(* Title: HOL/Library/Efficient_Nat.thy
Author: Stefan Berghofer, Florian Haftmann, TU Muenchen
*)
header {* Implementation of natural numbers by target-language integers *}
theory Efficient_Nat
imports Code_Integer Main
begin
text {*
When generating code for functions on natural numbers, the
canonical representation using @{term "0::nat"} and
@{term "Suc"} is unsuitable for computations involving large
numbers. The efficiency of the generated code can be improved
drastically by implementing natural numbers by target-language
integers. To do this, just include this theory.
*}
subsection {* Basic arithmetic *}
text {*
Most standard arithmetic functions on natural numbers are implemented
using their counterparts on the integers:
*}
code_datatype number_nat_inst.number_of_nat
lemma zero_nat_code [code, code_unfold_post]:
"0 = (Numeral0 :: nat)"
by simp
lemma one_nat_code [code, code_unfold_post]:
"1 = (Numeral1 :: nat)"
by simp
lemma Suc_code [code]:
"Suc n = n + 1"
by simp
lemma plus_nat_code [code]:
"n + m = nat (of_nat n + of_nat m)"
by simp
lemma minus_nat_code [code]:
"n - m = nat (of_nat n - of_nat m)"
by simp
lemma times_nat_code [code]:
"n * m = nat (of_nat n * of_nat m)"
unfolding of_nat_mult [symmetric] by simp
text {* Specialized @{term "op div \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat"}
and @{term "op mod \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat"} operations. *}
definition divmod_aux :: "nat \<Rightarrow> nat \<Rightarrow> nat \<times> nat" where
[code del]: "divmod_aux = divmod_nat"
lemma [code]:
"divmod_nat n m = (if m = 0 then (0, n) else divmod_aux n m)"
unfolding divmod_aux_def divmod_nat_div_mod by simp
lemma divmod_aux_code [code]:
"divmod_aux n m = (nat (of_nat n div of_nat m), nat (of_nat n mod of_nat m))"
unfolding divmod_aux_def divmod_nat_div_mod zdiv_int [symmetric] zmod_int [symmetric] by simp
lemma eq_nat_code [code]:
"eq_class.eq n m \<longleftrightarrow> eq_class.eq (of_nat n \<Colon> int) (of_nat m)"
by (simp add: eq)
lemma eq_nat_refl [code nbe]:
"eq_class.eq (n::nat) n \<longleftrightarrow> True"
by (rule HOL.eq_refl)
lemma less_eq_nat_code [code]:
"n \<le> m \<longleftrightarrow> (of_nat n \<Colon> int) \<le> of_nat m"
by simp
lemma less_nat_code [code]:
"n < m \<longleftrightarrow> (of_nat n \<Colon> int) < of_nat m"
by simp
subsection {* Case analysis *}
text {*
Case analysis on natural numbers is rephrased using a conditional
expression:
*}
lemma [code, code_unfold]:
"nat_case = (\<lambda>f g n. if n = 0 then f else g (n - 1))"
by (auto simp add: expand_fun_eq dest!: gr0_implies_Suc)
subsection {* Preprocessors *}
text {*
In contrast to @{term "Suc n"}, the term @{term "n + (1::nat)"} is no longer
a constructor term. Therefore, all occurrences of this term in a position
where a pattern is expected (i.e.\ on the left-hand side of a recursion
equation or in the arguments of an inductive relation in an introduction
rule) must be eliminated.
This can be accomplished by applying the following transformation rules:
*}
lemma Suc_if_eq: "(\<And>n. f (Suc n) \<equiv> h n) \<Longrightarrow> f 0 \<equiv> g \<Longrightarrow>
f n \<equiv> if n = 0 then g else h (n - 1)"
by (rule eq_reflection) (cases n, simp_all)
lemma Suc_clause: "(\<And>n. P n (Suc n)) \<Longrightarrow> n \<noteq> 0 \<Longrightarrow> P (n - 1) n"
by (cases n) simp_all
text {*
The rules above are built into a preprocessor that is plugged into
the code generator. Since the preprocessor for introduction rules
does not know anything about modes, some of the modes that worked
for the canonical representation of natural numbers may no longer work.
