src/HOL/Subst/Unify.ML

Algebra and Polynomial theories, by Clemens Ballarin

(*  Title:      Subst/Unify
    ID:         $Id$
    Author:     Konrad Slind, Cambridge University Computer Laboratory
    Copyright   1997  University of Cambridge

Unification algorithm
*)

(*---------------------------------------------------------------------------
 * This file defines a nested unification algorithm, then proves that it 
 * terminates, then proves 2 correctness theorems: that when the algorithm
 * succeeds, it 1) returns an MGU; and 2) returns an idempotent substitution.
 * Although the proofs may seem long, they are actually quite direct, in that
 * the correctness and termination properties are not mingled as much as in 
 * previous proofs of this algorithm. 
 *
 * Our approach for nested recursive functions is as follows: 
 *
 *    0. Prove the wellfoundedness of the termination relation.
 *    1. Prove the non-nested termination conditions.
 *    2. Eliminate (0) and (1) from the recursion equations and the 
 *       induction theorem.
 *    3. Prove the nested termination conditions by using the induction 
 *       theorem from (2) and by using the recursion equations from (2). 
 *       These are constrained by the nested termination conditions, but 
 *       things work out magically (by wellfoundedness of the termination 
 *       relation).
 *    4. Eliminate the nested TCs from the results of (2).
 *    5. Prove further correctness properties using the results of (4).
 *
 * Deeper nestings require iteration of steps (3) and (4).
 *---------------------------------------------------------------------------*)

open Unify;



(*---------------------------------------------------------------------------
 * The non-nested TC plus the wellfoundedness of unifyRel.
 *---------------------------------------------------------------------------*)
Tfl.tgoalw Unify.thy [] unify.rules;
(* Wellfoundedness of unifyRel *)
by (simp_tac (simpset() addsimps [unifyRel_def,
				 wf_inv_image, wf_lex_prod, wf_finite_psubset,
				 wf_measure]) 1);
(* TC *)
by Safe_tac;
by (simp_tac (simpset() addsimps [finite_psubset_def, finite_vars_of,
				 lex_prod_def, measure_def, inv_image_def]) 1);
by (rtac (monotone_vars_of RS (subset_iff_psubset_eq RS iffD1) RS disjE) 1);
by (Blast_tac 1);
by (asm_simp_tac (simpset() addsimps [less_eq, less_add_Suc1]) 1);
qed "tc0";


(*---------------------------------------------------------------------------
 * Termination proof.
 *---------------------------------------------------------------------------*)

Goalw [unifyRel_def, measure_def] "trans unifyRel";
by (REPEAT (resolve_tac [trans_inv_image, trans_lex_prod, 
			 trans_finite_psubset, trans_less_than,
			 trans_inv_image] 1));
qed "trans_unifyRel";


(*---------------------------------------------------------------------------
 * The following lemma is used in the last step of the termination proof for 
 * the nested call in Unify.  Loosely, it says that unifyRel doesn't care
 * about term structure.
 *---------------------------------------------------------------------------*)
Goalw [unifyRel_def,lex_prod_def, inv_image_def]
  "((X,Y), (Comb A (Comb B C), Comb D (Comb E F))) : unifyRel  ==> \
\  ((X,Y), (Comb (Comb A B) C, Comb (Comb D E) F)) : unifyRel";
by (asm_full_simp_tac (simpset() addsimps [measure_def, 
                          less_eq, inv_image_def,add_assoc]) 1);
by (subgoal_tac "(vars_of A Un vars_of B Un vars_of C Un \
                \  (vars_of D Un vars_of E Un vars_of F)) = \
                \ (vars_of A Un (vars_of B Un vars_of C) Un \
                \  (vars_of D Un (vars_of E Un vars_of F)))" 1);
by (Blast_tac 2);
by (Asm_simp_tac 1);
qed "Rassoc";


