(* Author: Lukas Bulwahn, TU Muenchen *)
header {* Lazy sequences *}
theory Lazy_Sequence
imports List Code_Numeral
begin
datatype 'a lazy_sequence = Empty | Insert 'a "'a lazy_sequence"
definition Lazy_Sequence :: "(unit => ('a * 'a lazy_sequence) option) => 'a lazy_sequence"
where
"Lazy_Sequence f = (case f () of None => Empty | Some (x, xq) => Insert x xq)"
code_datatype Lazy_Sequence
primrec yield :: "'a lazy_sequence => ('a * 'a lazy_sequence) option"
where
"yield Empty = None"
| "yield (Insert x xq) = Some (x, xq)"
lemma [simp]: "yield xq = Some (x, xq') ==> size xq' < size xq"
by (cases xq) auto
lemma yield_Seq [code]:
"yield (Lazy_Sequence f) = f ()"
unfolding Lazy_Sequence_def by (cases "f ()") auto
lemma Seq_yield:
"Lazy_Sequence (%u. yield f) = f"
unfolding Lazy_Sequence_def by (cases f) auto
lemma lazy_sequence_size_code [code]:
"lazy_sequence_size s xq = (case yield xq of None => 0 | Some (x, xq') => s x + lazy_sequence_size s xq' + 1)"
by (cases xq) auto
lemma size_code [code]:
"size xq = (case yield xq of None => 0 | Some (x, xq') => size xq' + 1)"
by (cases xq) auto
lemma [code]: "eq_class.eq xq yq = (case (yield xq, yield yq) of
(None, None) => True | (Some (x, xq'), Some (y, yq')) => (HOL.eq x y) \<and> (eq_class.eq xq yq) | _ => False)"
apply (cases xq) apply (cases yq) apply (auto simp add: eq_class.eq_equals)
apply (cases yq) apply (auto simp add: eq_class.eq_equals) done
lemma seq_case [code]:
"lazy_sequence_case f g xq = (case (yield xq) of None => f | Some (x, xq') => g x xq')"
by (cases xq) auto
lemma [code]: "lazy_sequence_rec f g xq = (case (yield xq) of None => f | Some (x, xq') => g x xq' (lazy_sequence_rec f g xq'))"
by (cases xq) auto
definition empty :: "'a lazy_sequence"
where
[code]: "empty = Lazy_Sequence (%u. None)"
definition single :: "'a => 'a lazy_sequence"
where
[code]: "single x = Lazy_Sequence (%u. Some (x, empty))"
primrec append :: "'a lazy_sequence => 'a lazy_sequence => 'a lazy_sequence"
where
"append Empty yq = yq"
| "append (Insert x xq) yq = Insert x (append xq yq)"
lemma [code]:
"append xq yq = Lazy_Sequence (%u. case yield xq of
None => yield yq
| Some (x, xq') => Some (x, append xq' yq))"
unfolding Lazy_Sequence_def
apply (cases "xq")
apply auto
apply (cases "yq")
apply auto
done
primrec flat :: "'a lazy_sequence lazy_sequence => 'a lazy_sequence"
where
"flat Empty = Empty"
| "flat (Insert xq xqq) = append xq (flat xqq)"
lemma [code]:
"flat xqq = Lazy_Sequence (%u. case yield xqq of
None => None
| Some (xq, xqq') => yield (append xq (flat xqq')))"
apply (cases "xqq")
apply (auto simp add: Seq_yield)
unfolding Lazy_Sequence_def
by auto
primrec map :: "('a => 'b) => 'a lazy_sequence => 'b lazy_sequence"
where
"map f Empty = Empty"
| "map f (Insert x xq) = Insert (f x) (map f xq)"
lemma [code]:
"map f xq = Lazy_Sequence (%u. Option.map (%(x, xq'). (f x, map f xq')) (yield xq))"
apply (cases xq)
apply (auto simp add: Seq_yield)
unfolding Lazy_Sequence_def
apply auto
done
definition bind :: "'a lazy_sequence => ('a => 'b lazy_sequence) => 'b lazy_sequence"
where
[code]: "bind xq f = flat (map f xq)"
definition if_seq :: "bool => unit lazy_sequence"
where
"if_seq b = (if b then single () else empty)"
function iterate_upto :: "(code_numeral => 'a) => code_numeral => code_numeral => 'a Lazy_Sequence.lazy_sequence"