*}
(*<*)
setup {*
let
fun remove_suc thy thms =
let
val vname = Name.variant (map fst
(fold (Term.add_var_names o Thm.full_prop_of) thms [])) "n";
val cv = cterm_of thy (Var ((vname, 0), HOLogic.natT));
fun lhs_of th = snd (Thm.dest_comb
(fst (Thm.dest_comb (cprop_of th))));
fun rhs_of th = snd (Thm.dest_comb (cprop_of th));
fun find_vars ct = (case term_of ct of
(Const (@{const_name Suc}, _) $ Var _) => [(cv, snd (Thm.dest_comb ct))]
| _ $ _ =>
let val (ct1, ct2) = Thm.dest_comb ct
in
map (apfst (fn ct => Thm.capply ct ct2)) (find_vars ct1) @
map (apfst (Thm.capply ct1)) (find_vars ct2)
end
| _ => []);
val eqs = maps
(fn th => map (pair th) (find_vars (lhs_of th))) thms;
fun mk_thms (th, (ct, cv')) =
let
val th' =
Thm.implies_elim
(Conv.fconv_rule (Thm.beta_conversion true)
(Drule.instantiate'
[SOME (ctyp_of_term ct)] [SOME (Thm.cabs cv ct),
SOME (Thm.cabs cv' (rhs_of th)), NONE, SOME cv']
@{thm Suc_if_eq})) (Thm.forall_intr cv' th)
in
case map_filter (fn th'' =>
SOME (th'', singleton
(Variable.trade (K (fn [th'''] => [th''' RS th'])) (Variable.thm_context th'')) th'')
handle THM _ => NONE) thms of
[] => NONE
| thps =>
let val (ths1, ths2) = split_list thps
in SOME (subtract Thm.eq_thm (th :: ths1) thms @ ths2) end
end
in get_first mk_thms eqs end;
fun eqn_suc_base_preproc thy thms =
let
val dest = fst o Logic.dest_equals o prop_of;
val contains_suc = exists_Const (fn (c, _) => c = @{const_name Suc});
in
if forall (can dest) thms andalso exists (contains_suc o dest) thms
then thms |> perhaps_loop (remove_suc thy) |> (Option.map o map) Drule.zero_var_indexes
else NONE
end;
val eqn_suc_preproc = Code_Preproc.simple_functrans eqn_suc_base_preproc;
fun remove_suc_clause thy thms =
let
val vname = Name.variant (map fst
(fold (Term.add_var_names o Thm.full_prop_of) thms [])) "x";
fun find_var (t as Const (@{const_name Suc}, _) $ (v as Var _)) = SOME (t, v)
| find_var (t $ u) = (case find_var t of NONE => find_var u | x => x)
| find_var _ = NONE;
fun find_thm th =
let val th' = Conv.fconv_rule ObjectLogic.atomize th
in Option.map (pair (th, th')) (find_var (prop_of th')) end
in
case get_first find_thm thms of
NONE => thms
| SOME ((th, th'), (Sucv, v)) =>
let
val cert = cterm_of (Thm.theory_of_thm th);
val th'' = ObjectLogic.rulify (Thm.implies_elim
(Conv.fconv_rule (Thm.beta_conversion true)
(Drule.instantiate' []
[SOME (cert (lambda v (Abs ("x", HOLogic.natT,
abstract_over (Sucv,
HOLogic.dest_Trueprop (prop_of th')))))),
SOME (cert v)] @{thm Suc_clause}))
(Thm.forall_intr (cert v) th'))
in
remove_suc_clause thy (map (fn th''' =>
if (op = o pairself prop_of) (th''', th) then th'' else th''') thms)
end
end;
fun clause_suc_preproc thy ths =
let
val dest = fst o HOLogic.dest_mem o HOLogic.dest_Trueprop
in
if forall (can (dest o concl_of)) ths andalso
exists (fn th => exists (exists_Const (fn (c, _) => c = @{const_name Suc}))
(map_filter (try dest) (concl_of th :: prems_of th))) ths
then remove_suc_clause thy ths else ths
end;
in
Code_Preproc.add_functrans ("eqn_Suc", eqn_suc_preproc)
#> Codegen.add_preprocessor clause_suc_preproc
end;
*}
(*>*)
subsection {* Target language setup *}
text {*
For ML, we map @{typ nat} to target language integers, where we
assert that values are always non-negative.
*}
code_type nat
(SML "IntInf.int")
(OCaml "Big'_int.big'_int")
types_code
nat ("int")
attach (term_of) {*
val term_of_nat = HOLogic.mk_number HOLogic.natT;
*}
attach (test) {*
fun gen_nat i =
let val n = random_range 0 i
in (n, fn () => term_of_nat n) end;
*}
text {*
For Haskell we define our own @{typ nat} type. The reason
is that we have to distinguish type class instances
for @{typ nat} and @{typ int}.
*}
code_include Haskell "Nat" {*
newtype Nat = Nat Integer deriving (Show, Eq);
instance Num Nat where {
fromInteger k = Nat (if k >= 0 then k else 0);
Nat n + Nat m = Nat (n + m);
Nat n - Nat m = fromInteger (n - m);
Nat n * Nat m = Nat (n * m);
abs n = n;
signum _ = 1;
negate n = error "negate Nat";
};
instance Ord Nat where {
Nat n <= Nat m = n <= m;
Nat n < Nat m = n < m;
};
instance Real Nat where {
toRational (Nat n) = toRational n;
};
instance Enum Nat where {
toEnum k = fromInteger (toEnum k);
fromEnum (Nat n) = fromEnum n;
};
instance Integral Nat where {
toInteger (Nat n) = n;
divMod n m = quotRem n m;
quotRem (Nat n) (Nat m) = (Nat k, Nat l) where (k, l) = quotRem n m;
};
*}
code_reserved Haskell Nat
code_type nat
(Haskell "Nat.Nat")
code_instance nat :: eq
(Haskell -)
text {*
Natural numerals.
