src/Provers/hypsubst.ML

lucas - added more comments and an extra type to clarify the code.

(*  Title:      Provers/hypsubst.ML
    ID:         $Id$
    Authors:    Martin D Coen, Tobias Nipkow and Lawrence C Paulson
    Copyright   1995  University of Cambridge

Basic equational reasoning: (hyp_)subst method and symmetric attribute.

Tactic to substitute using (at least) the assumption x=t in the rest
of the subgoal, and to delete (at least) that assumption.  Original
version due to Martin Coen.

This version uses the simplifier, and requires it to be already present.

Test data:

Goal "!!x.[| Q(x,y,z); y=x; a=x; z=y; P(y) |] ==> P(z)";
Goal "!!x.[| Q(x,y,z); z=f(x); x=z |] ==> P(z)";
Goal "!!y. [| ?x=y; P(?x) |] ==> y = a";
Goal "!!z. [| ?x=y; P(?x) |] ==> y = a";

Goal "!!x a. [| x = f(b); g(a) = b |] ==> P(x)";

by (bound_hyp_subst_tac 1);
by (hyp_subst_tac 1);

Here hyp_subst_tac goes wrong; harder still to prove P(f(f(a))) & P(f(a))
Goal "P(a) --> (EX y. a=y --> P(f(a)))";

Goal "!!x. [| Q(x,h1); P(a,h2); R(x,y,h3); R(y,z,h4); x=f(y); \
\                 P(x,h5); P(y,h6); K(x,h7) |] ==> Q(x,c)";
by (blast_hyp_subst_tac (ref true) 1);
*)

signature HYPSUBST_DATA =
  sig
  structure Simplifier : SIMPLIFIER
  val dest_Trueprop    : term -> term
  val dest_eq          : term -> term*term*typ
  val dest_imp         : term -> term*term
  val eq_reflection    : thm               (* a=b ==> a==b *)
  val rev_eq_reflection: thm               (* a==b ==> a=b *)
  val imp_intr         : thm               (* (P ==> Q) ==> P-->Q *)
  val rev_mp           : thm               (* [| P;  P-->Q |] ==> Q *)
  val subst            : thm               (* [| a=b;  P(a) |] ==> P(b) *)
  val sym              : thm               (* a=b ==> b=a *)
  val thin_refl        : thm               (* [|x=x; PROP W|] ==> PROP W *)
  end;


signature HYPSUBST =
  sig
  val bound_hyp_subst_tac    : int -> tactic
  val hyp_subst_tac          : int -> tactic
  val blast_hyp_subst_tac    : bool ref -> int -> tactic
    (*exported purely for debugging purposes*)
  val gen_hyp_subst_tac      : bool -> int -> tactic
  val vars_gen_hyp_subst_tac : bool -> int -> tactic
  val eq_var                 : bool -> bool -> term -> int * bool
  val inspect_pair           : bool -> bool -> term * term * typ -> bool
  val mk_eqs                 : bool -> thm -> thm list
  val stac		     : thm -> int -> tactic
  val hypsubst_setup         : (theory -> theory) list
  end;



functor HypsubstFun(Data: HYPSUBST_DATA): HYPSUBST =
struct

exception EQ_VAR;

fun loose (i,t) = 0 mem_int add_loose_bnos(t,i,[]);

local val odot = ord"."
in
(*Simplifier turns Bound variables to dotted Free variables:
  change it back (any Bound variable will do)
*)
fun contract t =
    case Pattern.eta_contract_atom t of
        Free(a,T) => if (ord a = odot) then Bound 0 else Free(a,T)
      | t'        => t'
end;

fun has_vars t = maxidx_of_term t <> ~1;

(*If novars then we forbid Vars in the equality.
  If bnd then we only look for Bound (or dotted Free) variables to eliminate.
  When can we safely delete the equality?
    Not if it equates two constants; consider 0=1.
    Not if it resembles x=t[x], since substitution does not eliminate x.
    Not if it resembles ?x=0; consider ?x=0 ==> ?x=1 or even ?x=0 ==> P
    Not if it involves a variable free in the premises,
        but we can't check for this -- hence bnd and bound_hyp_subst_tac
  Prefer to eliminate Bound variables if possible.
  Result:  true = use as is,  false = reorient first *)
fun inspect_pair bnd novars (t,u,T) =
  if novars andalso maxidx_of_typ T <> ~1
  then raise Match   (*variables in the type!*)
  else
  case (contract t, contract u) of
       (Bound i, _) => if loose(i,u) orelse novars andalso has_vars u
                       then raise Match
                       else true                (*eliminates t*)
     | (_, Bound i) => if loose(i,t) orelse novars andalso has_vars t
                       then raise Match
                       else false               (*eliminates u*)
     | (Free _, _) =>  if bnd orelse Logic.occs(t,u) orelse
                          novars andalso has_vars u
                       then raise Match
                       else true                (*eliminates t*)
     | (_, Free _) =>  if bnd orelse Logic.occs(u,t) orelse
                          novars andalso has_vars t
                       then raise Match
                       else false               (*eliminates u*)
     | _ => raise Match;

(*Locates a substitutable variable on the left (resp. right) of an equality
   assumption.  Returns the number of intervening assumptions. *)
fun eq_var bnd novars =
  let fun eq_var_aux k (Const("all",_) $ Abs(_,_,t)) = eq_var_aux k t
        | eq_var_aux k (Const("==>",_) $ A $ B) =
              ((k, inspect_pair bnd novars
                    (Data.dest_eq (Data.dest_Trueprop A)))
                      (*Exception comes from inspect_pair or dest_eq*)
               handle _ => eq_var_aux (k+1) B)
        | eq_var_aux k _ = raise EQ_VAR
  in  eq_var_aux 0  end;

