Theory Countable

theory Countable
imports Rat Nat_Bijection
(*  Title:      HOL/Library/Countable.thy
Author: Alexander Krauss, TU Muenchen
Author: Brian Huffman, Portland State University
*)


header {* Encoding (almost) everything into natural numbers *}

theory Countable
imports Main Rat Nat_Bijection
begin

subsection {* The class of countable types *}

class countable =
assumes ex_inj: "∃to_nat :: 'a => nat. inj to_nat"

lemma countable_classI:
fixes f :: "'a => nat"
assumes "!!x y. f x = f y ==> x = y"
shows "OFCLASS('a, countable_class)"
proof (intro_classes, rule exI)
show "inj f"
by (rule injI [OF assms]) assumption
qed


subsection {* Conversion functions *}

definition to_nat :: "'a::countable => nat" where
"to_nat = (SOME f. inj f)"

definition from_nat :: "nat => 'a::countable" where
"from_nat = inv (to_nat :: 'a => nat)"

lemma inj_to_nat [simp]: "inj to_nat"
by (rule exE_some [OF ex_inj]) (simp add: to_nat_def)

lemma inj_on_to_nat[simp, intro]: "inj_on to_nat S"
using inj_to_nat by (auto simp: inj_on_def)

lemma surj_from_nat [simp]: "surj from_nat"
unfolding from_nat_def by (simp add: inj_imp_surj_inv)

lemma to_nat_split [simp]: "to_nat x = to_nat y <-> x = y"
using injD [OF inj_to_nat] by auto

lemma from_nat_to_nat [simp]:
"from_nat (to_nat x) = x"
by (simp add: from_nat_def)


subsection {* Countable types *}

instance nat :: countable
by (rule countable_classI [of "id"]) simp

subclass (in finite) countable
proof
have "finite (UNIV::'a set)" by (rule finite_UNIV)
with finite_conv_nat_seg_image [of "UNIV::'a set"]
obtain n and f :: "nat => 'a"
where "UNIV = f ` {i. i < n}" by auto
then have "surj f" unfolding surj_def by auto
then have "inj (inv f)" by (rule surj_imp_inj_inv)
then show "∃to_nat :: 'a => nat. inj to_nat" by (rule exI[of inj])
qed

text {* Pairs *}

instance prod :: (countable, countable) countable
by (rule countable_classI [of "λ(x, y). prod_encode (to_nat x, to_nat y)"])
(auto simp add: prod_encode_eq)


text {* Sums *}

instance sum :: (countable, countable) countable
by (rule countable_classI [of "(λx. case x of Inl a => to_nat (False, to_nat a)
| Inr b => to_nat (True, to_nat b))"
])
(simp split: sum.split_asm)


text {* Integers *}

instance int :: countable
by (rule countable_classI [of "int_encode"])
(simp add: int_encode_eq)


text {* Options *}

instance option :: (countable) countable
by (rule countable_classI [of "option_case 0 (Suc o to_nat)"])
(simp split: option.split_asm)


text {* Lists *}

instance list :: (countable) countable
by (rule countable_classI [of "list_encode o map to_nat"])
(simp add: list_encode_eq)


text {* Further *}

instance String.literal :: countable
by (rule countable_classI [of "to_nat o explode"])
(auto simp add: explode_inject)

text {* Functions *}

instance "fun" :: (finite, countable) countable
proof
obtain xs :: "'a list" where xs: "set xs = UNIV"
using finite_list [OF finite_UNIV] ..
show "∃to_nat::('a => 'b) => nat. inj to_nat"
proof
show "inj (λf. to_nat (map f xs))"
by (rule injI, simp add: xs fun_eq_iff)
qed
qed


subsection {* The Rationals are Countably Infinite *}

definition nat_to_rat_surj :: "nat => rat" where
"nat_to_rat_surj n = (let (a,b) = prod_decode n
in Fract (int_decode a) (int_decode b))"


lemma surj_nat_to_rat_surj: "surj nat_to_rat_surj"
unfolding surj_def
proof
fix r::rat
show "∃n. r = nat_to_rat_surj n"
proof (cases r)
fix i j assume [simp]: "r = Fract i j" and "j > 0"
have "r = (let m = int_encode i; n = int_encode j
in nat_to_rat_surj(prod_encode (m,n)))"

by (simp add: Let_def nat_to_rat_surj_def)
thus "∃n. r = nat_to_rat_surj n" by(auto simp:Let_def)
qed
qed

lemma Rats_eq_range_nat_to_rat_surj: "\<rat> = range nat_to_rat_surj"
by (simp add: Rats_def surj_nat_to_rat_surj)

context field_char_0
begin

lemma Rats_eq_range_of_rat_o_nat_to_rat_surj:
"\<rat> = range (of_rat o nat_to_rat_surj)"
using surj_nat_to_rat_surj
by (auto simp: Rats_def image_def surj_def)
(blast intro: arg_cong[where f = of_rat])

lemma surj_of_rat_nat_to_rat_surj:
"r∈\<rat> ==> ∃n. r = of_rat(nat_to_rat_surj n)"
by(simp add: Rats_eq_range_of_rat_o_nat_to_rat_surj image_def)

end

instance rat :: countable
proof
show "∃to_nat::rat => nat. inj to_nat"
proof
have "surj nat_to_rat_surj"
by (rule surj_nat_to_rat_surj)
then show "inj (inv nat_to_rat_surj)"
by (rule surj_imp_inj_inv)
qed
qed


