structure Quotient =

struct

(* constructs the term lambda (c::rty => bool). EX (x::rty). c = rel x *)

fun typedef_term rel rty lthy =

let

  val [x, c] = [("x", rty), ("c", HOLogic.mk_setT rty)]

               |> Variable.variant_frees lthy [rel]

               |> map Free

in

  lambda c

    (HOLogic.exists_const rty $

       lambda x (HOLogic.mk_eq (c, (rel $ x))))

end

(* makes the new type definitions and proves non-emptyness*)

fun typedef_make (qty_name, mx, rel, rty) lthy =

let

  val typedef_tac =

     EVERY1 [rewrite_goal_tac @{thms mem_def},

             rtac @{thm exI},

             rtac @{thm exI},

             rtac @{thm refl}]

  val tfrees = map fst (Term.add_tfreesT rty [])

in

  LocalTheory.theory_result

    (Typedef.add_typedef false NONE

       (qty_name, tfrees, mx)

         (typedef_term rel rty lthy)

           NONE typedef_tac) lthy

end

(* tactic to prove the QUOT_TYPE theorem for the new type *)

fun typedef_quot_type_tac equiv_thm (typedef_info: Typedef.info) =

let

  val unfold_mem = MetaSimplifier.rewrite_rule @{thms mem_def}

  val rep_thm = #Rep typedef_info |> unfold_mem

  val rep_inv = #Rep_inverse typedef_info

  val abs_inv = #Abs_inverse typedef_info |> unfold_mem

  val rep_inj = #Rep_inject typedef_info

in

  EVERY1 [rtac @{thm QUOT_TYPE.intro},

          rtac equiv_thm,

          rtac rep_thm,

          rtac rep_inv,

          rtac abs_inv,

          rtac @{thm exI}, 

          rtac @{thm refl},

          rtac rep_inj]

end

(* proves the QUOT_TYPE theorem *)

fun typedef_quot_type_thm (rel, abs, rep, equiv_thm, typedef_info) lthy =

let

  val quot_type_const = Const (@{const_name "QUOT_TYPE"}, dummyT)

  val goal = HOLogic.mk_Trueprop (quot_type_const $ rel $ abs $ rep)

             |> Syntax.check_term lthy

in

  Goal.prove lthy [] [] goal

    (K (typedef_quot_type_tac equiv_thm typedef_info))

end

(* proves the quotient theorem *)

fun typedef_quotient_thm (rel, abs, rep, abs_def, rep_def, quot_type_thm) lthy =

let

  val quotient_const = Const (@{const_name "QUOTIENT"}, dummyT)

  val goal = HOLogic.mk_Trueprop (quotient_const $ rel $ abs $ rep)

             |> Syntax.check_term lthy

  val typedef_quotient_thm_tac =

    EVERY1 [K (rewrite_goals_tac [abs_def, rep_def]),

            rtac @{thm QUOT_TYPE.QUOTIENT},

            rtac quot_type_thm]

in

  Goal.prove lthy [] [] goal

    (K typedef_quotient_thm_tac)

end

(* two wrappers for define and note *)

fun define (name, mx, rhs) lthy =

let

  val ((rhs, (_ , thm)), lthy') =

     LocalTheory.define Thm.internalK ((name, mx), (Attrib.empty_binding, rhs)) lthy

in

  ((rhs, thm), lthy')

end

fun note (name, thm) lthy =

let

  val ((_,[thm']), lthy') = LocalTheory.note Thm.theoremK ((name, []), [thm]) lthy

in

  (thm', lthy')

end

(* main function for constructing the quotient type *)

fun mk_typedef_main (qty_name, mx, rel, rty, equiv_thm) lthy =

let

  (* generates typedef *)

  val ((_, typedef_info), lthy1) = typedef_make (qty_name, mx, rel, rty) lthy

  (* abs and rep functions *)

  val abs_ty = #abs_type typedef_info

  val rep_ty = #rep_type typedef_info

  val abs_name = #Abs_name typedef_info

  val rep_name = #Rep_name typedef_info

  val abs = Const (abs_name, rep_ty --> abs_ty)

  val rep = Const (rep_name, abs_ty --> rep_ty)

