Theory Hoare_Total

theory Hoare_Total
imports Hoare_Examples
(* Author: Tobias Nipkow *)

subsection "Hoare Logic for Total Correctness"

theory Hoare_Total
imports Hoare_Examples

subsubsection "Hoare Logic for Total Correctness --- Separate Termination Relation"

text‹Note that this definition of total validity @{text"⊨t"} only
works if execution is deterministic (which it is in our case).›

definition hoare_tvalid :: "assn ⇒ com ⇒ assn ⇒ bool"
  ("⊨t {(1_)}/ (_)/ {(1_)}" 50) where
"⊨t {P}c{Q}  ⟷  (∀s. P s ⟶ (∃t. (c,s) ⇒ t ∧ Q t))"

text‹Provability of Hoare triples in the proof system for total
correctness is written @{text"⊢t {P}c{Q}"} and defined
inductively. The rules for @{text"⊢t"} differ from those for
@{text"⊢"} only in the one place where nontermination can arise: the
@{term While}-rule.›

  hoaret :: "assn ⇒ com ⇒ assn ⇒ bool" ("⊢t ({(1_)}/ (_)/ {(1_)})" 50)

Skip:  "⊢t {P} SKIP {P}"  |

Assign:  "⊢t {λs. P(s[a/x])} x::=a {P}"  |

Seq: "⟦ ⊢t {P1} c1 {P2}; ⊢t {P2} c2 {P3} ⟧ ⟹ ⊢t {P1} c1;;c2 {P3}"  |

If: "⟦ ⊢t {λs. P s ∧ bval b s} c1 {Q}; ⊢t {λs. P s ∧ ¬ bval b s} c2 {Q} ⟧
  ⟹ ⊢t {P} IF b THEN c1 ELSE c2 {Q}"  |

    ⊢t {λs. P s ∧ bval b s ∧ T s n} c {λs. P s ∧ (∃n'<n. T s n')})
   ⟹ ⊢t {λs. P s ∧ (∃n. T s n)} WHILE b DO c {λs. P s ∧ ¬bval b s}"  |

conseq: "⟦ ∀s. P' s ⟶ P s; ⊢t {P}c{Q}; ∀s. Q s ⟶ Q' s  ⟧ ⟹
           ⊢t {P'}c{Q'}"

text‹The @{term While}-rule is like the one for partial correctness but it
requires additionally that with every execution of the loop body some measure
relation @{term[source]"T :: state ⇒ nat ⇒ bool"} decreases.
The following functional version is more intuitive:›

lemma While_fun:
  "⟦ ⋀n::nat. ⊢t {λs. P s ∧ bval b s ∧ n = f s} c {λs. P s ∧ f s < n}⟧
   ⟹ ⊢t {P} WHILE b DO c {λs. P s ∧ ¬bval b s}"
  by (rule While [where T="λs n. n = f s", simplified])

text‹Building in the consequence rule:›

lemma strengthen_pre:
  "⟦ ∀s. P' s ⟶ P s;  ⊢t {P} c {Q} ⟧ ⟹ ⊢t {P'} c {Q}"
by (metis conseq)

lemma weaken_post:
  "⟦ ⊢t {P} c {Q};  ∀s. Q s ⟶ Q' s ⟧ ⟹  ⊢t {P} c {Q'}"
by (metis conseq)

lemma Assign': "∀s. P s ⟶ Q(s[a/x]) ⟹ ⊢t {P} x ::= a {Q}"
by (simp add: strengthen_pre[OF _ Assign])

lemma While_fun':
assumes "⋀n::nat. ⊢t {λs. P s ∧ bval b s ∧ n = f s} c {λs. P s ∧ f s < n}"
    and "∀s. P s ∧ ¬ bval b s ⟶ Q s"
shows "⊢t {P} WHILE b DO c {Q}"
by(blast intro: assms(1) weaken_post[OF While_fun assms(2)])

text‹Our standard example:›

lemma "⊢t {λs. s ''x'' = i} ''y'' ::= N 0;; wsum {λs. s ''y'' = sum i}"
apply(rule Seq)
 prefer 2
 apply(rule While_fun' [where P = "λs. (s ''y'' = sum i - sum(s ''x''))"
    and f = "λs. nat(s ''x'')"])
   apply(rule Seq)
   prefer 2
   apply(rule Assign)
  apply(rule Assign')
  apply simp
apply(rule Assign')
apply simp

text‹The soundness theorem:›

theorem hoaret_sound: "⊢t {P}c{Q}  ⟹  ⊨t {P}c{Q}"
proof(unfold hoare_tvalid_def, induction rule: hoaret.induct)
  case (While P b T c)
  have "⟦ P s; T s n ⟧ ⟹ ∃t. (WHILE b DO c, s) ⇒ t ∧ P t ∧ ¬ bval b t" for s n
  proof(induction "n" arbitrary: s rule: less_induct)
    case (less n) thus ?case by (metis While.IH WhileFalse WhileTrue)
  thus ?case by auto
  case If thus ?case by auto blast
qed fastforce+

