(*  Title:      nominal_eqvt.ML

    Author:     Stefan Berghofer (original code)

    Author:     Christian Urban

    Automatic proofs for equivariance of inductive predicates.

*)

signature NOMINAL_EQVT =

sig

  val raw_equivariance: term list -> thm -> thm list -> Proof.context -> thm list

  val equivariance_cmd: string -> Proof.context -> local_theory

end

structure Nominal_Eqvt : NOMINAL_EQVT =

struct

open Nominal_Permeq;

open Nominal_ThmDecls;

val atomize_conv = 

  Raw_Simplifier.rewrite_cterm (true, false, false) (K (K NONE))

    (HOL_basic_ss addsimps @{thms induct_atomize});

val atomize_intr = Conv.fconv_rule (Conv.prems_conv ~1 atomize_conv);

fun atomize_induct ctxt = Conv.fconv_rule (Conv.prems_conv ~1

  (Conv.params_conv ~1 (K (Conv.prems_conv ~1 atomize_conv)) ctxt));

(** equivariance tactics **)

fun eqvt_rel_single_case_tac ctxt pred_names pi intro  = 

  let

    val thy = Proof_Context.theory_of ctxt

    val cpi = Thm.cterm_of thy pi

    val pi_intro_rule = Drule.instantiate' [] [NONE, SOME cpi] @{thm permute_boolI}

    val eqvt_sconfig = eqvt_strict_config addexcls pred_names

    val simps1 = HOL_basic_ss addsimps @{thms permute_fun_def permute_self split_paired_all}

    val simps2 = HOL_basic_ss addsimps @{thms permute_bool_def  permute_minus_cancel(2)}

  in

    eqvt_tac ctxt eqvt_sconfig THEN'

    SUBPROOF (fn {prems, context as ctxt, ...} =>

      let

        val prems' = map (transform_prem2 ctxt pred_names) prems

        val prems'' = map (fn thm => eqvt_rule ctxt eqvt_sconfig (thm RS pi_intro_rule)) prems'

        val prems''' = map (simplify simps2 o simplify simps1) prems''

      in

        HEADGOAL (rtac intro THEN_ALL_NEW resolve_tac (prems' @ prems'' @ prems''')) 

      end) ctxt

  end

fun eqvt_rel_tac ctxt pred_names pi induct intros =

  let

    val cases = map (eqvt_rel_single_case_tac ctxt pred_names pi) intros

  in

    EVERY' ((DETERM o rtac induct) :: cases)

  end

(** equivariance procedure *)

fun prepare_goal pi pred =

  let

    val (c, xs) = strip_comb pred;

  in

    HOLogic.mk_imp (pred, list_comb (c, map (mk_perm pi) xs))

  end

(* stores thm under name.eqvt and adds [eqvt]-attribute *)

fun get_name (Const (a, _)) = a

  | get_name (Free  (a, _)) = a

fun raw_equivariance pred_trms raw_induct intrs ctxt = 

  let

    val is_already_eqvt = 

      filter (is_eqvt ctxt) pred_trms

      |> map (Syntax.string_of_term ctxt)

    val _ = if null is_already_eqvt then ()

      else error ("Already equivariant: " ^ commas is_already_eqvt)

    val pred_names = map get_name pred_trms

    val raw_induct' = atomize_induct ctxt raw_induct

    val intrs' = map atomize_intr intrs

    val (([raw_concl], [raw_pi]), ctxt') = 

      ctxt 

      |> Variable.import_terms false [concl_of raw_induct'] 

      ||>> Variable.variant_fixes ["p"]

    val pi = Free (raw_pi, @{typ perm})

    val preds = map (fst o HOLogic.dest_imp)

      (HOLogic.dest_conj (HOLogic.dest_Trueprop raw_concl));

    val goal = HOLogic.mk_Trueprop 

      (foldr1 HOLogic.mk_conj (map (prepare_goal pi) preds)) 

  in 

    Goal.prove ctxt' [] [] goal 

      (fn {context,...} => eqvt_rel_tac context pred_names pi raw_induct' intrs' 1)

      |> Datatype_Aux.split_conj_thm 

      |> Proof_Context.export ctxt' ctxt

      |> map (fn th => th RS mp)

      |> map zero_var_indexes

  end

fun note_named_thm (name, thm) ctxt = 

  let

    val thm_name = Binding.qualified_name 

      (Long_Name.qualify (Long_Name.base_name name) "eqvt")

    val attr = Attrib.internal (K eqvt_add)

    val ((_, [thm']), ctxt') =  Local_Theory.note ((thm_name, [attr]), [thm]) ctxt

  in

    (thm', ctxt')

  end

fun equivariance_cmd pred_name ctxt =

  let

    val thy = Proof_Context.theory_of ctxt

    val ({names, ...}, {preds, raw_induct, intrs, ...}) =

      Inductive.the_inductive ctxt (Sign.intern_const thy pred_name)

    val thms = raw_equivariance preds raw_induct intrs ctxt 

  in

    fold_map note_named_thm (names ~~ thms) ctxt |> snd

  end

val _ =

  Outer_Syntax.local_theory ("equivariance", Keyword.thy_decl)

    "Proves equivariance for inductive predicate involving nominal datatypes." 

      (Parse.xname >> equivariance_cmd)

end (* structure *)