  (* ABS and REP definitions *)

  val ABS_const = Const (@{const_name "QUOT_TYPE.ABS"}, dummyT )

  val REP_const = Const (@{const_name "QUOT_TYPE.REP"}, dummyT )

  val ABS_trm = Syntax.check_term lthy1 (ABS_const $ rel $ abs)

  val REP_trm = Syntax.check_term lthy1 (REP_const $ rep)

  val ABS_name = Binding.prefix_name "ABS_" qty_name

  val REP_name = Binding.prefix_name "REP_" qty_name

  val (((ABS, ABS_def), (REP, REP_def)), lthy2) =

         lthy1 |> define (ABS_name, NoSyn, ABS_trm)

               ||>> define (REP_name, NoSyn, REP_trm)

  (* quot_type theorem *)

  val quot_thm = typedef_quot_type_thm (rel, abs, rep, equiv_thm, typedef_info) lthy2

  val quot_thm_name = Binding.prefix_name "QUOT_TYPE_" qty_name

  (* quotient theorem *)

  val quotient_thm = typedef_quotient_thm (rel, ABS, REP, ABS_def, REP_def, quot_thm) lthy2

  val quotient_thm_name = Binding.prefix_name "QUOTIENT_" qty_name

  (* interpretation *)

  val bindd = ((Binding.make ("", Position.none)), ([]: Attrib.src list))

  val ((_, [eqn1pre]), lthy3) = Variable.import true [ABS_def] lthy2;

  val eqn1i = Thm.prop_of (symmetric eqn1pre)

  val ((_, [eqn2pre]), lthy4) = Variable.import true [REP_def] lthy3;

  val eqn2i = Thm.prop_of (symmetric eqn2pre)

  val exp_morphism = ProofContext.export_morphism lthy4 (ProofContext.init (ProofContext.theory_of lthy4));

  val exp_term = Morphism.term exp_morphism;

  val exp = Morphism.thm exp_morphism;

  val mthd = Method.SIMPLE_METHOD ((rtac quot_thm 1) THEN

    ALLGOALS (simp_tac (HOL_basic_ss addsimps [(symmetric (exp ABS_def)), (symmetric (exp REP_def))])))

  val mthdt = Method.Basic (fn _ => mthd)

  val bymt = Proof.global_terminal_proof (mthdt, NONE)

  val exp_i = [(@{const_name QUOT_TYPE}, ((("QUOT_TYPE_I_" ^ (Binding.name_of qty_name)), true),

    Expression.Named [

     ("R", rel),

     ("Abs", abs),

     ("Rep", rep)

    ]))]

in

  lthy4

  |> note (quot_thm_name, quot_thm)

  ||>> note (quotient_thm_name, quotient_thm)

  ||> LocalTheory.theory (fn thy =>

      let

        val global_eqns = map exp_term [eqn2i, eqn1i];

        (* Not sure if the following context should not be used *)

        val (global_eqns2, lthy5) = Variable.import_terms true global_eqns lthy4;

        val global_eqns3 = map (fn t => (bindd, t)) global_eqns2;

      in ProofContext.theory_of (bymt (Expression.interpretation (exp_i, []) global_eqns3 thy)) end)

end

(* syntax setup *)

local structure P = OuterParse in

fun mk_typedef (qty_name, mx, rty, rel_trm) lthy = 

let

  val EQUIV_ty = ([rty, rty] ---> @{typ bool}) --> @{typ bool}

  val EQUIV_goal = HOLogic.mk_Trueprop (Const (@{const_name EQUIV}, EQUIV_ty) $ rel_trm)

  fun after_qed thms lthy =

  let

    val thm = the_single (flat thms)

  in

    mk_typedef_main (Binding.name qty_name, mx, rel_trm, rty, thm) lthy |> snd   

  end

in

  Proof.theorem_i NONE after_qed [[(EQUIV_goal, [])]] lthy

end

(* FIXME: allow more than one type definition *)

val quottype_parser = 

    P.short_ident -- P.opt_infix -- 

       (P.$$$ "=" |-- P.typ) -- 

         (P.$$$ "/" |-- P.term)

fun mk_typedef_cmd (((qstr, mx), rstr), rel_str) lthy = 

let

  val rty = Syntax.parse_typ lthy rstr

  val rel_trm = Syntax.parse_term lthy rel_str

                |> Syntax.check_term lthy

in

  mk_typedef (qstr, mx, rty, rel_trm) lthy  

end

val _ = OuterKeyword.keyword "/"

val _ = 

    OuterSyntax.local_theory_to_proof "quotient" 

      "quotient type definitions (requires equivalence proofs)"

         OuterKeyword.thy_goal (quottype_parser >> mk_typedef_cmd)	     

end; (* local *)

end; (* structure *)

open Quotient