(*---------------------------------------------------------------------------
 * This lemma proves the nested termination condition for the base cases 
 * 3, 4, and 6. 
 *---------------------------------------------------------------------------*)
Goal "~(Var x <: M) ==> \
\   ((N1 <| [(x,M)], N2 <| [(x,M)]), (Comb M N1, Comb(Var x) N2)) : unifyRel \
\ & ((N1 <| [(x,M)], N2 <| [(x,M)]), (Comb(Var x) N1, Comb M N2)) : unifyRel";
by (case_tac "Var x = M" 1);
by (hyp_subst_tac 1);
by (Simp_tac 1);
by (case_tac "x: (vars_of N1 Un vars_of N2)" 1);
(*uterm_less case*)
by (asm_simp_tac
    (simpset() addsimps [less_eq, unifyRel_def, lex_prod_def,
			measure_def, inv_image_def]) 1);
by (Blast_tac 1);
(*finite_psubset case*)
by (simp_tac
    (simpset() addsimps [unifyRel_def, lex_prod_def,
			measure_def, inv_image_def]) 1);
by (simp_tac (simpset() addsimps [finite_psubset_def, finite_vars_of,
				 psubset_def, set_eq_subset]) 1);
by (Blast_tac 1);
(** LEVEL 9 **)
(*Final case, also finite_psubset*)
by (simp_tac
    (simpset() addsimps [finite_vars_of, unifyRel_def, finite_psubset_def,
			lex_prod_def, measure_def, inv_image_def]) 1);
by (cut_inst_tac [("s","[(x,M)]"), ("v", "x"), ("t","N2")] Var_elim 1);
by (cut_inst_tac [("s","[(x,M)]"), ("v", "x"), ("t","N1")] Var_elim 3);
by (ALLGOALS (asm_simp_tac(simpset() addsimps [srange_iff, vars_iff_occseq])));
by (REPEAT_FIRST (resolve_tac [conjI, disjI1, psubsetI]));
by (ALLGOALS (asm_full_simp_tac 
	      (simpset() addsimps [srange_iff, set_eq_subset]))); 
by (ALLGOALS
    (fast_tac (claset() addEs [Var_intro RS disjE]
	               addss (simpset() addsimps [srange_iff]))));
qed "var_elimR";


(*---------------------------------------------------------------------------
 * Eliminate tc0 from the recursion equations and the induction theorem.
 *---------------------------------------------------------------------------*)
val wfr = tc0 RS conjunct1
and tc  = tc0 RS conjunct2;
val unifyRules0 = map (normalize_thm [fn th => wfr RS th, fn th => tc RS th]) 
                     unify.rules;

val unifyInduct0 = [wfr,tc] MRS unify.induct;


(*---------------------------------------------------------------------------
 * The nested TC. Proved by recursion induction.
 *---------------------------------------------------------------------------*)
val [_,_,tc3] = unify.tcs;
goalw_cterm [] (cterm_of (sign_of Unify.thy) (HOLogic.mk_Trueprop tc3));
(*---------------------------------------------------------------------------
 * The extracted TC needs the scope of its quantifiers adjusted, so our 
 * first step is to restrict the scopes of N1 and N2.
 *---------------------------------------------------------------------------*)
by (subgoal_tac "!M1 M2 theta.  \
 \   unify(M1, M2) = Some theta --> \
 \   (!N1 N2. ((N1<|theta, N2<|theta), Comb M1 N1, Comb M2 N2) : unifyRel)" 1);
by (Blast_tac 1);
by (rtac allI 1); 
by (rtac allI 1);
(* Apply induction *)
by (res_inst_tac [("u","M1"),("v","M2")] unifyInduct0 1);
by (ALLGOALS 
    (asm_simp_tac (simpset() addsimps (var_elimR::unifyRules0))));
(*Const-Const case*)
by (simp_tac
    (simpset() addsimps [unifyRel_def, lex_prod_def, measure_def,
			inv_image_def, less_eq]) 1);
(** LEVEL 7 **)
(*Comb-Comb case*)
by (asm_simp_tac (simpset() addsplits [option.split]) 1);
by (strip_tac 1);
by (rtac (trans_unifyRel RS transD) 1);
by (Blast_tac 1);
by (simp_tac (HOL_ss addsimps [subst_Comb RS sym]) 1);
by (rtac Rassoc 1);
by (Blast_tac 1);
qed_spec_mp "unify_TC";


