src/Pure/Isar/obtain.ML

support for "if prems" in local goal statements;

(*  Title:      Pure/Isar/obtain.ML
    Author:     Markus Wenzel, TU Muenchen

The 'obtain' and 'guess' language elements -- generalized existence at
the level of proof texts: 'obtain' involves a proof that certain
fixes/assumes may be introduced into the present context; 'guess' is
similar, but derives these elements from the course of reasoning!

  <chain_facts>
  obtain x where "A x" <proof> ==

  have "!!thesis. (!!x. A x ==> thesis) ==> thesis"
  proof succeed
    fix thesis
    assume that [intro?]: "!!x. A x ==> thesis"
    <chain_facts>
    show thesis
      apply (insert that)
      <proof>
  qed
  fix x assm <<obtain_export>> "A x"


  <chain_facts>
  guess x <proof body> <proof end> ==

  {
    fix thesis
    <chain_facts> have "PROP ?guess"
      apply magic      -- {* turns goal into "thesis ==> #thesis" *}
      <proof body>
      apply_end magic  -- {* turns final "(!!x. P x ==> thesis) ==> #thesis" into
        "#((!!x. A x ==> thesis) ==> thesis)" which is a finished goal state *}
      <proof end>
  }
  fix x assm <<obtain_export>> "A x"
*)

signature OBTAIN =
sig
  val thatN: string
  val obtain: string -> (binding * typ option * mixfix) list ->
    (Thm.binding * (term * term list) list) list -> bool -> Proof.state -> Proof.state
  val obtain_cmd: string -> (binding * string option * mixfix) list ->
    (Attrib.binding * (string * string list) list) list -> bool -> Proof.state -> Proof.state
  val result: (Proof.context -> tactic) -> thm list -> Proof.context ->
    ((string * cterm) list * thm list) * Proof.context
  val guess: (binding * typ option * mixfix) list -> bool -> Proof.state -> Proof.state
  val guess_cmd: (binding * string option * mixfix) list -> bool -> Proof.state -> Proof.state
end;

structure Obtain: OBTAIN =
struct

(** obtain_export **)

(*
  [x, A x]
     :
     B
  --------
     B
*)
fun eliminate_term ctxt xs tm =
  let
    val vs = map (dest_Free o Thm.term_of) xs;
    val bads = Term.fold_aterms (fn t as Free v =>
      if member (op =) vs v then insert (op aconv) t else I | _ => I) tm [];
    val _ = null bads orelse
      error ("Result contains obtained parameters: " ^
        space_implode " " (map (Syntax.string_of_term ctxt) bads));
  in tm end;

fun eliminate ctxt rule xs As thm =
  let
    val _ = eliminate_term ctxt xs (Thm.full_prop_of thm);
    val _ = Object_Logic.is_judgment ctxt (Thm.concl_of thm) orelse
      error "Conclusion in obtained context must be object-logic judgment";

    val ((_, [thm']), ctxt') = Variable.import true [thm] ctxt;
    val prems = Drule.strip_imp_prems (#prop (Thm.crep_thm thm'));
  in
    ((Drule.implies_elim_list thm' (map Thm.assume prems)
        |> Drule.implies_intr_list (map (Drule.norm_hhf_cterm ctxt') As)
        |> Drule.forall_intr_list xs)
      COMP rule)
    |> Drule.implies_intr_list prems
    |> singleton (Variable.export ctxt' ctxt)
  end;

fun obtain_export ctxt rule xs _ As =
  (eliminate ctxt rule xs As, eliminate_term ctxt xs);



(** obtain **)

fun bind_judgment ctxt name =
  let
    val ([x], ctxt') = Proof_Context.add_fixes [(Binding.name name, NONE, NoSyn)] ctxt;
    val (t as _ $ Free v) = Object_Logic.fixed_judgment ctxt x;
  in ((v, t), ctxt') end;

val thatN = "that";

local

fun gen_obtain prep_att prep_var prep_propp
    name raw_vars raw_asms int state =
  let
    val _ = Proof.assert_forward_or_chain state;
    val ctxt = Proof.context_of state;
    val chain_facts = if can Proof.assert_chain state then Proof.the_facts state else [];

    (*obtain vars*)
    val ((xs', vars), fix_ctxt) = ctxt
      |> fold_map prep_var raw_vars
      |-> (fn vars => Proof_Context.add_fixes vars ##>> pair vars);
    val xs = map (Variable.check_name o #1) vars;

    (*obtain asms*)
    val (propss, binds) = prep_propp fix_ctxt (map snd raw_asms);
    val props = flat propss;
    val declare_asms =
      fold Variable.declare_term props #>
      fold Variable.bind_term binds;
    val asms =
      map (fn ((b, raw_atts), _) => (b, map (prep_att fix_ctxt) raw_atts)) raw_asms ~~
      map (map (rpair [])) propss;

