src/HOL/Tools/Function/function_elims.ML

generate elim rules for elimination of function equalities;
added fun_cases command;
recover proper cases rules for mutual recursive case (no sum types)

(*  Title:      HOL/Tools/Function/function_elims.ML
    Author:     Manuel Eberl <[e-mail]>, TU München

Generates the pelims rules for a function. These are of the shape
[|f x y z = w; !!…. [|x = …; y = …; z = …; w = …|] ==> P; …|] ==> P
and are derived from the cases rule. There is at least one pelim rule for
each function (cf. mutually recursive functions)
There may be more than one pelim rule for a function in case of functions
that return a boolean. For such a function, e.g. P x, not only the normal
elim rule with the premise P x = z is generated, but also two additional
elim rules with P x resp. ¬P x as premises.
*)

signature FUNCTION_ELIMS =
sig
  val dest_funprop : term -> (term * term list) * term
  val mk_partial_elim_rules :
          local_theory -> Function_Common.function_result -> thm list list
end;

structure Function_Elims : FUNCTION_ELIMS =
struct

open Function_Lib
open Function_Common

(* Extracts a function and its arguments from a proposition that is
   either of the form "f x y z = ..." or, in case of function that
   returns a boolean, "f x y z" *)
fun dest_funprop (Const ("HOL.eq", _) $ lhs $ rhs) = (strip_comb lhs, rhs)
  | dest_funprop (Const ("HOL.Not", _) $ trm) = (strip_comb trm, @{term "False"})
  | dest_funprop trm = (strip_comb trm, @{term "True"});

local
  fun propagate_tac i thm =
    let fun inspect eq = case eq of
                Const ("HOL.Trueprop",_) $ (Const ("HOL.eq",_) $ Free x $ t) =>
                    if Logic.occs (Free x, t) then raise Match else true
              | Const ("HOL.Trueprop",_) $ (Const ("HOL.eq",_) $ t $ Free x) =>
                    if Logic.occs (Free x, t) then raise Match else false
              | _ => raise Match;
        fun mk_eq thm = (if inspect (prop_of thm) then
                            [thm RS eq_reflection]
                        else
                            [Thm.symmetric (thm RS eq_reflection)])
                        handle Match => [];
        val ss = Simplifier.global_context (Thm.theory_of_thm thm) empty_ss
                 |> Simplifier.set_mksimps (K mk_eq)
    in
      asm_lr_simp_tac ss i thm
    end;

  val eqBoolI = @{lemma "!!P. P ==> P = True" "!!P. ~P ==> P = False" by iprover+}
  val boolE = @{thms HOL.TrueE HOL.FalseE}
  val boolD = @{lemma "!!P. True = P ==> P" "!!P. False = P ==> ~P" by iprover+}
  val eqBool = @{thms HOL.eq_True HOL.eq_False HOL.not_False_eq_True HOL.not_True_eq_False}

  fun bool_subst_tac ctxt i =
      REPEAT (EqSubst.eqsubst_asm_tac ctxt [1] eqBool i)
      THEN REPEAT (dresolve_tac boolD i)
      THEN REPEAT (eresolve_tac boolE i)

  fun mk_bool_elims ctxt elim =
    let val tac = ALLGOALS (bool_subst_tac ctxt)
        fun mk_bool_elim b =
          elim
          |> Thm.forall_elim b
          |> Tactic.rule_by_tactic ctxt (TRY (resolve_tac eqBoolI 1))
          |> Tactic.rule_by_tactic ctxt tac
    in
        map mk_bool_elim [@{cterm True}, @{cterm False}]
    end;

in

  fun mk_partial_elim_rules ctxt result=
    let val FunctionResult {fs, G, R, dom, psimps, simple_pinducts, cases,
                            termination, domintros, ...} = result;
        val n_fs = length fs;

        fun mk_partial_elim_rule (idx,f) =
          let fun mk_funeq 0 T (acc_vars, acc_lhs) =
                  let val y = Free("y",T) in
                    (y :: acc_vars, (HOLogic.mk_Trueprop (HOLogic.mk_eq (acc_lhs, y))), T)
                  end
                | mk_funeq n (Type("fun",[S,T])) (acc_vars, acc_lhs) =
                  let val xn = Free ("x" ^ Int.toString n,S) in
                    mk_funeq (n - 1) T (xn :: acc_vars, acc_lhs $ xn)
                  end
                | mk_funeq _ _ _ = raise (TERM ("Not a function.", [f]))

              val f_simps = filter (fn r => (prop_of r |> Logic.strip_assums_concl
                                             |> HOLogic.dest_Trueprop
                                             |> dest_funprop |> fst |> fst) = f)
                                   psimps

              val arity = hd f_simps |> prop_of |> Logic.strip_assums_concl
                                     |> HOLogic.dest_Trueprop
                                     |> snd o fst o dest_funprop |> length;
              val (free_vars,prop,ranT) = mk_funeq arity (fastype_of f) ([],f)
              val (rhs_var, arg_vars) = case free_vars of x::xs => (x, rev xs)
              val args = HOLogic.mk_tuple arg_vars;
              val domT = R |> dest_Free |> snd |> hd o snd o dest_Type

              val sumtree_inj = SumTree.mk_inj domT n_fs (idx+1) args;

              val thy = Proof_Context.theory_of ctxt;
              val cprop = cterm_of thy prop

              val asms = [cprop, cterm_of thy (HOLogic.mk_Trueprop (dom $ sumtree_inj))];
              val asms_thms = map Thm.assume asms;

              fun prep_subgoal i =
                REPEAT (eresolve_tac @{thms Pair_inject} i)
                THEN Method.insert_tac (case asms_thms of
                                          thm::thms => (thm RS sym) :: thms) i
                THEN propagate_tac i
                THEN TRY
                    ((EqSubst.eqsubst_asm_tac ctxt [1] psimps i) THEN atac i)
                THEN bool_subst_tac ctxt i;

            val tac = ALLGOALS prep_subgoal;

            val elim_stripped =
                  nth cases idx
                  |> Thm.forall_elim @{cterm "P::bool"}
                  |> Thm.forall_elim (cterm_of thy args)
                  |> Tactic.rule_by_tactic ctxt tac
                  |> fold_rev Thm.implies_intr asms
                  |> Thm.forall_intr (cterm_of thy rhs_var)

            val bool_elims = (case ranT of
                                Type ("HOL.bool", []) => mk_bool_elims ctxt elim_stripped
                                | _ => []);

            fun unstrip rl =
                  rl  |> (fn thm => List.foldr (uncurry Thm.forall_intr) thm
                             (map (cterm_of thy) arg_vars))
                      |> Thm.forall_intr @{cterm "P::bool"}

        in
          map unstrip (elim_stripped :: bool_elims)
        end;

    in
      map_index mk_partial_elim_rule fs
    end;
  end;
end;