(*---------------------------------------------------------------------------
 * Now for elimination of nested TC from unify.rules and induction. 
 *---------------------------------------------------------------------------*)

(*Desired rule, copied from the theory file.  Could it be made available?*)
Goal "unify(Comb M1 N1, Comb M2 N2) =      \
\      (case unify(M1,M2)               \
\        of None => None                \
\         | Some theta => (case unify(N1 <| theta, N2 <| theta)        \
\                            of None => None    \
\                             | Some sigma => Some (theta <> sigma)))";
by (asm_simp_tac (simpset() addsimps (unify_TC::unifyRules0)
			   addsplits [option.split]) 1);
qed "unifyCombComb";


val unifyRules = rev (unifyCombComb :: tl (rev unifyRules0));

Addsimps unifyRules;

bind_thm ("unifyInduct",
	  rule_by_tactic
	     (ALLGOALS (full_simp_tac (simpset() addsimps [unify_TC])))
	     unifyInduct0);


(*---------------------------------------------------------------------------
 * Correctness. Notice that idempotence is not needed to prove that the 
 * algorithm terminates and is not needed to prove the algorithm correct, 
 * if you are only interested in an MGU.  This is in contrast to the
 * approach of M&W, who used idempotence and MGU-ness in the termination proof.
 *---------------------------------------------------------------------------*)

Goal "!theta. unify(M,N) = Some theta --> MGUnifier theta M N";
by (res_inst_tac [("u","M"),("v","N")] unifyInduct 1);
by (ALLGOALS Asm_simp_tac);
(*Const-Const case*)
by (simp_tac (simpset() addsimps [MGUnifier_def,Unifier_def]) 1);
(*Const-Var case*)
by (stac mgu_sym 1);
by (simp_tac (simpset() addsimps [MGUnifier_Var]) 1);
(*Var-M case*)
by (simp_tac (simpset() addsimps [MGUnifier_Var]) 1);
(*Comb-Var case*)
by (stac mgu_sym 1);
by (simp_tac (simpset() addsimps [MGUnifier_Var]) 1);
(** LEVEL 8 **)
(*Comb-Comb case*)
by (asm_simp_tac (simpset() addsplits [option.split]) 1);
by (strip_tac 1);
by (rotate_tac ~2 1);
by (asm_full_simp_tac 
    (simpset() addsimps [MGUnifier_def, Unifier_def, MoreGeneral_def]) 1);
by (Safe_tac THEN rename_tac "theta sigma gamma" 1);
by (eres_inst_tac [("x","gamma")] allE 1 THEN mp_tac 1);
by (etac exE 1 THEN rename_tac "delta" 1);
by (eres_inst_tac [("x","delta")] allE 1);
by (subgoal_tac "N1 <| theta <| delta = N2 <| theta <| delta" 1);
(*Proving the subgoal*)
by (full_simp_tac (simpset() addsimps [subst_eq_iff]) 2
    THEN blast_tac (claset() addIs [trans,sym] delrules [impCE]) 2);
by (blast_tac (claset() addIs [subst_trans, subst_cong, 
			      comp_assoc RS subst_sym]) 1);
qed_spec_mp "unify_gives_MGU";


(*---------------------------------------------------------------------------
 * Unify returns idempotent substitutions, when it succeeds.
 *---------------------------------------------------------------------------*)
Goal "!theta. unify(M,N) = Some theta --> Idem theta";
by (res_inst_tac [("u","M"),("v","N")] unifyInduct 1);
by (ALLGOALS 
    (asm_simp_tac 
       (simpset() addsimps [Var_Idem] addsplits [option.split])));
(*Comb-Comb case*)
by Safe_tac;
by (REPEAT (dtac spec 1 THEN mp_tac 1));
by (safe_tac (claset() addSDs [rewrite_rule [MGUnifier_def] unify_gives_MGU]));
by (rtac Idem_comp 1);
by (atac 1);
by (atac 1);
by (best_tac (claset() addss (simpset() addsimps 
			     [MoreGeneral_def, subst_eq_iff, Idem_def])) 1);
qed_spec_mp "unify_gives_Idem";