    (*obtain params*)
    val (params, params_ctxt) =
      declare_asms fix_ctxt |> fold_map Proof_Context.inferred_param xs' |>> map Free;
    val cparams = map (Thm.cterm_of params_ctxt) params;
    val binds' = (map o apsnd) (fold_rev Term.dependent_lambda_name (xs ~~ params)) binds;

    val _ = Variable.warn_extra_tfrees fix_ctxt params_ctxt;

    (*obtain statements*)
    val thesisN = singleton (Name.variant_list xs) Auto_Bind.thesisN;
    val (thesis_var, thesis) = #1 (bind_judgment params_ctxt thesisN);

    val that_name = if name = "" then thatN else name;
    val that_prop =
      Logic.list_rename_params xs
        (fold_rev Logic.all params (Logic.list_implies (props, thesis)));
    val obtain_prop =
      Logic.list_rename_params [Auto_Bind.thesisN]
        (Logic.all (Free thesis_var) (Logic.mk_implies (that_prop, thesis)));

    fun after_qed _ =
      Proof.local_qed (NONE, false)
      #> `Proof.the_fact #-> (fn rule =>
        Proof.fix vars
        #> Proof.map_context declare_asms
        #> Proof.assm (obtain_export params_ctxt rule cparams) asms);
  in
    state
    |> Proof.enter_forward
    |> Proof.have NONE (K I) [] [] [(Thm.empty_binding, [(obtain_prop, [])])] int
    |> Proof.map_context (fold Variable.bind_term binds')
    |> Proof.proof (SOME Method.succeed_text) |> Seq.hd
    |> Proof.fix [(Binding.name thesisN, NONE, NoSyn)]
    |> Proof.assume
      [((Binding.name that_name, [Context_Rules.intro_query NONE]), [(that_prop, [])])]
    |> `Proof.the_facts
    ||> Proof.chain_facts chain_facts
    ||> Proof.show NONE after_qed [] [] [(Thm.empty_binding, [(thesis, [])])] false
    |-> Proof.refine_insert
  end;

in

val obtain = gen_obtain (K I) Proof_Context.cert_var Proof_Context.cert_propp;
val obtain_cmd = gen_obtain Attrib.attribute_cmd Proof_Context.read_var Proof_Context.read_propp;

end;



(** tactical result **)

fun check_result ctxt thesis th =
  (case Thm.prems_of th of
    [prem] =>
      if Thm.concl_of th aconv thesis andalso
        Logic.strip_assums_concl prem aconv thesis then th
      else error ("Guessed a different clause:\n" ^ Display.string_of_thm ctxt th)
  | [] => error "Goal solved -- nothing guessed"
  | _ => error ("Guess split into several cases:\n" ^ Display.string_of_thm ctxt th));

fun result tac facts ctxt =
  let
    val ((thesis_var, thesis), thesis_ctxt) = bind_judgment ctxt Auto_Bind.thesisN;
    val st = Goal.init (Thm.cterm_of ctxt thesis);
    val rule =
      (case SINGLE (Method.insert_tac facts 1 THEN tac thesis_ctxt) st of
        NONE => raise THM ("Obtain.result: tactic failed", 0, facts)
      | SOME th =>
          check_result thesis_ctxt thesis (Raw_Simplifier.norm_hhf thesis_ctxt (Goal.conclude th)));

    val closed_rule = Thm.forall_intr (Thm.cterm_of ctxt (Free thesis_var)) rule;
    val ((_, [rule']), ctxt') = Variable.import false [closed_rule] ctxt;
    val obtain_rule =
      Thm.forall_elim (Thm.cterm_of ctxt (Logic.varify_global (Free thesis_var))) rule';
    val ((params, stmt), fix_ctxt) = Variable.focus_cterm (Thm.cprem_of obtain_rule 1) ctxt';
    val (prems, ctxt'') =
      Assumption.add_assms (obtain_export fix_ctxt obtain_rule (map #2 params))
        (Drule.strip_imp_prems stmt) fix_ctxt;
  in ((params, prems), ctxt'') end;



(** guess **)

local

fun unify_params vars thesis_var raw_rule ctxt =
  let
    val thy = Proof_Context.theory_of ctxt;
    val string_of_term = Syntax.string_of_term (Config.put show_types true ctxt);

    fun err msg th = error (msg ^ ":\n" ^ Display.string_of_thm ctxt th);

    val maxidx = fold (Term.maxidx_typ o snd o fst) vars ~1;
    val rule = Thm.incr_indexes (maxidx + 1) raw_rule;

    val params = Rule_Cases.strip_params (Logic.nth_prem (1, Thm.prop_of rule));
    val m = length vars;
    val n = length params;
    val _ = m <= n orelse err "More variables than parameters in obtained rule" rule;

