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"

 text_raw{*\snip{AExpaexpdef}{2}{1}{% *}

 datatype aexp = N int | V vname | Plus aexp aexp

 text_raw{*}%endsnip*}

 text_raw{*\snip{AExpavaldef}{1}{2}{% *}

 fun aval :: "aexp \<Rightarrow> state \<Rightarrow> val" where

 "aval (N n) s = n" |

 "aval (V x) s = s x" |

 "aval (Plus a\<^sub>1 a\<^sub>2) s = aval a\<^sub>1 s + aval a\<^sub>2 s"

 text_raw{*}%endsnip*}

 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 {* \noindent

   We can now write a series of updates to the function @{text "\<lambda>x. 0"} compactly:

 *}

 lemma "<a := 1, b := 2> = (<> (a := 1)) (b := (2::int))"

   by (rule refl)

 value "aval (Plus (V ''x'') (N 5)) <''x'' := 7>"

 text {* In  the @{term[source] "<a := b>"} syntax, variables that are not mentioned are 0 by default:

 *}

 value "aval (Plus (V ''x'') (N 5)) <''y'' := 7>"

 text{* Note that this @{text"<\<dots>>"} syntax works for any function space

 @{text"\<tau>\<^sub>1 \<Rightarrow> \<tau>\<^sub>2"} where @{text "\<tau>\<^sub>2"} has a @{text 0}. *}

 subsection "Constant Folding"

 text{* Evaluate constant subsexpressions: *}

 text_raw{*\snip{AExpasimpconstdef}{0}{2}{% *}

 fun asimp_const :: "aexp \<Rightarrow> aexp" where

 "asimp_const (N n) = N n" |

 "asimp_const (V x) = V x" |

 "asimp_const (Plus a\<^sub>1 a\<^sub>2) =

   (case (asimp_const a\<^sub>1, asimp_const a\<^sub>2) of

     (N n\<^sub>1, N n\<^sub>2) \<Rightarrow> N(n\<^sub>1+n\<^sub>2) |

     (b\<^sub>1,b\<^sub>2) \<Rightarrow> Plus b\<^sub>1 b\<^sub>2)"

 text_raw{*}%endsnip*}

 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: *}

 text_raw{*\snip{AExpplusdef}{0}{2}{% *}

 fun plus :: "aexp \<Rightarrow> aexp \<Rightarrow> aexp" where

 "plus (N i\<^sub>1) (N i\<^sub>2) = N(i\<^sub>1+i\<^sub>2)" |

 "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 a\<^sub>1 a\<^sub>2 = Plus a\<^sub>1 a\<^sub>2"

 text_raw{*}%endsnip*}

 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

 text_raw{*\snip{AExpasimpdef}{2}{0}{% *}

 fun asimp :: "aexp \<Rightarrow> aexp" where

 "asimp (N n) = N n" |

 "asimp (V x) = V x" |

 "asimp (Plus a\<^sub>1 a\<^sub>2) = plus (asimp a\<^sub>1) (asimp a\<^sub>2)"

 text_raw{*}%endsnip*}

 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