(*For the simpset.  Adds ALL suitable equalities, even if not first!
  No vars are allowed here, as simpsets are built from meta-assumptions*)
fun mk_eqs bnd th =
    [ if inspect_pair bnd false (Data.dest_eq
                                   (Data.dest_Trueprop (#prop (rep_thm th))))
      then th RS Data.eq_reflection
      else symmetric(th RS Data.eq_reflection) (*reorient*) ]
    handle _ => [];  (*Exception comes from inspect_pair or dest_eq*)

local open Simplifier
in

  (*Select a suitable equality assumption; substitute throughout the subgoal
    If bnd is true, then it replaces Bound variables only. *)
  fun gen_hyp_subst_tac bnd =
    let val tac = SUBGOAL (fn (Bi, i) =>
      let val (k, _) = eq_var bnd true Bi
          val hyp_subst_ss = empty_ss setmksimps (mk_eqs bnd)
      in EVERY [rotate_tac k i, asm_lr_simp_tac hyp_subst_ss i,
        etac thin_rl i, rotate_tac (~k) i]
      end handle THM _ => no_tac | EQ_VAR => no_tac)
    in REPEAT_DETERM1 o tac end;

end;

val ssubst = standard (Data.sym RS Data.subst);

val imp_intr_tac = rtac Data.imp_intr;

(*Old version of the tactic above -- slower but the only way
  to handle equalities containing Vars.*)
fun vars_gen_hyp_subst_tac bnd = SUBGOAL(fn (Bi,i) =>
      let val n = length(Logic.strip_assums_hyp Bi) - 1
          val (k,symopt) = eq_var bnd false Bi
      in
         DETERM
           (EVERY [REPEAT_DETERM_N k (etac Data.rev_mp i),
                   rotate_tac 1 i,
                   REPEAT_DETERM_N (n-k) (etac Data.rev_mp i),
                   etac (if symopt then ssubst else Data.subst) i,
                   REPEAT_DETERM_N n (imp_intr_tac i THEN rotate_tac ~1 i)])
      end
      handle THM _ => no_tac | EQ_VAR => no_tac);

(*Substitutes for Free or Bound variables*)
val hyp_subst_tac = FIRST' [ematch_tac [Data.thin_refl],
        gen_hyp_subst_tac false, vars_gen_hyp_subst_tac false];

(*Substitutes for Bound variables only -- this is always safe*)
val bound_hyp_subst_tac =
    gen_hyp_subst_tac true ORELSE' vars_gen_hyp_subst_tac true;


(** Version for Blast_tac.  Hyps that are affected by the substitution are
    moved to the front.  Defect: even trivial changes are noticed, such as
    substitutions in the arguments of a function Var. **)

(*final re-reversal of the changed assumptions*)
fun reverse_n_tac 0 i = all_tac
  | reverse_n_tac 1 i = rotate_tac ~1 i
  | reverse_n_tac n i =
      REPEAT_DETERM_N n (rotate_tac ~1 i THEN etac Data.rev_mp i) THEN
      REPEAT_DETERM_N n (imp_intr_tac i THEN rotate_tac ~1 i);

(*Use imp_intr, comparing the old hyps with the new ones as they come out.*)
fun all_imp_intr_tac hyps i =
  let fun imptac (r, [])    st = reverse_n_tac r i st
        | imptac (r, hyp::hyps) st =
           let val (hyp',_) = List.nth (prems_of st, i-1) |>
                              Logic.strip_assums_concl    |>
                              Data.dest_Trueprop          |> Data.dest_imp
               val (r',tac) = if Pattern.aeconv (hyp,hyp')
                              then (r, imp_intr_tac i THEN rotate_tac ~1 i)
                              else (*leave affected hyps at end*)
                                   (r+1, imp_intr_tac i)
           in
               case Seq.pull(tac st) of
                   NONE       => Seq.single(st)
                 | SOME(st',_) => imptac (r',hyps) st'
           end handle _ => error "?? in blast_hyp_subst_tac"
  in  imptac (0, rev hyps)  end;


fun blast_hyp_subst_tac trace = SUBGOAL(fn (Bi,i) =>
      let val (k,symopt) = eq_var false false Bi
          val hyps0 = map Data.dest_Trueprop (Logic.strip_assums_hyp Bi)
          (*omit selected equality, returning other hyps*)
          val hyps = List.take(hyps0, k) @ List.drop(hyps0, k+1)
          val n = length hyps
      in
         if !trace then tracing "Substituting an equality" else ();
         DETERM
           (EVERY [REPEAT_DETERM_N k (etac Data.rev_mp i),
                   rotate_tac 1 i,
                   REPEAT_DETERM_N (n-k) (etac Data.rev_mp i),
                   etac (if symopt then ssubst else Data.subst) i,
                   all_imp_intr_tac hyps i])
      end
      handle THM _ => no_tac | EQ_VAR => no_tac);


(*apply an equality or definition ONCE;
  fails unless the substitution has an effect*)
fun stac th =
  let val th' = th RS Data.rev_eq_reflection handle THM _ => th
  in CHANGED_GOAL (rtac (th' RS ssubst)) end;


(* proof methods *)

val subst_meth = Method.thm_args (Method.SIMPLE_METHOD' HEADGOAL o stac);
val hyp_subst_meth =
  Method.no_args (Method.SIMPLE_METHOD' HEADGOAL (CHANGED_PROP o hyp_subst_tac));


(* theory setup *)

val hypsubst_setup =
 [Method.add_methods
  [("hypsubst", hyp_subst_meth, "substitution using an assumption (improper)"),
   ("simplesubst", subst_meth, "simple substitution")]];

end;