src/HOLCF/cont_proc.ML

tuned;

(*  Title:      HOLCF/cont_proc.ML
    ID:         $Id$
    Author:     Brian Huffman
*)

signature CONT_PROC =
sig
  val is_lcf_term: term -> bool
  val cont_thms: term -> thm list
  val all_cont_thms: term -> thm list
  val cont_tac: int -> tactic
  val cont_proc: theory -> simproc
  val setup: theory -> theory
end;

structure ContProc: CONT_PROC =
struct

(** theory context references **)

val cont_K = thm "cont_const";
val cont_I = thm "cont_id";
val cont_A = thm "cont2cont_Rep_CFun";
val cont_L = thm "cont2cont_LAM";
val cont_R = thm "cont_Rep_CFun2";

(* checks whether a term contains no dangling bound variables *)
val is_closed_term =
  let
    fun bound_less i (t $ u) =
          bound_less i t andalso bound_less i u
      | bound_less i (Abs (_, _, t)) = bound_less (i+1) t
      | bound_less i (Bound n) = n < i
      | bound_less i _ = true; (* Const, Free, and Var are OK *)
  in bound_less 0 end;

(* checks whether a term is written entirely in the LCF sublanguage *)
fun is_lcf_term (Const ("Cfun.Rep_CFun", _) $ t $ u) =
      is_lcf_term t andalso is_lcf_term u
  | is_lcf_term (Const ("Cfun.Abs_CFun", _) $ Abs (_, _, t)) = is_lcf_term t
  | is_lcf_term (Const ("Cfun.Abs_CFun", _) $ _) = false
  | is_lcf_term (Bound _) = true
  | is_lcf_term t = is_closed_term t;

(*
  efficiently generates a cont thm for every LAM abstraction in a term,
  using forward proof and reusing common subgoals
*)
local
  fun var 0 = [SOME cont_I]
    | var n = NONE :: var (n-1);

  fun k NONE     = cont_K
    | k (SOME x) = x;

  fun ap NONE NONE = NONE
    | ap x    y    = SOME (k y RS (k x RS cont_A));

  fun zip []      []      = []
    | zip []      (y::ys) = (ap NONE y   ) :: zip [] ys
    | zip (x::xs) []      = (ap x    NONE) :: zip xs []
    | zip (x::xs) (y::ys) = (ap x    y   ) :: zip xs ys

  fun lam [] = ([], cont_K)
    | lam (x::ys) =
    let
      (* should use "standard" for thms that are used multiple times *)
      (* it seems to allow for sharing in explicit proof objects *)
      val x' = standard (k x);
      val Lx = x' RS cont_L;
    in (map (fn y => SOME (k y RS Lx)) ys, x') end;

  (* first list: cont thm for each dangling bound variable *)
  (* second list: cont thm for each LAM in t *)
  (* if b = false, only return cont thm for outermost LAMs *)
  fun cont_thms1 b (Const ("Cfun.Rep_CFun", _) $ f $ t) =
    let
      val (cs1,ls1) = cont_thms1 b f;
      val (cs2,ls2) = cont_thms1 b t;
    in (zip cs1 cs2, if b then ls1 @ ls2 else []) end
    | cont_thms1 b (Const ("Cfun.Abs_CFun", _) $ Abs (_, _, t)) =
    let
      val (cs, ls) = cont_thms1 b t;
      val (cs', l) = lam cs;
    in (cs', l::ls) end
    | cont_thms1 _ (Bound n) = (var n, [])
    | cont_thms1 _ _ = ([], []);
in
  (* precondition: is_lcf_term t = true *)
  fun cont_thms t = snd (cont_thms1 false t);
  fun all_cont_thms t = snd (cont_thms1 true t);
end;

(*
  Given the term "cont f", the procedure tries to construct the
  theorem "cont f == True". If this theorem cannot be completely
  solved by the introduction rules, then the procedure returns a
  conditional rewrite rule with the unsolved subgoals as premises.
*)

local
  val rules = [cont_K, cont_I, cont_R, cont_A, cont_L];
  
  val prev_cont_thms : thm list ref = ref [];

  fun old_cont_tac i thm =
    case !prev_cont_thms of
      [] => no_tac thm
    | (c::cs) => (prev_cont_thms := cs; rtac c i thm);

  fun new_cont_tac f' i thm =
    case all_cont_thms f' of
      [] => no_tac thm
    | (c::cs) => (prev_cont_thms := cs; rtac c i thm);

  fun cont_tac_of_term (Const ("Cont.cont", _) $ f) =
    let
      val f' = Const ("Cfun.Abs_CFun", dummyT) $ f;
    in
      if is_lcf_term f'
      then old_cont_tac ORELSE' new_cont_tac f'
      else REPEAT_ALL_NEW (resolve_tac rules)
    end
    | cont_tac_of_term _ = K no_tac;
in
  val cont_tac =
    SUBGOAL (fn (t, i) => cont_tac_of_term (HOLogic.dest_Trueprop t) i);
end;

local
  fun solve_cont thy _ t =
    let
      val tr = instantiate' [] [SOME (cterm_of thy t)] Eq_TrueI;
    in Option.map fst (Seq.pull (cont_tac 1 tr)) end
in
  fun cont_proc thy =
    Simplifier.simproc thy "cont_proc" ["cont f"] solve_cont;
end;

val setup =
  (fn thy =>
    (Simplifier.change_simpset_of thy
      (fn ss => ss addsimprocs [cont_proc thy]); thy));

end;