*}
lemma [code_unfold_post]:
"nat (number_of i) = number_nat_inst.number_of_nat i"
-- {* this interacts as desired with @{thm nat_number_of_def} *}
by (simp add: number_nat_inst.number_of_nat)
setup {*
fold (Numeral.add_code @{const_name number_nat_inst.number_of_nat}
false true) ["SML", "OCaml", "Haskell"]
*}
text {*
Since natural numbers are implemented
using integers in ML, the coercion function @{const "of_nat"} of type
@{typ "nat \<Rightarrow> int"} is simply implemented by the identity function.
For the @{const "nat"} function for converting an integer to a natural
number, we give a specific implementation using an ML function that
returns its input value, provided that it is non-negative, and otherwise
returns @{text "0"}.
*}
definition int :: "nat \<Rightarrow> int" where
[code del]: "int = of_nat"
lemma int_code' [code]:
"int (number_of l) = (if neg (number_of l \<Colon> int) then 0 else number_of l)"
unfolding int_nat_number_of [folded int_def] ..
lemma nat_code' [code]:
"nat (number_of l) = (if neg (number_of l \<Colon> int) then 0 else number_of l)"
unfolding nat_number_of_def number_of_is_id neg_def by simp
lemma of_nat_int [code_unfold_post]:
"of_nat = int" by (simp add: int_def)
lemma of_nat_aux_int [code_unfold]:
"of_nat_aux (\<lambda>i. i + 1) k 0 = int k"
by (simp add: int_def Nat.of_nat_code)
code_const int
(SML "_")
(OCaml "_")
consts_code
int ("(_)")
nat ("\<module>nat")
attach {*
fun nat i = if i < 0 then 0 else i;
*}
code_const nat
(SML "IntInf.max/ (/0,/ _)")
(OCaml "Big'_int.max'_big'_int/ Big'_int.zero'_big'_int")
text {* For Haskell, things are slightly different again. *}
code_const int and nat
(Haskell "toInteger" and "fromInteger")
text {* Conversion from and to indices. *}
code_const Code_Numeral.of_nat
(SML "IntInf.toInt")
(OCaml "_")
(Haskell "fromEnum")
code_const Code_Numeral.nat_of
(SML "IntInf.fromInt")
(OCaml "_")
(Haskell "toEnum")
text {* Using target language arithmetic operations whenever appropriate *}
code_const "op + \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat"
(SML "IntInf.+ ((_), (_))")
(OCaml "Big'_int.add'_big'_int")
(Haskell infixl 6 "+")
code_const "op * \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat"
(SML "IntInf.* ((_), (_))")
(OCaml "Big'_int.mult'_big'_int")
(Haskell infixl 7 "*")
code_const divmod_aux
(SML "IntInf.divMod/ ((_),/ (_))")
(OCaml "Big'_int.quomod'_big'_int")
(Haskell "divMod")
code_const "eq_class.eq \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool"
(SML "!((_ : IntInf.int) = _)")
(OCaml "Big'_int.eq'_big'_int")
(Haskell infixl 4 "==")
code_const "op \<le> \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool"
(SML "IntInf.<= ((_), (_))")
(OCaml "Big'_int.le'_big'_int")
(Haskell infix 4 "<=")
code_const "op < \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool"
(SML "IntInf.< ((_), (_))")
(OCaml "Big'_int.lt'_big'_int")
(Haskell infix 4 "<")
consts_code
"0::nat" ("0")
"1::nat" ("1")
Suc ("(_ +/ 1)")
"op + \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat" ("(_ +/ _)")
"op * \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat" ("(_ */ _)")
"op \<le> \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool" ("(_ <=/ _)")
"op < \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool" ("(_ </ _)")
text {* Evaluation *}
lemma [code, code del]:
"(Code_Evaluation.term_of \<Colon> nat \<Rightarrow> term) = Code_Evaluation.term_of" ..
code_const "Code_Evaluation.term_of \<Colon> nat \<Rightarrow> term"
(SML "HOLogic.mk'_number/ HOLogic.natT")
text {* Module names *}
code_modulename SML
Efficient_Nat Arith
code_modulename OCaml
Efficient_Nat Arith
code_modulename Haskell
Efficient_Nat Arith
hide const int
end