src/HOL/Isar_examples/Hoare.thy

updated;

(*  Title:      HOL/Isar_examples/Hoare.thy
    ID:         $Id$
    Author:     Markus Wenzel, TU Muenchen

A formulation of Hoare logic suitable for Isar.
*)

header {* Hoare Logic *}

theory Hoare = Main
files ("~~/src/HOL/Hoare/Hoare.ML"):

subsection {* Abstract syntax and semantics *}

text {*
 The following abstract syntax and semantics of Hoare Logic over
 \texttt{WHILE} programs closely follows the existing tradition in
 Isabelle/HOL of formalizing the presentation given in
 \cite[\S6]{Winskel:1993}.  See also
 \url{http://isabelle.in.tum.de/library/Hoare/} and
 \cite{Nipkow:1998:Winskel}.
*}

types
  'a bexp = "'a set"
  'a assn = "'a set"

datatype 'a com =
    Basic "'a => 'a"
  | Seq "'a com" "'a com"    ("(_;/ _)" [60, 61] 60)
  | Cond "'a bexp" "'a com" "'a com"
  | While "'a bexp" "'a assn" "'a com"

syntax
  "_skip" :: "'a com"    ("SKIP")
translations
  "SKIP" == "Basic id"

types
  'a sem = "'a => 'a => bool"

consts
  iter :: "nat => 'a bexp => 'a sem => 'a sem"
primrec
  "iter 0 b S s s' = (s ~: b & s = s')"
  "iter (Suc n) b S s s' =
    (s : b & (EX s''. S s s'' & iter n b S s'' s'))"

consts
  Sem :: "'a com => 'a sem"
primrec
  "Sem (Basic f) s s' = (s' = f s)"
  "Sem (c1; c2) s s' = (EX s''. Sem c1 s s'' & Sem c2 s'' s')"
  "Sem (Cond b c1 c2) s s' =
    (if s : b then Sem c1 s s' else Sem c2 s s')"
  "Sem (While b x c) s s' = (EX n. iter n b (Sem c) s s')"

constdefs
  Valid :: "'a bexp => 'a com => 'a bexp => bool"
    ("(3|- _/ (2_)/ _)" [100, 55, 100] 50)
  "|- P c Q == ALL s s'. Sem c s s' --> s : P --> s' : Q"

syntax (xsymbols)
  Valid :: "'a bexp => 'a com => 'a bexp => bool"
    ("(3\<turnstile> _/ (2_)/ _)" [100, 55, 100] 50)

lemma ValidI [intro?]:
    "(!!s s'. Sem c s s' ==> s : P ==> s' : Q) ==> |- P c Q"
  by (simp add: Valid_def)

lemma ValidD [dest?]:
    "|- P c Q ==> Sem c s s' ==> s : P ==> s' : Q"
  by (simp add: Valid_def)


subsection {* Primitive Hoare rules *}

text {*
 From the semantics defined above, we derive the standard set of
 primitive Hoare rules; e.g.\ see \cite[\S6]{Winskel:1993}.  Usually,
 variant forms of these rules are applied in actual proof, see also
 \S\ref{sec:hoare-isar} and \S\ref{sec:hoare-vcg}.

 \medskip The \name{basic} rule represents any kind of atomic access
 to the state space.  This subsumes the common rules of \name{skip}
 and \name{assign}, as formulated in \S\ref{sec:hoare-isar}.
*}

theorem basic: "|- {s. f s : P} (Basic f) P"
proof
  fix s s' assume s: "s : {s. f s : P}"
  assume "Sem (Basic f) s s'"
  hence "s' = f s" by simp
  with s show "s' : P" by simp
qed

text {*
 The rules for sequential commands and semantic consequences are
 established in a straight forward manner as follows.
*}

theorem seq: "|- P c1 Q ==> |- Q c2 R ==> |- P (c1; c2) R"
proof
  assume cmd1: "|- P c1 Q" and cmd2: "|- Q c2 R"
  fix s s' assume s: "s : P"
  assume "Sem (c1; c2) s s'"
  then obtain s'' where sem1: "Sem c1 s s''" and sem2: "Sem c2 s'' s'"
    by auto
  from cmd1 sem1 s have "s'' : Q" ..
  with cmd2 sem2 show "s' : R" ..
qed

