Theory Vars

(* Author: Tobias Nipkow *)

section "Definite Initialization Analysis"

theory Vars imports Com

subsection "The Variables in an Expression"

text‹We need to collect the variables in both arithmetic and boolean
expressions. For a change we do not introduce two functions, e.g.\ avars› and bvars›, but we overload the name vars›
via a \emph{type class}, a device that originated with Haskell:›
class vars =
fixes vars :: "'a  vname set"

text‹This defines a type class ``vars'' with a single
function of (coincidentally) the same name. Then we define two separated
instances of the class, one for typaexp and one for typbexp:›

instantiation aexp :: vars

fun vars_aexp :: "aexp  vname set" where
"vars (N n) = {}" |
"vars (V x) = {x}" |
"vars (Plus a1 a2) = vars a1  vars a2"

instance ..


value "vars (Plus (V ''x'') (V ''y''))"

instantiation bexp :: vars

fun vars_bexp :: "bexp  vname set" where
"vars (Bc v) = {}" |
"vars (Not b) = vars b" |
"vars (And b1 b2) = vars b1  vars b2" |
"vars (Less a1 a2) = vars a1  vars a2"

instance ..


value "vars (Less (Plus (V ''z'') (V ''y'')) (V ''x''))"

  eq_on :: "('a  'b)  ('a  'b)  'a set  bool"
 ("(_ =/ _/ on _)" [50,0,50] 50) where
"f = g on X ==  x  X. f x = g x"

lemma aval_eq_if_eq_on_vars[simp]:
  "s1 = s2 on vars a  aval a s1 = aval a s2"
apply(induction a)
apply simp_all

lemma bval_eq_if_eq_on_vars:
  "s1 = s2 on vars b  bval b s1 = bval b s2"
proof(induction b)
  case (Less a1 a2)
  hence "aval a1 s1 = aval a1 s2" and "aval a2 s1 = aval a2 s2" by simp_all
  thus ?case by simp
qed simp_all

fun lvars :: "com  vname set" where
"lvars SKIP = {}" |
"lvars (x::=e) = {x}" |
"lvars (c1;;c2) = lvars c1  lvars c2" |
"lvars (IF b THEN c1 ELSE c2) = lvars c1  lvars c2" |
"lvars (WHILE b DO c) = lvars c"

fun rvars :: "com  vname set" where
"rvars SKIP = {}" |
"rvars (x::=e) = vars e" |
"rvars (c1;;c2) = rvars c1  rvars c2" |
"rvars (IF b THEN c1 ELSE c2) = vars b  rvars c1  rvars c2" |
"rvars (WHILE b DO c) = vars b  rvars c"

instantiation com :: vars

definition "vars_com c = lvars c  rvars c"

instance ..


lemma vars_com_simps[simp]:
  "vars SKIP = {}"
  "vars (x::=e) = {x}  vars e"
  "vars (c1;;c2) = vars c1  vars c2"
  "vars (IF b THEN c1 ELSE c2) = vars b  vars c1  vars c2"
  "vars (WHILE b DO c) = vars b  vars c"
by(auto simp: vars_com_def)

lemma finite_avars[simp]: "finite(vars(a::aexp))"
by(induction a) simp_all

lemma finite_bvars[simp]: "finite(vars(b::bexp))"
by(induction b) simp_all

lemma finite_lvars[simp]: "finite(lvars(c))"
by(induction c) simp_all

lemma finite_rvars[simp]: "finite(rvars(c))"
by(induction c) simp_all

lemma finite_cvars[simp]: "finite(vars(c::com))"
by(simp add: vars_com_def)