Theory Time_Monad

section ‹Time Monad›

theory Time_Monad
imports
  Main
  "HOL-Library.Monad_Syntax"
begin

datatype 'a tm = TM (val: 'a) nat

fun val :: "'a tm ⇒ 'a" where
"val (TM v n) = v"

fun time :: "'a tm ⇒ nat" where
"time (TM v n) = n"

definition bind_tm :: "'a tm ⇒ ('a ⇒ 'b tm) ⇒ 'b tm" where
"bind_tm s f = (case s of TM u m ⇒ case f u of TM v n ⇒ TM v (m+n))"

adhoc_overloading Monad_Syntax.bind bind_tm

definition "tick v = TM v 1"

definition "return v = TM v 0"

abbreviation eqtick :: "'a tm ⇒ 'a tm ⇒ bool" (infix "=1" 50) where
 "eqtick l r ≡ (l = (r ⤜ tick))"

(* warning: bind_tm is not a constant on purpose, does not work if it is: *)
translations "CONST eqtick l r" <= "(l = (bind_tm r CONST tick))"

lemmas tm_simps = bind_tm_def return_def tick_def

lemma time_return[simp]: "time (return x) = 0"
by(simp add: return_def)

lemma surj_TM: "v = val tm ⟹ t = time tm ⟹ tm = TM v t"
by (metis time.simps tm.exhaust val.simps)

text‹The following lemmas push @{const val} into a monadic term:›

lemma val_return[simp]: "val (return x) = x"
by(simp add: return_def)

lemma val_bind_tm[simp]: "val (bind_tm m f) = (let x = val m in val(f x))"
by(simp add: bind_tm_def split: tm.split)

lemma val_tick[simp]: "val (tick x) = x"
by(simp add: tick_def)

lemma val_let: "val (let x = t in f(x)) = (let x = t in val(f x))"
by simp

lemma let_id: "(let x = t in x) = t"
by simp

lemmas val_simps =
  val_return
  val_bind_tm
  val_tick
  val_let
  let_id
  if_distrib[of val]
  prod.case_distrib[of val]

lemmas val_cong = arg_cong[where f=val]

text‹The following congruence rule enables termination proofs for recursive functions using 
this monad.›

lemma bind_tm_cong[fundef_cong]:
  assumes "m1 = m2"
  assumes "f1 (val m1) = f2 (val m2)"
  shows "m1 ⤜ f1 = m2 ⤜ f2"
  using assms unfolding bind_tm_def
  by (cases m1;cases m2) auto

hide_const TM

end