theorem conseq: "P' <= P ==> |- P c Q ==> Q <= Q' ==> |- P' c Q'"
proof
  assume P'P: "P' <= P" and QQ': "Q <= Q'"
  assume cmd: "|- P c Q"
  fix s s' :: 'a
  assume sem: "Sem c s s'"
  assume "s : P'" with P'P have "s : P" ..
  with cmd sem have "s' : Q" ..
  with QQ' show "s' : Q'" ..
qed

text {*
 The rule for conditional commands is directly reflected by the
 corresponding semantics; in the proof we just have to look closely
 which cases apply.
*}

theorem cond:
  "|- (P Int b) c1 Q ==> |- (P Int -b) c2 Q ==> |- P (Cond b c1 c2) Q"
proof
  assume case_b: "|- (P Int b) c1 Q" and case_nb: "|- (P Int -b) c2 Q"
  fix s s' assume s: "s : P"
  assume sem: "Sem (Cond b c1 c2) s s'"
  show "s' : Q"
  proof cases
    assume b: "s : b"
    from case_b show ?thesis
    proof
      from sem b show "Sem c1 s s'" by simp
      from s b show "s : P Int b" by simp
    qed
  next
    assume nb: "s ~: b"
    from case_nb show ?thesis
    proof
      from sem nb show "Sem c2 s s'" by simp
      from s nb show "s : P Int -b" by simp
    qed
  qed
qed

text {*
 The \name{while} rule is slightly less trivial --- it is the only one
 based on recursion, which is expressed in the semantics by a
 Kleene-style least fixed-point construction.  The auxiliary statement
 below, which is by induction on the number of iterations is the main
 point to be proven; the rest is by routine application of the
 semantics of \texttt{WHILE}.
*}

theorem while: "|- (P Int b) c P ==> |- P (While b X c) (P Int -b)"
proof
  assume body: "|- (P Int b) c P"
  fix s s' assume s: "s : P"
  assume "Sem (While b X c) s s'"
  then obtain n where iter: "iter n b (Sem c) s s'" by auto
  have "!!s. iter n b (Sem c) s s' ==> s : P ==> s' : P Int -b"
  proof (induct n)
    case (0 s)
    thus ?case by auto
  next
    case (Suc n s)
    then obtain s'' where b: "s : b" and sem: "Sem c s s''"
      and iter: "iter n b (Sem c) s'' s'"
      by auto
    from Suc and b have "s : P Int b" by simp
    with body sem have "s'' : P" ..
    with iter show ?case by (rule Suc)
  qed
  from this iter s show "s' : P Int -b" .
qed


subsection {* Concrete syntax for assertions *}

text {*
 We now introduce concrete syntax for describing commands (with
 embedded expressions) and assertions. The basic technique is that of
 semantic ``quote-antiquote''.  A \emph{quotation} is a syntactic
 entity delimited by an implicit abstraction, say over the state
 space.  An \emph{antiquotation} is a marked expression within a
 quotation that refers the implicit argument; a typical antiquotation
 would select (or even update) components from the state.

 We will see some examples later in the concrete rules and
 applications.
*}

text {*
 The following specification of syntax and translations is for
 Isabelle experts only; feel free to ignore it.

 While the first part is still a somewhat intelligible specification
 of the concrete syntactic representation of our Hoare language, the
 actual ``ML drivers'' is quite involved.  Just note that the we
 re-use the basic quote/antiquote translations as already defined in
 Isabelle/Pure (see \verb,Syntax.quote_tr, and
 \verb,Syntax.quote_tr',).
*}

syntax
  "_quote"       :: "'b => ('a => 'b)"       ("(.'(_').)" [0] 1000)
  "_antiquote"   :: "('a => 'b) => 'b"       ("\<acute>_" [1000] 1000)
  "_Subst"       :: "'a bexp \<Rightarrow> 'b \<Rightarrow> idt \<Rightarrow> 'a bexp"
        ("_[_'/\<acute>_]" [1000] 999)
  "_Assert"      :: "'a => 'a set"           ("(.{_}.)" [0] 1000)
  "_Assign"      :: "idt => 'b => 'a com"    ("(\<acute>_ :=/ _)" [70, 65] 61)
  "_Cond"        :: "'a bexp => 'a com => 'a com => 'a com"
        ("(0IF _/ THEN _/ ELSE _/ FI)" [0, 0, 0] 61)
  "_While_inv"   :: "'a bexp => 'a assn => 'a com => 'a com"
        ("(0WHILE _/ INV _ //DO _ /OD)"  [0, 0, 0] 61)
  "_While"       :: "'a bexp => 'a com => 'a com"
        ("(0WHILE _ //DO _ /OD)"  [0, 0] 61)

