

Definition sum_program_spec := ∀l,
  {{ fun st => aslist (st X) = l }}
  sum_program
  {{ fun st => asnat (st Y) = sum l }}.

(* FILL IN HERE *)

☐

Exercise: 4 stars (list_reverse)

Recall the function rev from Poly.v, for reversing lists.

Fixpoint rev {X:Type} (l:list X) : list X :=
  match l with
  | nil => []
  | cons h t => snoc (rev t) h
  end.

Write an Imp program list_reverse_program that reverses lists. Formally prove that it satisfies the following specification:

    ∀l : list nat,
      {{ X =  l ∧ Y = nil }}
      list_reverse_program
      {{ Y = rev l }}.

You may find the lemmas append_nil and rev_equation useful.

Lemma rev_equation : ∀(A : Type) (h : A) (x y : list A),
rev (h :: x) ++ y = rev x ++ h :: y.
Proof.
intros. simpl. apply snoc_equation.
Qed.

(* FILL IN HERE *)

☐

Finally, for a bigger example, let's redo the proof of list_member_correct from above using our new tools.

Notice that the verify tactic leaves subgoals for each "interesting" use of hoare_consequence — that is, for each => that occurs in the decorated program, except for the ones that can be eliminated by repeated application of a few simple automated tactics. Each of these implications relies on a fact about lists, for example that l ++ [] = l. In other words, the Hoare logic infrastructure has taken care of the boilerplate reasoning about the execution of imperative programs, while the user has to prove lemmas that are specific to the problem domain (e.g. lists or numbers).

Formal Decorated Programs

Again, the definitions are copied verbatim from Hoare.v

Inductive dcom : Type :=
  | DCSkip : Assertion → dcom
  | DCSeq : dcom → dcom → dcom
  | DCAsgn : id → aexp → Assertion → dcom
  | DCIf : bexp → Assertion → dcom → Assertion → dcom
           → Assertion→ dcom
  | DCWhile : bexp → Assertion → dcom → Assertion → dcom
  | DCPre : Assertion → dcom → dcom
  | DCPost : dcom → Assertion → dcom.

Tactic Notation "dcom_cases" tactic(first) ident(c) :=
  first;
  [ Case_aux c "Skip" | Case_aux c "Seq" | Case_aux c "Asgn"
  | Case_aux c "If" | Case_aux c "While"
  | Case_aux c "Pre" | Case_aux c "Post" ].

Notation "'SKIP' {{ P }}"
      := (DCSkip P)
      (at level 10) : dcom_scope.
Notation "l '::=' a {{ P }}"
      := (DCAsgn l a P)
      (at level 60, a at next level) : dcom_scope.
Notation "'WHILE' b 'DO' {{ Pbody }} d 'END' {{ Ppost }}"
      := (DCWhile b Pbody d Ppost)
      (at level 80, right associativity) : dcom_scope.
Notation "'IFB' b 'THEN' {{ P }} d 'ELSE' {{ P' }} d' 'FI' {{ Q }}"
      := (DCIf b P d P' d' Q)
      (at level 80, right associativity) : dcom_scope.
Notation "'=>' {{ P }} d"
      := (DCPre P d)
      (at level 90, right associativity) : dcom_scope.
Notation "{{ P }} d"
      := (DCPre P d)
      (at level 90) : dcom_scope.
Notation "d '=>' {{ P }}"
      := (DCPost d P)
      (at level 91, right associativity) : dcom_scope.
Notation " d ; d' "
      := (DCSeq d d')
      (at level 80, right associativity) : dcom_scope.

Delimit Scope dcom_scope with dcom.

Example dec_while : dcom := (
  {{ fun st => True }}
  WHILE (BNot (BEq (AId X) (ANum 0)))
  DO
    {{ fun st => True ∧ bassn (BNot (BEq (AId X) (ANum 0))) st}}
    X ::= (AMinus (AId X) (ANum 1))
    {{ fun _ => True }}
  END
  {{ fun st => True ∧ ~bassn (BNot (BEq (AId X) (ANum 0))) st}} =>
  {{ fun st => asnat (st X) = 0 }}
) % dcom.

Fixpoint extract (d:dcom) : com :=
  match d with
  | DCSkip _ => SKIP
  | DCSeq d1 d2 => (extract d1 ; extract d2)
  | DCAsgn X a _ => X ::= a
  | DCIf b _ d1 _ d2 _ => IFB b THEN extract d1 ELSE extract d2 FI
  | DCWhile b _ d _ => WHILE b DO extract d END
  | DCPre _ d => extract d
  | DCPost d _ => extract d
  end.

