Theory Code_Lazy_Demo

(* Author: Andreas Lochbihler, Digital Asset *)

theory Code_Lazy_Demo imports
  "HOL-Library.Code_Lazy"
  "HOL-Library.Debug"
  "HOL-Library.RBT_Impl"
begin

text ‹This theory demonstrates the use of the \<^theory>‹HOL-Library.Code_Lazy› theory.›

section ‹Streams›

text ‹Lazy evaluation for streams›

codatatype 'a stream = 
  SCons (shd: 'a) (stl: "'a stream") (infixr "##" 65)

primcorec up :: "nat ⇒ nat stream" where
  "up n = n ## up (n + 1)"

primrec stake :: "nat ⇒ 'a stream ⇒ 'a list" where
  "stake 0 xs = []"
| "stake (Suc n) xs = shd xs # stake n (stl xs)"

code_thms up stake ― ‹The original code equations›

code_lazy_type stream

code_thms up stake ― ‹The lazified code equations›

value "stake 5 (up 3)"


section ‹Finite lazy lists›

text ‹Lazy types need not be infinite. We can also have lazy types that are finite.›

datatype 'a llist
  = LNil ("❙⟦❙⟧") 
  | LCons (lhd: 'a) (ltl: "'a llist") (infixr "###" 65)

syntax "_llist" :: "args => 'a list"    ("❙⟦(_)❙⟧")
translations
  "❙⟦x, xs❙⟧" == "x###❙⟦xs❙⟧"
  "❙⟦x❙⟧" == "x###❙⟦❙⟧"

fun lnth :: "nat ⇒ 'a llist ⇒ 'a" where
  "lnth 0 (x ### xs) = x"
| "lnth (Suc n) (x ### xs) = lnth n xs"

definition llist :: "nat llist" where
  "llist = ❙⟦1, 2, 3, hd [], 4❙⟧"

code_lazy_type llist

value [code] "llist"
value [code] "lnth 2 llist"
value [code] "let x = lnth 2 llist in (x, llist)"

fun lfilter :: "('a ⇒ bool) ⇒ 'a llist ⇒ 'a llist" where
  "lfilter P ❙⟦❙⟧ = ❙⟦❙⟧"
| "lfilter P (x ### xs) = 
   (if P x then x ### lfilter P xs else lfilter P xs)"

export_code lfilter in SML file_prefix lfilter

value [code] "lfilter odd llist"

value [code] "lhd (lfilter odd llist)"


section ‹Iterator for red-black trees›

text ‹Thanks to laziness, we do not need to program a complicated iterator for a tree. 
  A conversion function to lazy lists is enough.›

primrec lappend :: "'a llist ⇒ 'a llist ⇒ 'a llist"
  (infixr "@@" 65) where
  "❙⟦❙⟧ @@ ys = ys"
| "(x ### xs) @@ ys = x ### (xs @@ ys)"

primrec rbt_iterator :: "('a, 'b) rbt ⇒ ('a × 'b) llist" where
  "rbt_iterator rbt.Empty = ❙⟦❙⟧"
| "rbt_iterator (Branch _ l k v r) = 
   (let _ = Debug.flush (STR ''tick'') in 
   rbt_iterator l @@ (k, v) ### rbt_iterator r)"

definition tree :: "(nat, unit) rbt"
  where "tree = fold (λk. rbt_insert k ()) [0..<100] rbt.Empty"

definition find_min :: "('a :: linorder, 'b) rbt ⇒ ('a × 'b) option" where
  "find_min rbt = 
  (case rbt_iterator rbt of ❙⟦❙⟧ ⇒ None 
   | kv ### _ ⇒ Some kv)"

value "find_min tree" ― ‹Observe that \<^const>‹rbt_iterator› is evaluated only for going down 
  to the first leaf, not for the whole tree (as seen by the ticks).›

text ‹With strict lists, the whole tree is converted into a list.›

deactivate_lazy_type llist
value "find_min tree"
activate_lazy_type llist



section ‹Branching datatypes›

datatype tree
  = L              ("♠") 
  | Node tree tree (infix "△" 900)

notation (output) Node ("△(//❙l: _//❙r: _)")

code_lazy_type tree

fun mk_tree :: "nat ⇒ tree" where mk_tree_0:
  "mk_tree 0 = ♠"
| "mk_tree (Suc n) = (let t = mk_tree n in t △ t)"

declare mk_tree.simps [code]

code_thms mk_tree

function subtree :: "bool list ⇒ tree ⇒ tree" where
  "subtree [] t = t"
| "subtree (True # p) (l △ r) = subtree p l"
| "subtree (False # p) (l △ r) = subtree p r"
| "subtree _ ♠ = ♠"
  by pat_completeness auto
termination by lexicographic_order

value [code] "mk_tree 10"
value [code] "let t = mk_tree 10; _ = subtree [True, True, False, False] t in t"
  ― ‹Since \<^const>‹mk_tree› shares the two subtrees of a node thanks to the let binding,
      digging into one subtree spreads to the whole tree.›
value [code] "let t = mk_tree 3; _ = subtree [True, True, False, False] t in t"

lemma mk_tree_Suc_debug [code]: ― ‹Make the evaluation visible with tracing.›
  "mk_tree (Suc n) = 
  (let _ = Debug.flush (STR ''tick''); t = mk_tree n in t △ t)"
  by simp

value [code] "mk_tree 10"
  ― ‹The recursive call to \<^const>‹mk_tree› is not guarded by a lazy constructor,
      so all the suspensions are built up immediately.›

lemma mk_tree_Suc [code]: "mk_tree (Suc n) = mk_tree n △ mk_tree n"
  ― ‹In this code equation, there is no sharing and the recursive calls are guarded by a constructor.›
  by(simp add: Let_def)

value [code] "mk_tree 10"
value [code] "let t = mk_tree 10; _ = subtree [True, True, False, False] t in t"

lemma mk_tree_Suc_debug' [code]: 
  "mk_tree (Suc n) = (let _ = Debug.flush (STR ''tick'') in mk_tree n △ mk_tree n)"
  by(simp add: Let_def)

value [code] "mk_tree 10" ― ‹Only one tick thanks to the guarding constructor›
value [code] "let t = mk_tree 10; _ = subtree [True, True, False, False] t in t"
value [code] "let t = mk_tree 3; _ = subtree [True, True, False, False] t in t"


section ‹Pattern matching elimination›

text ‹The pattern matching elimination handles deep pattern matches and overlapping equations
 and only eliminates necessary pattern matches.›

function crazy :: "nat llist llist ⇒ tree ⇒ bool ⇒ unit" where
  "crazy (❙⟦0❙⟧ ### xs) _ _    = Debug.flush (1 :: integer)"
| "crazy xs          ♠ True = Debug.flush (2 :: integer)"
| "crazy xs          t  b   = Debug.flush (3 :: integer)"
  by pat_completeness auto
termination by lexicographic_order

code_thms crazy

end