header "Arithmetic and Boolean Expressions"
theory AExp imports Main begin
subsection "Arithmetic Expressions"
type_synonym vname = string
type_synonym val = int
type_synonym state = "vname \<Rightarrow> val"
datatype aexp = N int | V vname | Plus aexp aexp
fun aval :: "aexp \<Rightarrow> state \<Rightarrow> val" where
"aval (N n) s = n" |
"aval (V x) s = s x" |
"aval (Plus a1 a2) s = aval a1 s + aval a2 s"
value "aval (Plus (V ''x'') (N 5)) (\<lambda>x. if x = ''x'' then 7 else 0)"
text {* The same state more concisely: *}
value "aval (Plus (V ''x'') (N 5)) ((\<lambda>x. 0) (''x'':= 7))"
text {* A little syntax magic to write larger states compactly: *}
definition null_state ("<>") where
"null_state \<equiv> \<lambda>x. 0"
syntax
"_State" :: "updbinds => 'a" ("<_>")
translations
"_State ms" => "_Update <> ms"
text {*
We can now write a series of updates to the function @{text "\<lambda>x. 0"} compactly:
*}
lemma "<a := Suc 0, b := 2> = (<> (a := Suc 0)) (b := 2)"
by (rule refl)
value "aval (Plus (V ''x'') (N 5)) <''x'' := 7>"
text {* Variables that are not mentioned in the state are 0 by default in
the @{term "<a := b::int>"} syntax:
*}
value "aval (Plus (V ''x'') (N 5)) <''y'' := 7>"
text{* Note that this @{text"<\<dots>>"} syntax works for any function space
@{text"\<tau>\<^isub>1 \<Rightarrow> \<tau>\<^isub>2"} where @{text "\<tau>\<^isub>2"} has a @{text 0}. *}
subsection "Optimization"
text{* Evaluate constant subsexpressions: *}
fun asimp_const :: "aexp \<Rightarrow> aexp" where
"asimp_const (N n) = N n" |
"asimp_const (V x) = V x" |
"asimp_const (Plus a1 a2) =
(case (asimp_const a1, asimp_const a2) of
(N n1, N n2) \<Rightarrow> N(n1+n2) |
(a1',a2') \<Rightarrow> Plus a1' a2')"
theorem aval_asimp_const:
"aval (asimp_const a) s = aval a s"
apply(induction a)
apply (auto split: aexp.split)
done
text{* Now we also eliminate all occurrences 0 in additions. The standard
method: optimized versions of the constructors: *}
fun plus :: "aexp \<Rightarrow> aexp \<Rightarrow> aexp" where
"plus (N i1) (N i2) = N(i1+i2)" |
"plus (N i) a = (if i=0 then a else Plus (N i) a)" |
"plus a (N i) = (if i=0 then a else Plus a (N i))" |
"plus a1 a2 = Plus a1 a2"
lemma aval_plus[simp]:
"aval (plus a1 a2) s = aval a1 s + aval a2 s"
apply(induction a1 a2 rule: plus.induct)
apply simp_all (* just for a change from auto *)
done
fun asimp :: "aexp \<Rightarrow> aexp" where
"asimp (N n) = N n" |
"asimp (V x) = V x" |
"asimp (Plus a1 a2) = plus (asimp a1) (asimp a2)"
text{* Note that in @{const asimp_const} the optimized constructor was
inlined. Making it a separate function @{const plus} improves modularity of
the code and the proofs. *}
value "asimp (Plus (Plus (N 0) (N 0)) (Plus (V ''x'') (N 0)))"
theorem aval_asimp[simp]:
"aval (asimp a) s = aval a s"
apply(induction a)
apply simp_all
done
end