Fixpoint post (d:dcom) : Assertion :=
  match d with
  | DCSkip P => P
  | DCSeq d1 d2 => post d2
  | DCAsgn X a Q => Q
  | DCIf _ _ d1 _ d2 Q => Q
  | DCWhile b Pbody c Ppost => Ppost
  | DCPre _ d => post d
  | DCPost c Q => Q
  end.

Fixpoint pre (d:dcom) : Assertion :=
  match d with
  | DCSkip P => fun st => True
  | DCSeq c1 c2 => pre c1
  | DCAsgn X a Q => fun st => True
  | DCIf _ _ t _ e _ => fun st => True
  | DCWhile b Pbody c Ppost => fun st => True
  | DCPre P c => P
  | DCPost c Q => pre c
  end.

Definition dec_correct (d:dcom) :=
  {{pre d}} (extract d) {{post d}}.

Fixpoint verification_conditions (P : Assertion) (d:dcom) : Prop :=
  match d with
  | DCSkip Q =>
      (P ⇾ Q)
  | DCSeq d1 d2 =>
      verification_conditions P d1
      ∧ verification_conditions (post d1) d2
  | DCAsgn X a Q =>
      (P ⇾ assn_sub X a Q)
  | DCIf b P1 d1 P2 d2 Q =>
      ((fun st => P st ∧ bassn b st) ⇾ P1)
      ∧ ((fun st => P st ∧ ~ (bassn b st)) ⇾ P2)
      ∧ (Q = post d1) ∧ (Q = post d2)
      ∧ verification_conditions P1 d1
      ∧ verification_conditions P2 d2
  | DCWhile b Pbody d Ppost =>
      (* post d is the loop invariant and the initial precondition *)
      (P ⇾ post d)
      ∧ (Pbody = (fun st => post d st ∧ bassn b st))
      ∧ (Ppost = (fun st => post d st ∧ ~(bassn b st)))
      ∧ verification_conditions Pbody d
  | DCPre P' d =>
      (P ⇾ P') ∧ verification_conditions P' d
  | DCPost d Q =>
      verification_conditions P d ∧ (post d ⇾ Q)
  end.

Theorem verification_correct : ∀d P,
  verification_conditions P d → {{P}} (extract d) {{post d}}.
Proof.
  dcom_cases (induction d) Case; intros P H; simpl in *.
  Case "Skip".
    eapply hoare_consequence_pre.
      apply hoare_skip.
      assumption.
  Case "Seq".
    inversion H as [H1 H2]. clear H.
    eapply hoare_seq.
      apply IHd2. apply H2.
      apply IHd1. apply H1.
  Case "Asgn".
    eapply hoare_consequence_pre.
      apply hoare_asgn.
      assumption.
  Case "If".
    inversion H as [HPre1 [HPre2 [Hd1 [Hd2 [HThen HElse]]]]]; clear H.
    subst.
    apply hoare_if.
      eapply hoare_consequence_pre. apply IHd1. eassumption. assumption.
      rewrite Hd2.
      eapply hoare_consequence_pre. apply IHd2. eassumption. assumption.
  Case "While".
    inversion H as [Hpre [Hbody [Hpost Hd]]]; subst; clear H.
    eapply hoare_consequence_pre.
    apply hoare_while with (P := post d).
      apply IHd. apply Hd.
      assumption.
  Case "Pre".
    inversion H as [HP Hd]; clear H.
    eapply hoare_consequence_pre. apply IHd. apply Hd. assumption.
  Case "Post".
    inversion H as [Hd HQ]; clear H.
    eapply hoare_consequence_post. apply IHd. apply Hd. assumption.
Qed.

Tactic Notation "verify" :=
  try apply verification_correct;
  repeat split;
  simpl; unfold assert_implies;
  unfold bassn in *; unfold beval in *; unfold aeval in *;
  unfold assn_sub; intros;
  repeat rewrite update_eq;
  repeat (rewrite update_neq; [| reflexivity]);
  simpl in *;
  repeat match goal with [H : _ ∧ _ ⊢ _] => destruct H end;
  repeat rewrite not_true_iff_false in *;
  repeat rewrite not_false_iff_true in *;
  repeat rewrite negb_true_iff in *;
  repeat rewrite negb_false_iff in *;
  repeat rewrite beq_nat_true_iff in *;
  repeat rewrite beq_nat_false_iff in *;
  try eauto; try omega.

Finally, for a bigger example, let's redo the proof of list_member_correct from above using our new tools.