where
"iterate_upto f n m = Lazy_Sequence.Lazy_Sequence (%u. if n > m then None else Some (f n, iterate_upto f (n + 1) m))"
by pat_completeness auto
termination by (relation "measure (%(f, n, m). Code_Numeral.nat_of (m + 1 - n))") auto
definition not_seq :: "unit lazy_sequence => unit lazy_sequence"
where
"not_seq xq = (case yield xq of None => single () | Some ((), xq) => empty)"
subsection {* Code setup *}
code_reflect
datatypes lazy_sequence = Lazy_Sequence
functions "Lazy_Sequence.map" yield
module_name Lazy_Sequence
(* FIXME formulate yieldn in the logic with type code_numeral *)
ML {*
signature LAZY_SEQUENCE =
sig
datatype 'a lazy_sequence = Lazy_Sequence of unit -> ('a * 'a lazy_sequence) option
val yield : 'a lazy_sequence -> ('a * 'a lazy_sequence) option
val yieldn : int -> 'a lazy_sequence -> ('a list * 'a lazy_sequence)
val map : ('a -> 'b) -> 'a lazy_sequence -> 'b lazy_sequence
end;
structure Lazy_Sequence : LAZY_SEQUENCE =
struct
open Lazy_Sequence;
fun map f = mapa f;
fun anamorph f k x = (if k = 0 then ([], x)
else case f x
of NONE => ([], x)
| SOME (v, y) => let
val (vs, z) = anamorph f (k - 1) y
in (v :: vs, z) end);
fun yieldn S = anamorph yield S;
end;
*}
section {* With Hit Bound Value *}
text {* assuming in negative context *}
types 'a hit_bound_lazy_sequence = "'a option lazy_sequence"
definition hit_bound :: "'a hit_bound_lazy_sequence"
where
[code]: "hit_bound = Lazy_Sequence (%u. Some (None, empty))"
definition hb_single :: "'a => 'a hit_bound_lazy_sequence"
where
[code]: "hb_single x = Lazy_Sequence (%u. Some (Some x, empty))"
primrec hb_flat :: "'a hit_bound_lazy_sequence hit_bound_lazy_sequence => 'a hit_bound_lazy_sequence"
where
"hb_flat Empty = Empty"
| "hb_flat (Insert xq xqq) = append (case xq of None => hit_bound | Some xq => xq) (hb_flat xqq)"
lemma [code]:
"hb_flat xqq = Lazy_Sequence (%u. case yield xqq of
None => None
| Some (xq, xqq') => yield (append (case xq of None => hit_bound | Some xq => xq) (hb_flat xqq')))"
apply (cases "xqq")
apply (auto simp add: Seq_yield)
unfolding Lazy_Sequence_def
by auto
primrec hb_map :: "('a => 'b) => 'a hit_bound_lazy_sequence => 'b hit_bound_lazy_sequence"
where
"hb_map f Empty = Empty"
| "hb_map f (Insert x xq) = Insert (Option.map f x) (hb_map f xq)"
lemma [code]:
"hb_map f xq = Lazy_Sequence (%u. Option.map (%(x, xq'). (Option.map f x, hb_map f xq')) (yield xq))"
apply (cases xq)
apply (auto simp add: Seq_yield)
unfolding Lazy_Sequence_def
apply auto
done
definition hb_bind :: "'a hit_bound_lazy_sequence => ('a => 'b hit_bound_lazy_sequence) => 'b hit_bound_lazy_sequence"
where
[code]: "hb_bind xq f = hb_flat (hb_map f xq)"
definition hb_if_seq :: "bool => unit hit_bound_lazy_sequence"
where
"hb_if_seq b = (if b then hb_single () else empty)"
definition hb_not_seq :: "unit hit_bound_lazy_sequence => unit lazy_sequence"
where
"hb_not_seq xq = (case yield xq of None => single () | Some (x, xq) => empty)"
hide_type (open) lazy_sequence
hide_const (open) Empty Insert Lazy_Sequence yield empty single append flat map bind if_seq iterate_upto not_seq
hide_fact yield.simps empty_def single_def append.simps flat.simps map.simps bind_def iterate_upto.simps if_seq_def not_seq_def
end