The completeness proof proceeds along the same lines as the one for partial
correctness. First we have to strengthen our notion of weakest precondition
to take termination into account:›

definition wpt :: "com ⇒ assn ⇒ assn" ("wpt") where
"wpt c Q  =  (λs. ∃t. (c,s) ⇒ t ∧ Q t)"

lemma [simp]: "wpt SKIP Q = Q"
by(auto intro!: ext simp: wpt_def)

lemma [simp]: "wpt (x ::= e) Q = (λs. Q(s(x := aval e s)))"
by(auto intro!: ext simp: wpt_def)

lemma [simp]: "wpt (c1;;c2) Q = wpt c1 (wpt c2 Q)"
unfolding wpt_def
apply(rule ext)
apply auto

lemma [simp]:
 "wpt (IF b THEN c1 ELSE c2) Q = (λs. wpt (if bval b s then c1 else c2) Q s)"
apply(unfold wpt_def)
apply(rule ext)
apply auto

text‹Now we define the number of iterations @{term "WHILE b DO c"} needs to
terminate when started in state @{text s}. Because this is a truly partial
function, we define it as an (inductive) relation first:›

inductive Its :: "bexp ⇒ com ⇒ state ⇒ nat ⇒ bool" where
Its_0: "¬ bval b s ⟹ Its b c s 0" |
Its_Suc: "⟦ bval b s;  (c,s) ⇒ s';  Its b c s' n ⟧ ⟹ Its b c s (Suc n)"

text‹The relation is in fact a function:›

lemma Its_fun: "Its b c s n ⟹ Its b c s n' ⟹ n=n'"
proof(induction arbitrary: n' rule:Its.induct)
  case Its_0 thus ?case by(metis Its.cases)
  case Its_Suc thus ?case by(metis Its.cases big_step_determ)

text‹For all terminating loops, @{const Its} yields a result:›

lemma WHILE_Its: "(WHILE b DO c,s) ⇒ t ⟹ ∃n. Its b c s n"
proof(induction "WHILE b DO c" s t rule: big_step_induct)
  case WhileFalse thus ?case by (metis Its_0)
  case WhileTrue thus ?case by (metis Its_Suc)

lemma wpt_is_pre: "⊢t {wpt c Q} c {Q}"
proof (induction c arbitrary: Q)
  case SKIP show ?case by (auto intro:hoaret.Skip)
  case Assign show ?case by (auto intro:hoaret.Assign)
  case Seq thus ?case by (auto intro:hoaret.Seq)
  case If thus ?case by (auto intro:hoaret.If hoaret.conseq)
  case (While b c)
  let ?w = "WHILE b DO c"
  let ?T = "Its b c"
  have 1: "∀s. wpt ?w Q s ⟶ wpt ?w Q s ∧ (∃n. Its b c s n)"
    unfolding wpt_def by (metis WHILE_Its)
  let ?R = "λn s'. wpt ?w Q s' ∧ (∃n'<n. ?T s' n')"
  have "∀s. wpt ?w Q s ∧ bval b s ∧ ?T s n ⟶ wpt c (?R n) s" for n
  proof -
    have "wpt c (?R n) s" if "bval b s" and "?T s n" and "(?w, s) ⇒ t" and "Q t" for s t
    proof -
      from ‹bval b s› and ‹(?w, s) ⇒ t› obtain s' where
        "(c,s) ⇒ s'" "(?w,s') ⇒ t" by auto
      from ‹(?w, s') ⇒ t› obtain n' where "?T s' n'"
        by (blast dest: WHILE_Its)
      with ‹bval b s› and ‹(c, s) ⇒ s'› have "?T s (Suc n')" by (rule Its_Suc)
      with ‹?T s n› have "n = Suc n'" by (rule Its_fun)
      with ‹(c,s) ⇒ s'› and ‹(?w,s') ⇒ t› and ‹Q t› and ‹?T s' n'›
      show ?thesis by (auto simp: wpt_def)
    thus ?thesis
      unfolding wpt_def by auto
      (* by (metis WhileE Its_Suc Its_fun WHILE_Its lessI) *) 
  note 2 = hoaret.While[OF strengthen_pre[OF this While.IH]]
  have "∀s. wpt ?w Q s ∧ ¬ bval b s ⟶ Q s"
    by (auto simp add:wpt_def)
  with 1 2 show ?case by (rule conseq)

text‹\noindent In the @{term While}-case, @{const Its} provides the obvious
termination argument.

The actual completeness theorem follows directly, in the same manner
as for partial correctness:›

theorem hoaret_complete: "⊨t {P}c{Q} ⟹ ⊢t {P}c{Q}"
apply(rule strengthen_pre[OF _ wpt_is_pre])
apply(auto simp: hoare_tvalid_def wpt_def)

corollary hoaret_sound_complete: "⊢t {P}c{Q} ⟷ ⊨t {P}c{Q}"
by (metis hoaret_sound hoaret_complete)