    fun unify ((x, T), (y, U)) (tyenv, max) = Sign.typ_unify thy (T, U) (tyenv, max)
      handle Type.TUNIFY =>
        err ("Failed to unify variable " ^
          string_of_term (Free (x, Envir.norm_type tyenv T)) ^ " against parameter " ^
          string_of_term (Syntax_Trans.mark_bound_abs (y, Envir.norm_type tyenv U)) ^ " in") rule;
    val (tyenv, _) = fold unify (map #1 vars ~~ take m params)
      (Vartab.empty, Int.max (maxidx, Thm.maxidx_of rule));
    val norm_type = Envir.norm_type tyenv;

    val xs = map (apsnd norm_type o fst) vars;
    val ys = map (apsnd norm_type) (drop m params);
    val ys' = map Name.internal (Name.variant_list (map fst xs) (map fst ys)) ~~ map #2 ys;
    val terms = map (Drule.mk_term o Thm.cterm_of ctxt o Free) (xs @ ys');

    val instT =
      fold (Term.add_tvarsT o #2) params []
      |> map (TVar #> (fn T => apply2 (Thm.ctyp_of ctxt) (T, norm_type T)));
    val closed_rule = rule
      |> Thm.forall_intr (Thm.cterm_of ctxt (Free thesis_var))
      |> Thm.instantiate (instT, []);

    val ((_, rule' :: terms'), ctxt') = Variable.import false (closed_rule :: terms) ctxt;
    val vars' =
      map (dest_Free o Thm.term_of o Drule.dest_term) terms' ~~
      (map snd vars @ replicate (length ys) NoSyn);
    val rule'' = Thm.forall_elim (Thm.cterm_of ctxt' (Logic.varify_global (Free thesis_var))) rule';
  in ((vars', rule''), ctxt') end;

fun inferred_type (binding, _, mx) ctxt =
  let
    val x = Variable.check_name binding;
    val ((_, T), ctxt') = Proof_Context.inferred_param x ctxt
  in ((x, T, mx), ctxt') end;

fun polymorphic ctxt vars =
  let val Ts = map Logic.dest_type (Variable.polymorphic ctxt (map (Logic.mk_type o #2) vars))
  in map2 (fn (x, _, mx) => fn T => ((x, T), mx)) vars Ts end;

fun gen_guess prep_var raw_vars int state =
  let
    val _ = Proof.assert_forward_or_chain state;
    val ctxt = Proof.context_of state;
    val chain_facts = if can Proof.assert_chain state then Proof.the_facts state else [];

    val (thesis_var, thesis) = #1 (bind_judgment ctxt Auto_Bind.thesisN);
    val vars = ctxt
      |> fold_map prep_var raw_vars |-> fold_map inferred_type
      |> fst |> polymorphic ctxt;

    fun guess_context raw_rule state' =
      let
        val ((parms, rule), ctxt') =
          unify_params vars thesis_var raw_rule (Proof.context_of state');
        val (xs, _) = Variable.add_fixes (map (#1 o #1) parms) ctxt';
        val ps = xs ~~ map (#2 o #1) parms;
        val ts = map Free ps;
        val asms =
          Logic.strip_assums_hyp (Logic.nth_prem (1, Thm.prop_of rule))
          |> map (fn asm => (Term.betapplys (fold_rev Term.abs ps asm, ts), []));
        val _ = not (null asms) orelse error "Trivial result -- nothing guessed";
      in
        state'
        |> Proof.map_context (K ctxt')
        |> Proof.fix (map (fn ((x, T), mx) => (Binding.name x, SOME T, mx)) parms)
        |> `Proof.context_of |-> (fn fix_ctxt => Proof.assm
          (obtain_export fix_ctxt rule (map (Thm.cterm_of ctxt) ts))
            [(Thm.empty_binding, asms)])
        |> Proof.map_context (fold Variable.unbind_term Auto_Bind.no_facts)
      end;

    val goal = Var (("guess", 0), propT);
    val pos = Position.thread_data ();
    fun print_result ctxt' (k, [(s, [_, th])]) =
      Proof_Display.print_results int pos ctxt' (k, [(s, [th])]);
    val before_qed =
      Method.primitive_text (fn ctxt =>
        Goal.conclude #> Raw_Simplifier.norm_hhf ctxt #>
          (fn th => Goal.protect 0 (Conjunction.intr (Drule.mk_term (Thm.cprop_of th)) th)));
    fun after_qed [[_, res]] =
      Proof.end_block #> guess_context (check_result ctxt thesis res);
  in
    state
    |> Proof.enter_forward
    |> Proof.begin_block
    |> Proof.fix [(Binding.name Auto_Bind.thesisN, NONE, NoSyn)]
    |> Proof.chain_facts chain_facts
    |> Proof.internal_goal print_result Proof_Context.mode_schematic "guess"
      (SOME before_qed) after_qed [] [] [(Thm.empty_binding, [(Logic.mk_term goal, []), (goal, [])])]
    |> Proof.refine (Method.primitive_text (fn _ => fn _ =>
        Goal.init (Thm.cterm_of ctxt thesis))) |> Seq.hd
  end;

in

val guess = gen_guess Proof_Context.cert_var;
val guess_cmd = gen_guess Proof_Context.read_var;

end;

end;