(*  Title:      HOL/Tools/Function/partial_function.ML

    Author:     Alexander Krauss, TU Muenchen

Partial function definitions based on least fixed points in ccpos.

*)

signature PARTIAL_FUNCTION =

sig

  val setup: theory -> theory

  val init: term -> term -> thm -> declaration

  val add_partial_function: string -> (binding * typ option * mixfix) list ->

    Attrib.binding * term -> local_theory -> local_theory

  val add_partial_function_cmd: string -> (binding * string option * mixfix) list ->

    Attrib.binding * string -> local_theory -> local_theory

end;

structure Partial_Function: PARTIAL_FUNCTION =

struct

(*** Context Data ***)

structure Modes = Generic_Data

(

  type T = ((term * term) * thm) Symtab.table;

  val empty = Symtab.empty;

  val extend = I;

  fun merge (a, b) = Symtab.merge (K true) (a, b);

)

fun init fixp mono fixp_eq phi =

  let

    val term = Morphism.term phi;

    val data' = ((term fixp, term mono), Morphism.thm phi fixp_eq);

    val mode = (* extract mode identifier from morphism prefix! *)

      Binding.prefix_of (Morphism.binding phi (Binding.empty))

      |> map fst |> space_implode ".";

  in

    if mode = "" then I

    else Modes.map (Symtab.update (mode, data'))

  end

val known_modes = Symtab.keys o Modes.get o Context.Proof;

val lookup_mode = Symtab.lookup o Modes.get o Context.Proof;

structure Mono_Rules = Named_Thms

(

  val name = "partial_function_mono";

  val description = "monotonicity rules for partial function definitions";

);

(*** Automated monotonicity proofs ***)

fun strip_cases ctac = ctac #> Seq.map snd;

(*rewrite conclusion with k-th assumtion*)

fun rewrite_with_asm_tac ctxt k =

  Subgoal.FOCUS (fn {context=ctxt', prems, ...} =>

    Local_Defs.unfold_tac ctxt' [nth prems k]) ctxt;

fun dest_case thy t =

  case strip_comb t of

    (Const (case_comb, _), args) =>

      (case Datatype.info_of_case thy case_comb of

         NONE => NONE

       | SOME {case_rewrites, ...} =>

           let

             val lhs = prop_of (hd case_rewrites)

               |> HOLogic.dest_Trueprop |> HOLogic.dest_eq |> fst;

             val arity = length (snd (strip_comb lhs));

             val conv = funpow (length args - arity) Conv.fun_conv

               (Conv.rewrs_conv (map mk_meta_eq case_rewrites));

           in

             SOME (nth args (arity - 1), conv)

           end)

  | _ => NONE;

(*split on case expressions*)

val split_cases_tac = Subgoal.FOCUS_PARAMS (fn {context=ctxt, ...} =>

  SUBGOAL (fn (t, i) => case t of

    _ $ (_ $ Abs (_, _, body)) =>

      (case dest_case (ProofContext.theory_of ctxt) body of

         NONE => no_tac

       | SOME (arg, conv) =>

           let open Conv in

              if not (null (loose_bnos arg)) then no_tac

              else ((DETERM o strip_cases o Induct.cases_tac ctxt false [[SOME arg]] NONE [])

                THEN_ALL_NEW (rewrite_with_asm_tac ctxt 0)

                THEN_ALL_NEW etac @{thm thin_rl}

                THEN_ALL_NEW (CONVERSION

                  (params_conv ~1 (fn ctxt' =>

                    arg_conv (arg_conv (abs_conv (K conv) ctxt'))) ctxt))) i

           end)

  | _ => no_tac) 1);

(*monotonicity proof: apply rules + split case expressions*)

fun mono_tac ctxt =

  K (Local_Defs.unfold_tac ctxt [@{thm curry_def}])

  THEN' (TRY o REPEAT_ALL_NEW

   (resolve_tac (Mono_Rules.get ctxt)

     ORELSE' split_cases_tac ctxt));

(*** Auxiliary functions ***)

(*positional instantiation with computed type substitution.

  internal version of  attribute "[of s t u]".*)

fun cterm_instantiate' cts thm =

  let

    val thy = Thm.theory_of_thm thm;

    val vs = rev (Term.add_vars (prop_of thm) [])

      |> map (Thm.cterm_of thy o Var);

  in

    cterm_instantiate (zip_options vs cts) thm

  end;

(*Returns t $ u, but instantiates the type of t to make the

application type correct*)

fun apply_inst ctxt t u =

  let

    val thy = ProofContext.theory_of ctxt;

    val T = domain_type (fastype_of t);

    val T' = fastype_of u;

    val subst = Type.typ_match (Sign.tsig_of thy) (T, T') Vartab.empty

      handle Type.TYPE_MATCH => raise TYPE ("apply_inst", [T, T'], [t, u])

  in

    map_types (Envir.norm_type subst) t $ u

  end;

fun head_conv cv ct =

  if can Thm.dest_comb ct then Conv.fun_conv (head_conv cv) ct else cv ct;