syntax (xsymbols)
  "_Assert"      :: "'a => 'a set"            ("(\<lbrace>_\<rbrace>)" [0] 1000)

translations
  ".{b}."                   => "Collect .(b)."
  "B [a/\<acute>x]"                => ".{\<acute>(_update_name x a) \<in> B}."
  "\<acute>x := a"                 => "Basic .(\<acute>(_update_name x a))."
  "IF b THEN c1 ELSE c2 FI" => "Cond .{b}. c1 c2"
  "WHILE b INV i DO c OD"   => "While .{b}. i c"
  "WHILE b DO c OD"         == "WHILE b INV arbitrary DO c OD"

parse_translation {*
  let
    fun quote_tr [t] = Syntax.quote_tr "_antiquote" t
      | quote_tr ts = raise TERM ("quote_tr", ts);
  in [("_quote", quote_tr)] end
*}

text {*
 As usual in Isabelle syntax translations, the part for printing is
 more complicated --- we cannot express parts as macro rules as above.
 Don't look here, unless you have to do similar things for yourself.
*}

print_translation {*
  let
    fun quote_tr' f (t :: ts) =
          Term.list_comb (f $ Syntax.quote_tr' "_antiquote" t, ts)
      | quote_tr' _ _ = raise Match;

    val assert_tr' = quote_tr' (Syntax.const "_Assert");

    fun bexp_tr' name ((Const ("Collect", _) $ t) :: ts) =
          quote_tr' (Syntax.const name) (t :: ts)
      | bexp_tr' _ _ = raise Match;

    fun upd_tr' (x_upd, T) =
      (case try (unsuffix RecordPackage.updateN) x_upd of
        Some x => (x, if T = dummyT then T else Term.domain_type T)
      | None => raise Match);

    fun update_name_tr' (Free x) = Free (upd_tr' x)
      | update_name_tr' ((c as Const ("_free", _)) $ Free x) =
          c $ Free (upd_tr' x)
      | update_name_tr' (Const x) = Const (upd_tr' x)
      | update_name_tr' _ = raise Match;

    fun assign_tr' (Abs (x, _, f $ t $ Bound 0) :: ts) =
          quote_tr' (Syntax.const "_Assign" $ update_name_tr' f)
            (Abs (x, dummyT, t) :: ts)
      | assign_tr' _ = raise Match;
  in
    [("Collect", assert_tr'), ("Basic", assign_tr'),
      ("Cond", bexp_tr' "_Cond"), ("While", bexp_tr' "_While_inv")]
  end
*}


subsection {* Rules for single-step proof \label{sec:hoare-isar} *}

text {*
 We are now ready to introduce a set of Hoare rules to be used in
 single-step structured proofs in Isabelle/Isar.  We refer to the
 concrete syntax introduce above.

 \medskip Assertions of Hoare Logic may be manipulated in
 calculational proofs, with the inclusion expressed in terms of sets
 or predicates.  Reversed order is supported as well.
*}

lemma [trans]: "|- P c Q ==> P' <= P ==> |- P' c Q"
  by (unfold Valid_def) blast
lemma [trans] : "P' <= P ==> |- P c Q ==> |- P' c Q"
  by (unfold Valid_def) blast

lemma [trans]: "Q <= Q' ==> |- P c Q ==> |- P c Q'"
  by (unfold Valid_def) blast
lemma [trans]: "|- P c Q ==> Q <= Q' ==> |- P c Q'"
  by (unfold Valid_def) blast

lemma [trans]:
    "|- .{\<acute>P}. c Q ==> (!!s. P' s --> P s) ==> |- .{\<acute>P'}. c Q"
  by (simp add: Valid_def)
lemma [trans]:
    "(!!s. P' s --> P s) ==> |- .{\<acute>P}. c Q ==> |- .{\<acute>P'}. c Q"
  by (simp add: Valid_def)