subsection {* Automatically proving countability of datatypes *}

inductive finite_item :: "'a Datatype.item => bool" where
undefined: "finite_item undefined"
| In0: "finite_item x ==> finite_item (Datatype.In0 x)"
| In1: "finite_item x ==> finite_item (Datatype.In1 x)"
| Leaf: "finite_item (Datatype.Leaf a)"
| Scons: "[|finite_item x; finite_item y|] ==> finite_item (Datatype.Scons x y)"

function
nth_item :: "nat => ('a::countable) Datatype.item"
where
"nth_item 0 = undefined"
| "nth_item (Suc n) =
(case sum_decode n of
Inl i =>
(case sum_decode i of
Inl j => Datatype.In0 (nth_item j)
| Inr j => Datatype.In1 (nth_item j))
| Inr i =>
(case sum_decode i of
Inl j => Datatype.Leaf (from_nat j)
| Inr j =>
(case prod_decode j of
(a, b) => Datatype.Scons (nth_item a) (nth_item b))))"

by pat_completeness auto

lemma le_sum_encode_Inl: "x ≤ y ==> x ≤ sum_encode (Inl y)"
unfolding sum_encode_def by simp

lemma le_sum_encode_Inr: "x ≤ y ==> x ≤ sum_encode (Inr y)"
unfolding sum_encode_def by simp

termination
by (relation "measure id")
(auto simp add: sum_encode_eq [symmetric] prod_encode_eq [symmetric]
le_imp_less_Suc le_sum_encode_Inl le_sum_encode_Inr
le_prod_encode_1 le_prod_encode_2)

lemma nth_item_covers: "finite_item x ==> ∃n. nth_item n = x"
proof (induct set: finite_item)
case undefined
have "nth_item 0 = undefined" by simp
thus ?case ..
next
case (In0 x)
then obtain n where "nth_item n = x" by fast
hence "nth_item (Suc (sum_encode (Inl (sum_encode (Inl n)))))
= Datatype.In0 x"
by simp
thus ?case ..
next
case (In1 x)
then obtain n where "nth_item n = x" by fast
hence "nth_item (Suc (sum_encode (Inl (sum_encode (Inr n)))))
= Datatype.In1 x"
by simp
thus ?case ..
next
case (Leaf a)
have "nth_item (Suc (sum_encode (Inr (sum_encode (Inl (to_nat a))))))
= Datatype.Leaf a"
by simp
thus ?case ..
next
case (Scons x y)
then obtain i j where "nth_item i = x" and "nth_item j = y" by fast
hence "nth_item
(Suc (sum_encode (Inr (sum_encode (Inr (prod_encode (i, j)))))))
= Datatype.Scons x y"
by simp
thus ?case ..
qed

theorem countable_datatype:
fixes Rep :: "'b => ('a::countable) Datatype.item"
fixes Abs :: "('a::countable) Datatype.item => 'b"
fixes rep_set :: "('a::countable) Datatype.item => bool"
assumes type: "type_definition Rep Abs (Collect rep_set)"
assumes finite_item: "!!x. rep_set x ==> finite_item x"
shows "OFCLASS('b, countable_class)"
proof
def f "λy. LEAST n. nth_item n = Rep y"
{
fix y :: 'b
have "rep_set (Rep y)"
using type_definition.Rep [OF type] by simp
hence "finite_item (Rep y)"
by (rule finite_item)
hence "∃n. nth_item n = Rep y"
by (rule nth_item_covers)
hence "nth_item (f y) = Rep y"
unfolding f_def by (rule LeastI_ex)
hence "Abs (nth_item (f y)) = y"
using type_definition.Rep_inverse [OF type] by simp
}
hence "inj f"
by (rule inj_on_inverseI)
thus "∃f::'b => nat. inj f"
by - (rule exI)
qed

ML {*
fun countable_tac ctxt =
SUBGOAL (fn (goal, i) =>
let
val ty_name =
(case goal of
(_ $ Const (@{const_name TYPE}, Type (@{type_name itself}, [Type (n, _)]))) => n
| _ => raise Match)
val typedef_info = hd (Typedef.get_info ctxt ty_name)
val typedef_thm = #type_definition (snd typedef_info)
val pred_name =
(case HOLogic.dest_Trueprop (concl_of typedef_thm) of
(typedef $ rep $ abs $ (collect $ Const (n, _))) => n
| _ => raise Match)
val induct_info = Inductive.the_inductive ctxt pred_name
val pred_names = #names (fst induct_info)
val induct_thms = #inducts (snd induct_info)
val alist = pred_names ~~ induct_thms
val induct_thm = the (AList.lookup (op =) alist pred_name)
val vars = rev (Term.add_vars (Thm.prop_of induct_thm) [])
val thy = Proof_Context.theory_of ctxt
val insts = vars |> map (fn (_, T) => try (Thm.cterm_of thy)
(Const (@{const_name Countable.finite_item}, T)))
val induct_thm' = Drule.instantiate' [] insts induct_thm
val rules = @{thms finite_item.intros}
in
SOLVED' (fn i => EVERY
[rtac @{thm countable_datatype} i,
rtac typedef_thm i,
etac induct_thm' i,
REPEAT (resolve_tac rules i ORELSE atac i)]) 1
end)
*}


method_setup countable_datatype = {*
Scan.succeed (fn ctxt => SIMPLE_METHOD' (countable_tac ctxt))
*}
"prove countable class instances for datatypes"

hide_const (open) finite_item nth_item


subsection {* Countable datatypes *}

instance typerep :: countable
by countable_datatype

end