(*** currying transformation ***)

fun curry_const (A, B, C) =

  Const (@{const_name Product_Type.curry},

    [HOLogic.mk_prodT (A, B) --> C, A, B] ---> C);

fun mk_curry f =

  case fastype_of f of

    Type ("fun", [Type (_, [S, T]), U]) =>

      curry_const (S, T, U) $ f

  | T => raise TYPE ("mk_curry", [T], [f]);

(* iterated versions. Nonstandard left-nested tuples arise naturally

from "split o split o split"*)

fun curry_n arity = funpow (arity - 1) mk_curry;

fun uncurry_n arity = funpow (arity - 1) HOLogic.mk_split;

val curry_uncurry_ss = HOL_basic_ss addsimps

  [@{thm Product_Type.curry_split}, @{thm Product_Type.split_curry}]

(*** partial_function definition ***)

fun gen_add_partial_function prep mode fixes_raw eqn_raw lthy =

  let

    val ((fixp, mono), fixp_eq) = the (lookup_mode lthy mode)

      handle Option.Option => error (cat_lines ["Unknown mode " ^ quote mode ^ ".",

        "Known modes are " ^ commas_quote (known_modes lthy) ^ "."]);

    val ((fixes, [(eq_abinding, eqn)]), _) = prep fixes_raw [eqn_raw] lthy;

    val ((_, plain_eqn), _) = Function_Lib.focus_term eqn lthy;

    val ((f_binding, fT), mixfix) = the_single fixes;

    val fname = Binding.name_of f_binding;

    val cert = cterm_of (ProofContext.theory_of lthy);

    val (lhs, rhs) = HOLogic.dest_eq (HOLogic.dest_Trueprop plain_eqn);

    val (head, args) = strip_comb lhs;

    val F = fold_rev lambda (head :: args) rhs;

    val arity = length args;

    val (aTs, bTs) = chop arity (binder_types fT);

    val tupleT = foldl1 HOLogic.mk_prodT aTs;

    val fT_uc = tupleT :: bTs ---> body_type fT;

    val f_uc = Var ((fname, 0), fT_uc);

    val x_uc = Var (("x", 0), tupleT);

    val uncurry = lambda head (uncurry_n arity head);

    val curry = lambda f_uc (curry_n arity f_uc);

    val F_uc =

      lambda f_uc (uncurry_n arity (F $ curry_n arity f_uc));

    val mono_goal = apply_inst lthy mono (lambda f_uc (F_uc $ f_uc $ x_uc))

      |> HOLogic.mk_Trueprop

      |> Logic.all x_uc;

    val mono_thm = Goal.prove_internal [] (cert mono_goal)

        (K (mono_tac lthy 1))

      |> Thm.forall_elim (cert x_uc);

    val f_def_rhs = curry_n arity (apply_inst lthy fixp F_uc);

    val f_def_binding = Binding.conceal (Binding.name (Thm.def_name fname));

    val ((f, (_, f_def)), lthy') = Local_Theory.define

      ((f_binding, mixfix), ((f_def_binding, []), f_def_rhs)) lthy;

    val eqn = HOLogic.mk_eq (list_comb (f, args),

        Term.betapplys (F, f :: args))

      |> HOLogic.mk_Trueprop;

    val unfold =

      (cterm_instantiate' (map (SOME o cert) [uncurry, F, curry]) fixp_eq

        OF [mono_thm, f_def])

      |> Tactic.rule_by_tactic lthy (Simplifier.simp_tac curry_uncurry_ss 1);

    val rec_rule = let open Conv in

      Goal.prove lthy' (map (fst o dest_Free) args) [] eqn (fn _ =>

        CONVERSION ((arg_conv o arg1_conv o head_conv o rewr_conv) (mk_meta_eq unfold)) 1

        THEN rtac @{thm refl} 1) end;

  in

    lthy'

    |> Local_Theory.note (eq_abinding, [rec_rule])

    |-> (fn (_, rec') =>

      Spec_Rules.add Spec_Rules.Equational ([f], rec')

      #> Local_Theory.note ((Binding.qualify true fname (Binding.name "simps"), []), rec') #> snd)

  end;

val add_partial_function = gen_add_partial_function Specification.check_spec;

val add_partial_function_cmd = gen_add_partial_function Specification.read_spec;

val mode = Parse.$$$ "(" |-- Parse.xname --| Parse.$$$ ")";

val _ = Outer_Syntax.local_theory

  "partial_function" "define partial function" Keyword.thy_decl

  ((mode -- (Parse.fixes -- (Parse.where_ |-- Parse_Spec.spec)))

     >> (fn (mode, (fixes, spec)) => add_partial_function_cmd mode fixes spec));

val setup = Mono_Rules.setup;

end