lemma [trans]:
    "|- P c .{\<acute>Q}. ==> (!!s. Q s --> Q' s) ==> |- P c .{\<acute>Q'}."
  by (simp add: Valid_def)
lemma [trans]:
    "(!!s. Q s --> Q' s) ==> |- P c .{\<acute>Q}. ==> |- P c .{\<acute>Q'}."
  by (simp add: Valid_def)


text {*
 Identity and basic assignments.\footnote{The $\idt{hoare}$ method
 introduced in \S\ref{sec:hoare-vcg} is able to provide proper
 instances for any number of basic assignments, without producing
 additional verification conditions.}
*}

lemma skip [intro?]: "|- P SKIP P"
proof -
  have "|- {s. id s : P} SKIP P" by (rule basic)
  thus ?thesis by simp
qed

lemma assign: "|- P [\<acute>a/\<acute>x] \<acute>x := \<acute>a P"
  by (rule basic)

text {*
 Note that above formulation of assignment corresponds to our
 preferred way to model state spaces, using (extensible) record types
 in HOL \cite{Naraschewski-Wenzel:1998:HOOL}.  For any record field
 $x$, Isabelle/HOL provides a functions $x$ (selector) and
 $\idt{x{\dsh}update}$ (update).  Above, there is only a place-holder
 appearing for the latter kind of function: due to concrete syntax
 \isa{\'x := \'a} also contains \isa{x\_update}.\footnote{Note that due
 to the external nature of HOL record fields, we could not even state
 a general theorem relating selector and update functions (if this
 were required here); this would only work for any particular instance
 of record fields introduced so far.}
*}

text {*
 Sequential composition --- normalizing with associativity achieves
 proper of chunks of code verified separately.
*}

lemmas [trans, intro?] = seq

lemma seq_assoc [simp]: "( |- P c1;(c2;c3) Q) = ( |- P (c1;c2);c3 Q)"
  by (auto simp add: Valid_def)

text {*
 Conditional statements.
*}

lemmas [trans, intro?] = cond

lemma [trans, intro?]:
  "|- .{\<acute>P & \<acute>b}. c1 Q
      ==> |- .{\<acute>P & ~ \<acute>b}. c2 Q
      ==> |- .{\<acute>P}. IF \<acute>b THEN c1 ELSE c2 FI Q"
    by (rule cond) (simp_all add: Valid_def)

text {*
 While statements --- with optional invariant.
*}

lemma [intro?]:
    "|- (P Int b) c P ==> |- P (While b P c) (P Int -b)"
  by (rule while)

lemma [intro?]:
    "|- (P Int b) c P ==> |- P (While b arbitrary c) (P Int -b)"
  by (rule while)


lemma [intro?]:
  "|- .{\<acute>P & \<acute>b}. c .{\<acute>P}.
    ==> |- .{\<acute>P}. WHILE \<acute>b INV .{\<acute>P}. DO c OD .{\<acute>P & ~ \<acute>b}."
  by (simp add: while Collect_conj_eq Collect_neg_eq)

lemma [intro?]:
  "|- .{\<acute>P & \<acute>b}. c .{\<acute>P}.
    ==> |- .{\<acute>P}. WHILE \<acute>b DO c OD .{\<acute>P & ~ \<acute>b}."
  by (simp add: while Collect_conj_eq Collect_neg_eq)


subsection {* Verification conditions \label{sec:hoare-vcg} *}

text {*
 We now load the \emph{original} ML file for proof scripts and tactic
 definition for the Hoare Verification Condition Generator (see
 \url{http://isabelle.in.tum.de/library/Hoare/}).  As far as we are
 concerned here, the result is a proof method \name{hoare}, which may
 be applied to a Hoare Logic assertion to extract purely logical
 verification conditions.  It is important to note that the method
 requires \texttt{WHILE} loops to be fully annotated with invariants
 beforehand.  Furthermore, only \emph{concrete} pieces of code are
 handled --- the underlying tactic fails ungracefully if supplied with
 meta-variables or parameters, for example.
*}

ML {* val Valid_def = thm "Valid_def" *}
use "~~/src/HOL/Hoare/Hoare.ML"

method_setup hoare = {*
  Method.no_args
    (Method.SIMPLE_METHOD' HEADGOAL (hoare_tac (K all_tac))) *}
  "verification condition generator for Hoare logic"

end