Definition list_member_dec (n : nat) (l : list nat) : dcom := (
    {{ fun st => aslist (st X) = l ∧ asnat (st Y) = n ∧ asnat (st Z) = 0 }}
  WHILE BIsCons (AId X)
  DO {{ fun st => (asnat (st Y) = n
               ∧ (∃p, p ++ aslist (st X) = l
               ∧ (asnat (st Z) = 1 ↔ appears_in n p)))
               ∧ bassn (BIsCons (AId X)) st }}
    IFB (BEq (AId Y) (AHead (AId X))) THEN
        {{ fun st =>
          ((asnat (st Y) = n
            ∧ (∃p, p ++ aslist (st X) = l
                ∧ (asnat (st Z) = 1 ↔ appears_in n p)))
          ∧ bassn (BIsCons (AId X)) st)
          ∧ bassn (BEq (AId Y) (AHead (AId X))) st }}
        =>
        {{ fun st =>
            asnat (st Y) = n
            ∧ (∃p, p ++ tail (aslist (st X)) = l
                 ∧ (1 = 1 ↔ appears_in n p)) }}
      Z ::= ANum 1
        {{ fun st => asnat (st Y) = n
             ∧ (∃p, p ++ tail (aslist (st X)) = l
                  ∧ (asnat (st Z) = 1 ↔ appears_in n p)) }}
   ELSE
        {{ fun st =>
          ((asnat (st Y) = n
            ∧ (∃p, p ++ aslist (st X) = l
                  ∧ (asnat (st Z) = 1 ↔ appears_in n p)))
          ∧ bassn (BIsCons (AId X)) st)
          ∧ ~bassn (BEq (AId Y) (AHead (AId X))) st }}
        =>
        {{ fun st =>
          asnat (st Y) = n
          ∧ (∃p, p ++ tail (aslist (st X)) = l
               ∧ (asnat (st Z) = 1 ↔ appears_in n p)) }}
      SKIP
        {{ fun st => asnat (st Y) = n
            ∧ (∃p, p ++ tail (aslist (st X)) = l
                 ∧ (asnat (st Z) = 1 ↔ appears_in n p)) }}
   FI
     {{ fun st => asnat (st Y) = n
        ∧ (∃p, p ++ tail (aslist (st X)) = l
          ∧ (asnat (st Z) = 1 ↔ appears_in n p)) }}
   ;
   X ::= (ATail (AId X))
     {{ fun st =>
         asnat (st Y) = n ∧
         (∃p : list nat, p ++ aslist (st X) = l
           ∧ (asnat (st Z) = 1 ↔ appears_in n p)) }}
  END
   {{ fun st =>
     (asnat (st Y) = n
       ∧ (∃p, p ++ aslist (st X) = l
            ∧ (asnat (st Z) = 1 ↔ appears_in n p)))
     ∧ ~bassn (BIsCons (AId X)) st }}
   =>
   {{ fun st => asnat (st Z) = 1 ↔ appears_in n l }}
) %dcom.

Theorem list_member_correct' : ∀n l,
  dec_correct (list_member_dec n l).
Proof.
  intros n l. verify.
  Case "The loop precondition holds.".
    ∃[]. simpl. split.
      rewrite H. reflexivity.
      rewrite H1. split; intro Hc; inversion Hc.
  Case "IF taken".
    destruct H2 as [p [H3 H4]].
    (* We know X is non-nil. *)
    remember (aslist (st X)) as x.
    destruct x as [|h x'].
      inversion H1.
      ∃(snoc p h).
      simpl. split.
         rewrite snoc_equation. assumption.
         split.
           rewrite H in H0.
           simpl in H0. rewrite H0.
           intros _. apply appears_in_snoc1.
           intros _. reflexivity.
  Case "If not taken".
    destruct H2 as [p [H3 H4]].
    (* We know X is non-nil. *)
    remember (aslist (st X)) as x.
    destruct x as [|h x'].
      inversion H1.
      ∃(snoc p h).
      split. simpl.
        rewrite snoc_equation. assumption.
        split.
          intros. apply appears_in_snoc2. apply H4. assumption.
          intros Hai. destruct (appears_in_snoc3 _ _ _ Hai).
          SCase "later". apply H4. assumption.
          SCase "here (absurd)".
            subst. simpl in H0.
            apply ex_falso_quodlibet. apply H0. assumption.
  Case "Loop postcondition implies desired conclusion (->)".
    destruct H2 as [p [H3 H4]].
    destruct (aslist (st X)) as [|h x'].
       rewrite append_nil in H3. subst. apply H4. assumption.
       inversion H1.
  Case "loop postcondition implies desired conclusion (<-)".
    destruct H2 as [p [H3 H4]].
    destruct (aslist (st X)) as [|h x'].
       rewrite append_nil in H3. subst. apply H4. assumption.
       inversion H1.
Qed.